6 The innervation of the head and neck

The distinguished anatomist and zoologist, J.Z. Young, in his account of the evolution of the vertebrate head and neck (cephalization), suggested that the initial location of the head from a creature with repeated body segments (e.g. an earthworm-like animal) was merely related to the opening of the gut, the mouth, at one end of the creature as it moved forward. At about the same time, organs of special sense would evolve around the mouth so that the animal could better find, and select, its food and consequently the front end of the neural tube would evolve into a brain in order to ‘analyse’ the information received from these organs of special sense. In evolutionary terms, therefore, the mouth is the primary organ of the head; the special senses are secondary and the brain tertiary. This is all reflected in understanding the functions of the human head and neck. The primary function is metabolic, being concerned with the selection and ingestion of food and with respiration through the nose (a derivative of the foregut) and mouth. The head, having a very complex array of special senses and peripheral nerves, also functions overall as an ‘analyser’ of the external environment and as a means of recognizing other creatures (both friend and foe). Given these functions it is therefore not surprising that the mouth is so important biologically and physiologically, that it is associated with a complex sensory and motor innervation, and that clinically the treatment of the face, jaws, and mouth requires a thorough-going knowledge of the complex innervation.

This chapter serves as an ‘introduction’ to the peripheral nervous system of the head and neck in order to provide essentially background information preparatory to a more detailed perspective in relation to clinical situations in subsequent chapters. Specialist reference texts should be consulted if further detail is required^{1  –7}.

The face is bordered by the pinnae of the ears laterally, by the chin inferiorly, and by the hairline superiorly. This region is known to anatomists as the ‘superficial face’. This region continues superiorly into the scalp. The ‘deep face’ is the temporal fossa, the parotid region, and the infratemporal fossa. There are two major nerves innervating the superficial face. The fifth cranial nerve, the trigeminal nerve, for all three of its divisions is the ‘great sensory nerve’ of the head (only the mandibular division has a motor component primarily supplying the muscles of mastication). The seventh cranial nerve, the facial nerve, is the ‘great motor nerve’ of the head, supplying primarily the muscles of facial expression (only the nervus intermedius component has a major sensory element, associated with taste).

Three large areas of the face can be mapped out to indicate the peripheral nerve fields associated with the three divisions of the trigeminal nerve (Figure 6.1). Embryologically, each division of the trigeminal nerve is associated with a developing facial process which gives rise in the adult to a particular area of the face. The mandibular nerve is associated with the mandibular process of the first pharyngeal (branchial) arch, the maxillary nerve with the maxillary process from the first arch, and the ophthalmic nerve with the frontonasal process overlying the developing prosencephalon.

The area supplied by the mandibular nerve includes the skin overlying the mandible, the lower lip, the fleshy part of the cheek, part of the auricle, and part of the temple. It has three cutaneous branches:

The maxillary nerve supplies the skin of the lower eyelid, the prominence of the cheek, the ala part of the nose, part of the temple, and the upper lip. It has three cutaneous branches:

This nerve has three branches (frontal, nasociliary, and lacrimal) which are distributed to the face. The area supplied is extensive, including the forehead, the upper eyelid, and much of the external surface of the nose. Five nerves are concerned with the cutaneous innervation:

One part of the face which does not receive its cutaneous innervation from the trigeminal nerve is the angle of the mandible. The skin in this region is supplied by the great auricular nerve. This nerve is derived from the cervical plexus (anterior primary rami of the second and third cervical nerves). Appearing at the posterior border of the sternocleidomastoid muscle, it passes forwards and upwards across the muscle to reach the angle of the mandible.

The mandibular nerve (Figure 6.2) is the largest division of the trigeminal nerve and is the only one which contains motor as well as sensory fibres. Developmentally, it is the nerve of the first pharyngeal (branchial) arch and is thus responsible for supplying structures derived from it. Its sensory fibres supply the mandibular teeth and their supporting structures, the mucosa of the anterior two-thirds of the tongue and the floor of the mouth, and the skin of the lower part of the face (including the lower lip and parts of the temporal region and auricle). Its motor fibres supply the four muscles of mastication and the mylohyoid, anterior belly of digastric, tensor veli palatini, and tensor tympani muscles.

The mandibular nerve is formed in the infratemporal fossa by the union of the sensory and motor roots immediately after they leave the skull at the foramen ovale. Within the foramen ovale, the motor root (or roots) lie posteromedially to the sensory root and these roots are accompanied by emissary veins, the lesser petrosal nerve (from the glossopharyngeal nerve) going to the otic ganglion, and by the accessory meningeal artery. As the mandibular nerve leaves the foramen ovale, it lies on the tensor veli palatini muscle and is covered laterally by the upper head of the lateral pterygoid muscle (slightly anterior to the neck of the mandible). After a short course, the nerve divides into a smaller anterior trunk and a larger posterior trunk. Before this division, the main trunk gives off two branches—the meningeal branch and the nerve to medial pterygoid. The anterior trunk of the mandibular nerve is mainly motor, the posterior trunk mainly sensory.

The meningeal branch of the mandibular nerve (nervus spinosus) arises from the main trunk of the mandibular nerve. It is a ‘recurrent nerve’ as it runs back into the middle cranial fossa through the foramen spinosum. It supplies the dura mater lining the middle and anterior cranial fossae and the mucosa of the mastoid antrum and mastoid air cells.

The nerve to the medial pterygoid muscle enters the deep surface of the muscle and also gives slender branches that pass uninterrupted through the otic ganglion to supply the tensor tympani and tensor veli palatini muscles.

The masseteric nerve is usually the first branch of the anterior trunk of the mandibular nerve. It passes above the upper border of the lateral pterygoid muscle (accompanying the posterior deep temporal nerve) and then crosses the mandibular notch (between the condylar and coronoid processes) to be distributed into the masseter muscle. It also gives an articular branch to the temporomandibular joint. The nerve enters the masseter muscle as two branches. The upper branch is smaller and runs to the deeper layers of the muscle. The larger, lower trunk innervates the more superficial layers of the masseter muscle.

The deep temporal nerves also pass above the lateral pterygoid muscle. Anatomists have provided varying descriptions for them. Anterior, middle, and posterior deep temporal nerves may be recognized.

The nerve to the lateral pterygoid muscle may arise separately or may run with the buccal nerve before entering the deep surface of the lateral pterygoid muscle.

The buccal branch of the mandibular nerve is the only sensory branch of the anterior trunk of the mandibular nerve. On emerging between the upper and lower heads of the lateral pterygoid muscle, it passes downwards and forwards across the lower head to contact the medial surface of the temporalis muscle as it inserts onto the coronoid process of the mandible. It then clears the ramus of the mandible to lie on the lateral surface of the buccinator muscle in the cheek. At this point, it is close to the retromolar fossa of the mandible. It now gives branches to the skin of the cheek (see Figure 6.1) before piercing the buccinator to supply its lining mucosa, the buccal sulcus, and the buccal gingiva related to the mandibular molar and premolar teeth. It may also carry secretomotor fibres to minor salivary glands in the buccal mucosa, these being post-ganglionic fibres from the otic ganglion. The buccal branch of the mandibular nerve may be seen to ‘anastomose’ with the buccal branches of the facial nerve.

The auriculotemporal nerve is the first branch of the posterior trunk of the mandibular nerve. It is essentially sensory but it also distributes autonomic fibres to the parotid gland derived from the otic ganglion. It usually arises as two roots (approximately 75% of cases) that encircle the middle meningeal artery and unite behind the artery. The nerve then runs backwards under the lateral pterygoid muscle to lie beneath the mandibular condyle (between the condyle and the sphenomandibular ligament). On entering the parotid region, it turns to emerge superficially between the temporomandibular joint and the external acoustic meatus. From the upper surface of the parotid gland, the auriculotemporal nerve ascends on the side of the head with the superficial temporal vessels, passing over the posterior part of the zygomatic arch. It gives several branches along its course:

The lingual nerve is the second branch of the posterior trunk of the mandibular nerve. It is essentially a sensory nerve but, following union with the chorda tympani branch of the facial nerve, it also contains parasympathetic fibres. Initially, the nerve lies on the tensor veli palatini muscle deep to the lateral pterygoid muscle. Here, the chorda tympani nerve (which has entered the infratemporal fossa via the petrotympanic fissure and passed over the spine of the sphenoid bone) joins the posterior surface of the lingual nerve. After emerging from the inferior border of the lateral pterygoid muscle, the lingual nerve curves downwards and forwards in the space between the ramus of the mandible and the medial pterygoid muscle (pterygomandibular space). At this level, it lies anterior to, and slightly deeper than, the inferior alveolar nerve. The lingual nerve then leaves the infratemporal fossa, passing downwards and forwards to lie close to the lingual alveolar plate of the mandibular third molar. Before curving forwards into the tongue, the nerve is found above the origin of the mylohyoid muscle and lateral to the hyoglossus muscle.

The close relationship of the lingual nerve to the third molar tooth makes the nerve susceptible to damage during removal of the tooth. In addition, in about one in seven cases, the lingual nerve is actually located above the lingual bony plate in the third molar region and is liable to damage during surgery.

The lingual nerve supplies the mucosa covering the anterior two-thirds of the dorsum of the tongue, the ventral surface of the tongue, the floor of the mouth, and the lingual gingivae of the mandibular teeth. The chorda tympani fibres travelling with the lingual nerve are of two types: sensory and parasympathetic. The sensory fibres are associated with taste for the anterior two-thirds of the dorsum of the tongue. The parasympathetic fibres are preganglionic fibres that pass to the submandibular ganglion. Postganglionic fibres are distributed to the submandibular and sublingual salivary glands.

The chorda tympani branch of the facial nerve is distributed through the lingual nerve and has two types of fibres. Sensory fibres are associated with taste to the anterior two-thirds of the tongue. Parasympathetic fibres are preganglionic to the submandibular ganglion. Postganglionic fibres are secretomotor to the submandibular and sublingual glands.

The inferior alveolar nerve is the largest branch of the mandibular division of the trigeminal nerve. It is the third branch of the posterior trunk of the mandibular nerve. Although it is essentially a sensory nerve, it also carries motor fibres which are given off as the mylohyoid nerve. Indeed, the mylohyoid nerve contains all the motor fibres of the posterior trunk of the mandibular nerve. The inferior alveolar nerve descends deep to the lateral pterygoid muscle, posterior to the lingual nerve in the pterygoid hiatus. Here, it is crossed by the maxillary artery. On emerging at the inferior border of the muscle, it passes between the sphenomandibular ligament and the ramus of the mandible to enter the mandibular foramen. It is accompanied in its course by inferior alveolar blood vessels.

The mylohyoid nerve is given off just before the mandibular foramen. It pierces the sphenomandibular ligament and runs in a groove (the mylohyoid groove) which lies immediately below the mandibular foramen. The mylohyoid nerve supplies the mylohyoid muscle and the anterior belly of the digastric. The mylohyoid nerve may also contain sensory fibres that supply the skin of the chin and medial parts of the submandibular triangle in the suprahyoid region.

The main distribution of the inferior alveolar nerve is to the mandibular teeth and their supporting structures, there being molar and incisive branches. The mental nerve is a cutaneous branch that supplies the skin of the chin and the lower lip. It arises within the mandible in the premolar region, but soon exits onto the face via the mental foramen (see Figure 6.1).

The trigeminal nerve, although not specifically having parasympathetic fibres, is associated with the parasympathetic ganglia of the head and indeed conveys postganglionic parasympathetic fibres to target organs. The mandibular nerve in this context is associated with the otic ganglion in the infratemporal fossa and the submandibular ganglion in the floor of the mouth.

This parasympathetic ganglion lies immediately below the foramen ovale on the medial surface of the main trunk of the mandibular nerve. It is concerned primarily with supplying the parotid gland. Like other parasympathetic ganglia in the head, three types of fibres are associated with it: parasympathetic, sympathetic, and sensory fibres. However, only the parasympathetic fibres synapse in the ganglion. The preganglionic parasympathetic fibres originate from the inferior salivatory nucleus in the brainstem. The fibres pass out in the glossopharyngeal nerve, appearing as the lesser (superficial) petrosal nerve from the tympanic plexus in the middle ear cavity. The lesser petrosal nerve reaches the otic ganglion by a complex course. Passing through the petrous part of the temporal bone, the lesser petrosal nerve comes to lie in the floor of the middle cranial fossa. Here, it is lateral to the greater (superficial) petrosal branch of the facial nerve. The lesser petrosal nerve usually enters the infratemporal fossa through the foramen ovale to join the otic ganglion. On occasion, the lesser petrosal nerve passes through the sphenopetrosal fissure. The sympathetic root of the otic ganglion is derived from postganglionic fibres from the superior cervical ganglion. They are said to reach the otic ganglion from the plexus on the middle meningeal artery. Other descriptions have it that the sympathetic root arises from the deep petrosal nerve or directly from the internal carotid plexus. The sensory root is derived from the auriculotemporal nerve. The postganglionic parasympathetic fibres (with sympathetic and sensory components) reach the parotid gland by way of the auriculotemporal nerve. Parasympathetic fibres may also innervate the minor salivary glands in the cheek, passing with the buccal branch of the mandibular nerve.

This parasympathetic ganglion is found in the floor of the mouth, on the superficial surface of the hyoglossus muscle and under cover of the mylohyoid muscle. The ganglion lies between the lingual nerve and the deep part of the submandibular gland. Indeed, it is suspended by two roots from the lingual nerve. The prime function of the submandibular ganglion is to supply the submandibular and sublingual salivary glands.

In common with the other parasympathetic ganglia in the head, the submandibular ganglion has a parasympathetic, a sympathetic, and a sensory supply. Only the parasympathetic fibres synapse within the ganglion. Preganglionic parasympathetic fibres originate from the superior salivatory nucleus in the brain stem. The fibres pass with the nervus intermedius of the facial nerve into the internal acoustic meatus and exit the skull with the chorda tympani nerve at the petrotympanic fissure. The chorda tympani nerve joins the lingual nerve in the infratemporal fossa and by this route the parasympathetic fibres reach the submandibular ganglion. It is claimed that the preganglionic parasympathetic fibres pass via the posterior root linking the ganglion to the lingual nerve. The sympathetic supply to the ganglion is derived from the superior cervical ganglion. It reaches the submandibular ganglion via the sympathetic nerve plexus surrounding the facial artery. The sensory supply arises from the adjacent lingual nerve.

Branches from the ganglion pass directly to the submandibular gland. The sublingual gland, however, is supplied by fibres which re-enter the lingual nerve by the anterior connecting root.

The temporomandibular joint is richly innervated, particularly its upper aspect. Of special significance are the encapsulated proprioceptive nerve endings important in the reflex control of mastication. Free nerve endings associated with nociception are also present. Innervation for the joint is provided by the auriculotemporal, masseteric, and deep temporal nerves. The auriculotemporal branch of the mandibular division of the trigeminal nerve winds around the back of the temporomandibular joint, between it and the external acoustic meatus, before ascending in front of the tragus of the auricle to the temporal region. It provides multiple branches supplying the temporomandibular joint. The masseteric branch of the mandibular division of the trigeminal nerve passes through the mandibular notch to enter the posterior surface of the masseter muscle and, during its course, also gives multiple branches supplying the temporomandibular joint. The (posterior) deep temporal branch of the mandibular division of the trigeminal nerve arises in the infratemporal fossa and, passing up to supply the temporalis muscle, provides a branch to the temporomandibular joint. Additional sources of supply for the temporomandibular joint have been reported to be provided by the facial nerve and the otic ganglion.

This division of the trigeminal nerve (the fifth cranial nerve) contains only sensory fibres. Functionally, it supplies the maxillary teeth and their supporting structures, the hard and soft palate, the maxillary air sinus, much of the nasal cavity, and skin overlying the middle part of the face.

The maxillary nerve (Figure 6.3) arises from the trigeminal ganglion on the floor of the middle cranial fossa. It passes along the lateral dural wall of the cavernous sinus to exit the cranial cavity at the foramen rotundum. It emerges from the foramen rotundum in the upper part of the pterygopalatine fossa, where most of the branches are derived. These branches can be classified into those which come directly from the maxillary nerve and those which are associated with the pterygopalatine parasympathetic ganglion.

The meningeal nerve is the only branch from the main trunk of the maxillary nerve that does not originate in the pterygopalatine fossa; it arises within the middle cranial fossa, before the foramen rotundum. It runs with the middle meningeal artery and innervates the dura mater lining the middle cranial fossa.

The ganglionic branches are usually two in number and connect the maxillary nerve to the pterygopalatine ganglion.

The zygomatic nerve leaves the pterygopalatine fossa through the inferior orbital fissure. It passes along the lateral wall of the orbit before dividing into zygomaticotemporal and zygomaticofacial branches. These pass through the zygomatic bone to supply overlying skin (see Figure 6.1). The zygomaticotemporal nerve exits the zygomatic bone at its temporal (medial) surface. It pierces the temporal fascia to supply skin over the temple. The zygomaticotemporal nerve also gives a branch to the lacrimal nerve, which carries autonomic fibres to the lacrimal gland. The zygomaticofacial nerve leaves the zygomatic bone on its lateral surface to supply skin overlying the prominence of the cheek.

The posterior superior alveolar nerve(s) is one of three superior alveolar nerves that supply the maxillary teeth. The middle and anterior superior alveolar nerves are branches of the infraorbital nerve (see below). The posterior superior alveolar nerve(s) leaves the pterygopalatine fossa through the pterygomaxillary fissure. Thence, it runs onto the tuberosity of the maxilla and eventually pierces the bone to supply the maxillary molar teeth and the maxillary sinus. Before entering the maxilla, the nerve provides a gingival branch which innervates the buccal gingivae around the maxillary molars. The extra-bony course of the posterior superior alveolar nerve is variable. The nerve can subdivide into several branches just before, and just after, it enters the maxilla. Alternatively, it may arise as several distinct branches at the main trunk of the maxillary nerve.

The infraorbital nerve can be regarded as the terminal branch of the maxillary nerve proper. It leaves the pterygopalatine fossa to enter the orbit at the inferior orbital fissure. Initially lying in a groove in the floor of the orbit (the infraorbital groove), the infraorbital nerve runs into a canal (the infraorbital canal) and passes onto the face at the infraorbital foramen (see Figure 6.1 for the cutaneous distribution of the nerve). The middle and anterior superior alveolar nerves arise from the infraorbital nerve in the orbit.

The infraorbital nerve supplies the conjunctiva and skin of the lower eyelid. It also innervates the skin over the upper jaw (see Figure 6.1).

The branches of the maxillary nerve that arise with the pterygopalatine ganglion contain not only sensory fibres from the maxillary nerve, but also autonomic fibres from the ganglion, which are mainly distributed to glands and blood vessels.

The orbital nerve passes from the pterygopalatine ganglion into the orbit through the inferior orbital fissure. It supplies periosteum and, via sympathetic fibres, the orbitalis muscle. The orbital nerve can also supply part of the maxillary sinus and may pass through the posterior ethmoidal foramen to innervate posterior ethmoidal air cells and the sphenoid air sinus. It may also connect with the ciliary ganglion.

The nasopalatine nerve runs medially from the pterygopalatine ganglion into the nasal cavity through the sphenopalatine foramen. It passes across the roof of the nasal cavity to reach the back of the nasal septum. The nasopalatine nerve then passes downwards and forwards within a groove on the vomer to supply the posteroinferior part of the nasal septum. It passes through the incisive canal, where it usually forms a single nerve with its fellow of the opposite side, and emerges on the hard palate at the incisive fossa to supply the oral mucosa around the incisive papilla and palatal gingiva of the anterior teeth.

The posterior superior nasal nerve enters the back of the nasal cavity through the sphenopalatine foramen. It divides into lateral and medial branches. The lateral branches supply the posterosuperior part of the lateral wall of the nasal fossa. The medial branches cross the roof of the nasal cavity to supply the nasal septum overlying the posterior part of the perpendicular plate of the ethmoid.

The posterior inferior nasal nerve supplies the inferior part of the lateral wall of the nose in the region of the inferior nasal concha. It may arise directly from the pterygopalatine ganglion or appear as a branch from the anterior palatine nerve.

The greater (anterior) palatine nerve passes downwards from the pterygopalatine ganglion, through the palatine canal, and onto the hard palate at the palatine foramen. Within the greater palatine canal, it can give off nasal branches that innervate the posteroinferior part of the lateral wall of the nasal fossa. On the palate, it runs forwards at the interface between the palatine process and the alveolar process of the maxilla to supply much of the mucosa of the hard palate and palatal gingivae (except around the incisive papilla).

The lesser (posterior) palatine nerve(s) passes downwards from the pterygopalatine ganglion initially through the palatine canal. It then passes through the lesser palatine canal in the pyramidal process of the palatine bone and onto the palate at the lesser palatine foramen (or foramina). It runs backwards to supply the soft palate.

The pharyngeal branch originates from the pterygopalatine ganglion and passes through the palatovaginal canal to supply the mucosa of the nasopharynx. The palatovaginal canal is formed when the groove on the undersurface of the vaginal process of the sphenoid bone articulates with the upper surface of the sphenoid process of the palatine bone. The pharyngeal branch has also been reported to pass through the vomerovaginal canal, which generally transmits the pharyngeal branch of the sphenopalatine artery. The vomerovaginal canal lies between the upper surface of the vaginal process of the sphenoid bone and the ala of the vomer and is often continuous with the pterygoid canal.

The parasympathetic ganglion associated with the maxillary nerve is the pterygopalatine ganglion that is located within the pterygopalatine fossa.

This parasympathetic ganglion is situated below the maxillary nerve in the pterygopalatine fossa, connected by two ganglionic branches. It is concerned primarily with supplying the nose, palate, and lacrimal gland.

As with other parasympathetic ganglia in the head, three types of fibres enter the pterygopalatine ganglion: parasympathetic, sympathetic, and sensory fibres. However, only the parasympathetic fibres synapse in the ganglion. The preganglionic parasympathetic fibres originate from the superior salivatory nucleus in the brainstem. The fibres pass with the nervus intermedius of the facial nerve. They subsequently emerge as the greater (superficial) petrosal nerve. This occurs within the facial canal of the temporal bone, close to the geniculate ganglion of the facial nerve. The greater petrosal nerve then passes through the bone to appear on the floor of the middle cranial fossa. It then runs medially in a shallow groove towards the foramen lacerum. Passing within the foramen lacerum, the greater petrosal nerve enters the pterygoid canal which lies at the base of the pterygoid process. After passing along the pterygoid canal, the nerve emerges into the pterygopalatine fossa and joins the pterygopalatine ganglion. Postganglionic sympathetic fibres run to the pterygopalatine ganglion by a complex course. From the superior cervical ganglion, sympathetic fibres run to the internal carotid plexus surrounding the internal carotid artery. From this plexus, a branch called the deep petrosal nerve is given off that enters the pterygoid canal to reach the pterygopalatine ganglion. The greater petrosal nerve and the deep petrosal nerve join within the pterygoid canal to become the nerve of the pterygoid canal. The sensory fibres to the ganglion run in the ganglionic branches of the maxillary nerve.

The nerves leaving the pterygopalatine ganglion are the orbital nerve, the nasopalatine nerve, the greater and lesser palatine nerves, the posterior superior and inferior nasal nerves, and the pharyngeal nerve. These nerves are described above with the maxillary nerve. The parasympathetic component will be distributed within these nerves to supply the minor salivary glands. The parasympathetic component of the pterygopalatine ganglion is also responsible for supplying the lacrimal gland. The fibres first pass from the ganglion in one of the ganglionic branches to the maxillary nerve. They then travel with the zygomatic and zygomaticotemporal branches. Within the orbit, they pass from the zygomaticotemporal nerve to the lacrimal nerve (of the ophthalmic nerve) to reach the lacrimal gland.

The ophthalmic nerve (Figure 6.4) is a division of the trigeminal nerve (the fifth cranial nerve) and is a sensory nerve that travels through the orbit to supply primarily the upper part of the face. Developmentally, it is the nerve of the frontonasal process.

The trigeminal ganglion in the floor of the middle cranial fossa is the site where the ophthalmic nerve arises. The nerve passes along the lateral dural wall of the cavernous sinus and gives off three main branches just before the superior orbital fissure.

The lacrimal nerve enters the orbit through the superior orbital fissure, above the common tendinous ring of the recti muscles. Here, it is situated lateral to the frontal and trochlear nerves. The lacrimal nerve passes forwards along the lateral wall of the orbit on the superior border of the lateral rectus muscle. It passes through the lacrimal gland and the orbital septum to supply conjunctiva and skin covering the lateral part of the upper eyelid (see Figure 6.1). The lacrimal nerve communicates with the zygomatic branch of the maxillary nerve. By this means, parasympathetic fibres associated with the pterygopalatine ganglion are conveyed to the lacrimal gland.

The frontal nerve is the largest branch of the ophthalmic nerve. It enters the orbit through the superior orbital fissure, above the common tendinous ring of the recti muscles, and lies between the lacrimal nerve laterally and the trochlear nerve medially. The frontal nerve passes forwards on the levator palpebrae superioris muscle, towards the rim of the orbit. About halfway along this course, it divides into the supraorbital and supratrochlear nerves.

The supraorbital nerve is the larger of the terminal branches of the frontal nerve. It continues forwards along the levator palpebrae superioris muscle and leaves the orbit through the supraorbital notch (or foramen) to emerge onto the forehead. The supraorbital nerve supplies mucous membrane lining the frontal sinus, skin and conjunctiva covering the upper eyelid, and skin over the forehead and scalp (see Figure 6.1).

The supratrochlear nerve runs medially above the pulley for the superior oblique muscle. It gives a descending branch to the infratrochlear nerve and ascends onto the forehead through the frontal notch. It supplies skin and conjunctiva covering the upper eyelid, and skin over the forehead (see Figure 6.1).

The nasociliary nerve passes into the orbit through the superior orbital fissure, within the common tendinous ring of the recti muscles. Initially, the nerve lies lateral to the optic nerve. It then runs forwards and medially across the optic nerve and, coursing between the superior oblique and medial rectus muscles, comes to lie close to the medial wall of the orbit. Near the anterior ethmoidal foramen, the nasociliary nerve divides into its terminal branches: the anterior ethmoidal and infratrochlear nerves.

The first branches of the nasociliary nerve are sensory branches to the ciliary ganglion. They leave the ganglion in the short ciliary nerves, running to the eyeball to supply the cornea, the ciliary body, and the iris.

Two or three long ciliary branches arise from the nasociliary nerve as it crosses the optic nerve. These ciliary nerves pierce the sclera at the back of the eye and pass forwards to provide sensory innervation to the cornea and iris. They also distribute sympathetic fibres to the dilator pupillae muscle. The sympathetic fibres originate from the superior cervical ganglion. They are postganglionic fibres which travel in the plexus surrounding the internal carotid artery. They join the ophthalmic nerve in the cavernous sinus.

The posterior ethmoidal nerve passes beneath the superior oblique muscle and leaves the orbit through the posterior ethmoidal foramen to enter the nose. It supplies the sphenoidal sinus and the posterior ethmoidal air cells.

The anterior ethmoidal nerve exits the orbit through the anterior ethmoidal foramen. It enters the anterior cranial fossa where the cribriform plate of the ethmoid bone meets the orbital part of the frontal bone. It then runs into the roof of the nose through a small foramen at the side of the crista galli. The anterior ethmoidal nerve supplies the anterior and middle ethmoidal air cells and some of the mucosa covering the nasal septum and the lateral wall of the nose. It terminates on the face as the external nasal nerve (see Figure 6.1).

The infratrochlear nerve passes forwards along the medial wall of the orbit below the pulley of the superior oblique muscle. It passes above the medial palpebral ligament to reach the side of the nose. It supplies the lacrimal sac, the caruncle, the conjunctiva at the medial canthus, and the skin on the medial aspect of the upper eyelid.

The parasympathetic ganglion associated with the ophthalmic nerve is the ciliary ganglion that is located within the orbit and is described in this chapter elsewhere.

The facial nerve (Figure 6.5), the seventh cranial nerve, consists of two components: the motor facial nerve ‘proper’ and the nervus intermedius. The facial nerve ‘proper’ leaves the skull through the stylomastoid foramen. The nerve then passes over the styloid process of the temporal bone and its attached muscles to enter the parotid gland. Before entering the parotid, the facial nerve gives rise to a posterior auricular nerve and a digastric nerve.

The posterior auricular nerve supplies the auricular muscles (muscles of facial expression).

The digastric nerve supplies the posterior belly of the digastric muscle together with the accompanying stylohyoid muscle.

The facial nerve gives rise to five named branches: temporal, zygomatic, buccal, (marginal) mandibular, and cervical. These branches arise from two main trunks or divisions within the gland, namely the temporofacial and the cervicofacial divisions. The temporal and zygomatic branches usually arise from the temporofacial division while the mandibular and cervical branches arise from the cervicofacial division. The buccal branch has a variable origin.

Within the parotid gland, the facial nerve divides into the temporofacial and cervicofacial divisions at a point termed the pes anserinus (‘goose’s foot’). Variations of these two main divisions are common. There are frequently many ‘anastomoses’ between them, thus forming a ‘parotid plexus’ within the substance of the gland. Six distinctive patterns can, however, be recognized. For the Type I pattern (13%), no ‘anastomoses’ occur between adjacent branches. For the Type II pattern (20%), an arcade of ‘anastomoses’ exists between the branches of the temporofacial division and usually occurs beyond the anterior border of the parotid. For the Type III pattern (28%), a single large ‘anastomosis’ occurs between the temporofacial and cervicofacial divisions (again beyond the anterior margin of the gland). For the Type IV pattern (24%), there are ‘anastomotic’ loops between the temporal and zygomatic branches, as well as connections within the gland between the cervicofacial division and the buccal and zygomatic branches. For the Type V pattern (9%) there are connections between the cervicofacial division and the temporofacial division arising either from the buccal branch or directly from the point where the facial nerve trunk divides. Finally, for the Type VI pattern (6%), there is a marked plexus of ‘anastomoses’. This is the only pattern of the facial nerve in which the (marginal) mandibular branch is reinforced by an ‘anastomosis’ from an adjacent branch.

The temporal nerve (three or four branches) crosses the zygomatic arch just anteriorly to the superficial temporal vessels. The nerve supplies the auricular muscles and the muscles of the forehead and orbicularis oculi.

The zygomatic nerve (one, two, or three branches) runs below the inferior border of the zygomatic arch. It also innervates the orbicularis oculi muscle and the muscles of the nose and the upper lip.

The buccal branch of the facial nerve has a variable origin from the cervicofacial and/or the temporofacial divisions. Usually, there is only one branch, occasionally two. A line drawn from the labial commissure to the tragus indicates the level of the buccal nerve. Where the nerve is single, it is usually found below the origin of the parotid duct. Where there are two branches, one branch passes above and one below the parotid duct. The buccal nerve passes beneath the zygomaticus muscle to supply the buccinator muscle and the muscles of the upper lip.

The (marginal) mandibular nerve (usually two branches) has an important surgical relationship with the lower border of the mandible (hence its name), usually running 1–2 cm below the border. The mandibular nerve supplies the muscles of the lower lip and lies deep to the platysma muscle but superficial to the investing layer of deep cervical fascia.

The cervical nerve also passes deep to the platysma muscle but superficial to the investing layer of deep cervical fascia. It runs behind the superficial part of the submandibular gland where it divides into several branches that supply the platysma muscle.

The oral mucosa receives its sensory innervation primarily from the maxillary and mandibular divisions of the trigeminal nerve. The trigeminal nerve also supplies the teeth and their supporting tissues. The salivary glands are supplied by secretomotor, parasympathetic fibres from the facial and glossopharyngeal nerves. The motor innervation of the oral musculature is derived mainly from the mandibular, facial, accessory, and hypoglossal nerves.

The dentition in the lower jaw is innervated by the mandibular nerve. The teeth receive their nerve supply from the molar and incisive branches of the inferior alveolar nerve. The lingual gingivae are supplied mainly by the lingual branch of the mandibular nerve. The labial gingivae are innervated by the mental branch of the inferior alveolar nerve and the buccal gingivae by the buccal branch of the mandibular nerve.

The innervation of the dentition in the upper jaw is derived almost entirely from the maxillary nerve. The teeth are supplied by the anterior, middle, and posterior superior alveolar branches. The palatal gingivae are innervated by the nasopalatine and greater (anterior) palatine branches via the pterygopalatine ganglion. The labial and buccal gingivae are supplied by the infraorbital and the posterior superior alveolar branches.

Table 6.1 summarizes the nerve supply to the teeth and gingivae.

The inferior alveolar nerve (Figure 6.6) is the terminal branch of the posterior trunk of the mandibular nerve. It arises in the infratemporal fossa, deep to the lower head of the lateral pterygoid muscle. On emerging from beneath this muscle, the inferior alveolar nerve lies within the pterygomandibular space. Here, it gives off a mylohyoid branch which is a motor nerve to the anterior belly of the digastric muscle and the mylohyoid muscle (it may also have sensory fibres which enter the mandible in the mental region to participate in the nerve supply to the lower incisors). The inferior alveolar nerve enters the mandible through the mandibular foramen.

The course of the inferior alveolar nerve through the mandible is variable. The molar branches to the premolar and molar teeth come either directly from the inferior alveolar nerve in the mandibular canal by short or long branches, or indirectly from the nerve outside the mandibular canal by a series of alveolar branches. The mandibular canal may be closely related to the roots of the mandibular molars, even to the extent of occasionally perforating a root.

The main trunk of the inferior alveolar nerve divides near the premolars into mental and incisive nerves. The mental nerve runs for a short distance in a mental canal before leaving the body of the mandible at the mental foramen to emerge onto the face. It supplies the skin and mucosa of the lower lip, and the labial gingivae of the mandibular anterior teeth. The incisive nerve runs forwards in an incisive canal. This nerve usually innervates only the incisor and canine teeth, but occasionally it supplies the first premolar.

There are usually three superior alveolar nerves (see Figure 6.3) supplying the maxillary dentition: the posterior, middle, and anterior superior alveolar nerves.

The posterior superior alveolar nerve arises from the maxillary nerve in the pterygopalatine fossa. It descends onto the posterior wall of the maxilla, passing through the pterygomaxillary fissure, and divides into dental and gingival branches. The dental branches enter the maxilla and run in narrow canals above the roots of the molar teeth. The gingival branch does not enter the bone but runs along the outer surface of the maxillary tuberosity to supply the buccal gingivae of the maxillary molar teeth.

The middle superior alveolar nerve is found in about 70% of individuals. The nerve generally arises in the floor of the orbit from the infraorbital branch of the maxillary nerve. It can also arise directly from the maxillary nerve in the pterygopalatine fossa. The middle superior alveolar nerve runs in the posterior, lateral, or anterior wall of the maxillary air sinus. It terminates above the roots of the premolar teeth.

The anterior superior alveolar nerve arises from the infraorbital nerve within the infraorbital canal. It generally appears as a single nerve, occasionally as two or three branches. The nerve runs in the anterior wall of the maxillary sinus and terminates near the anterior nasal spine after giving off a small nasal branch.

The three superior alveolar nerves form a plexus just above the roots of the maxillary teeth. Indeed, it is difficult to trace the precise innervation of the teeth from a specific superior alveolar nerve. As a general rule, however, the incisors and canine are supplied by the anterior nerve, the molars by the posterior nerve, and the premolars by the middle nerve.

The mucosa of the upper lip is supplied by the infraorbital branch of the maxillary nerve. The lower lip is innervated by the mental branch of the mandibular nerve. The mucosa of the cheeks is innervated by the buccal branch of the mandibular nerve.

The buccal nerve is the terminal branch of the anterior trunk of the mandibular nerve. It arises in the infratemporal fossa, behind the upper head of the lateral pterygoid muscle. The buccal nerve passes between the two heads of the lateral pterygoid muscle and crosses the infratemporal fossa. It runs into the upper part of the retromolar fossa at the anterior border of the ramus of the mandible. The buccal nerve breaks up into several branches within the buccinator muscle. It innervates both the mucosa and the skin of the cheek and the buccal gingivae of the mandibular cheek teeth (perhaps even of the maxillary cheek teeth).

Concerning general sensation (i.e. excluding taste), three distinct nerve fields can be recognized on the dorsum of the tongue. The anterior part of the tongue, in front of the circumvallate papillae, is supplied by the lingual branches of the mandibular nerves (see Figure 6.2). Behind, and including, the circumvallate papillae, the tongue is innervated primarily by the glossopharyngeal nerves. Small areas on the posterior part of the tongue around the epiglottis are supplied by the superior laryngeal branches (internal branches) of the vagus nerves. Concerning taste, the anterior part of the tongue is innervated by the chorda tympani branches of the facial nerves. These are distributed through the lingual nerves. The posterior part of the tongue, including the circumvallate papillae, has a similar innervation for taste as that for general sensation.

The mucosa on the ventral surface of the tongue and on the floor of the mouth is supplied by the lingual branches of the mandibular nerves.

The lingual nerve is derived from the posterior trunk of the mandibular nerve within the infratemporal fossa. It receives the chorda tympani branch of the facial nerve beneath the lateral pterygoid muscle. At the level of the mandibular foramen, the lingual nerve lies on the medial pterygoid muscle and is anterior to the inferior alveolar nerve. The lingual nerve then leaves the infratemporal fossa, passing downwards and forwards to lie close to the lingual alveolar plate of the mandibular third molar tooth. Before curving forwards into the tongue, the nerve is found above the origin of the mylohyoid muscle and lateral to the hyoglossus muscle. On the superficial surface of the hyoglossus muscle, the lingual nerve twists twice around the submandibular salivary duct, first on the lateral side of the duct and then on the medial side. It enters the tongue behind the sublingual salivary gland. Suspended from the lingual nerve as it runs across the hyoglossus muscle is the submandibular parasympathetic ganglion.

The lingual nerve itself supplies the mucosa covering the anterior two-thirds of the dorsum of the tongue, the ventral surface of the tongue, the floor of the mouth, and the lingual gingivae of the mandibular teeth.

The chorda tympani fibres travelling with the lingual nerve are of two types: sensory and parasympathetic. The sensory fibres are associated with taste for the anterior two-thirds of the dorsum of the tongue. The parasympathetic fibres are preganglionic fibres that pass to the submandibular ganglion. Postganglionic fibres are distributed to the submandibular and sublingual salivary glands.

The sensory supply to the palate (Figure 6.3) is derived mainly from branches of the maxillary nerve via the pterygopalatine ganglion. A small area behind the incisor teeth is supplied by the nasopalatine nerves. The remainder of the hard palate is innervated by the greater palatine nerves. The soft palate is supplied by the lesser palatine nerves. There is evidence to suggest that some areas supplied by the lesser palatine nerves may also be innervated from the facial nerves. The posterior part of the soft palate and the uvula may be supplied by the glossopharyngeal nerves.

The nasopalatine nerve runs along the nasal septum from the pterygopalatine ganglion and emerges onto the hard palate at the incisive fossa behind the maxillary first incisor teeth. The nasopalatine nerve innervates the gingivae behind the maxillary incisor teeth.

The greater and lesser palatine nerves pass from the pterygopalatine ganglion, down the greater palatine canal at the back of the lateral wall of the nose. The greater palatine nerve runs through the greater palatine foramen and onto the back of the hard palate. It passes towards the front of the hard palate at the interface between the palatine and alveolar processes of the maxilla. In addition to supplying the mucosa of the palate, the greater palatine nerve innervates the palatal gingivae for the maxillary cheek teeth. The lesser palatine nerve emerges onto the palate at the lesser palatine foramen. It runs backwards into the soft palate.

The mucosa over the pillars of the fauces is supplied by the glossopharyngeal nerve.

The innervation of the various muscles associated with the mouth is derived from the mandibular division of the trigeminal, the facial, the cranial part of the accessory and the hypoglossal cranial nerves, and the first cervical spinal nerves. The innervation is summarized in Table 6.2.

The lesser petrosal branch of the glossopharyngeal nerve supplies the parotid gland via the otic parasympathetic ganglion. Postganglionic fibres pass to the gland through the auriculotemporal branch of the mandibular nerve.

The greater petrosal branch of the facial nerve probably supplies palatal and pharyngeal glands via the pterygopalatine parasympathetic ganglion. Postganglionic fibres reach the palate with the nasopalatine, greater palatine, and lesser palatine branches of the maxillary nerve.

The chorda tympani branch of the facial nerve provides secretomotor fibres parasympathetic to the submandibular and sublingual salivary glands via the submandibular ganglion. It probably also provides the innervation of minor salivary glands in the lips, cheeks and tongue.

Special sensation related to olfaction is associated with the olfactory nerves (i.e. the first cranial nerves). General sensation to the nasal mucosa is related to branches from the ophthalmic and maxillary divisions of the trigeminal nerves (i.e. the fifth cranial nerves).

The olfactory epithelium is located in the roof of the nasal cavity, extending onto the lateral walls of the nasal fossae (above the superior nasal conchae) and the uppermost part of the nasal septum. Filaments of the olfactory nerves (about 20 on each side) pass upwards through the cribiform plate of the ethmoid bone into the cranial cavity to synapse in the olfactory bulbs. Each filament is ensheathed by the meninges. Thus, a potential pathway exists for the spread of infection from the nose to the cranial cavity.

The anterior ethmoidal nerve is the only branch of the ophthalmic nerve which supplies the nasal mucosa. It arises from the nasociliary nerve and mainly supplies an area in front of the nasal conchae (it also innervates the anterior extremities of the middle and inferior conchae). After leaving the orbit through the anterior ethmoidal foramen, the anterior ethmoidal nerve enters the cranial cavity onto the cribiform plate of the ethmoid. It leaves the cranial cavity through a small slit near the crista galli and enters the roof of the nasal cavity. Here, the nerve runs in a groove on the inner surface of the nasal bone. The anterior ethmoidal nerve passes downwards and forwards and gives rise to lateral and medial internal nasal branches. The lateral internal nasal branches pass to the lateral wall of the nose whereas the medial internal nasal branches run to the nasal septum. When the anterior ethmoidal nerve emerges at the inferior margin of the nasal bone it becomes the external nasal nerve.

The maxillary nerve contributes many branches which supply the nasal mucosa. The infraorbital and the posterior superior alveolar nerves arise directly from the maxillary nerve. The posterior superior nasal, greater palatine and nasopalatine nerves arise indirectly by way of the pterygopalatine ganglion.

The infraorbital nerve is the terminal branch of the maxillary nerve. After passing onto the face at the infraorbital foramen, it provides a nasal branch which supplies the skin of the vestibule and the mobile part of the nasal septum. The anterior superior alveolar branch of the infraorbital nerve also supplies nasal mucosa. Its nasal branch passes through a small canal in the lateral wall of the nose (below the level of the inferior concha) to innervate the anterior part of the inferior meatus and the adjacent part of the floor of the nose and adjoining nasal septum.

The posterior superior nasal nerve originates at the pterygopalatine ganglion. It enters the back of the nasal cavity through the sphenopalatine foramen and gives off lateral and medial branches. The lateral branches supply the posterosuperior part of the lateral wall of the nose around the superior and middle nasal conchae. The medial branches cross the roof of the nasal cavity to supply the septum overlying the posterior part of the perpendicular plate of the ethmoid.

The greater (anterior) palatine nerve also arises from the pterygopalatine ganglion. It descends in the greater palatine canal where it gives off posterior inferior nasal branches. These branches pass through small openings in the perpendicular plate of the palatine bone to supply the posteroinferior portion of the lateral wall of the nose (below, and including, the middle meatus).

The nasopalatine nerve passes from the pterygopalatine ganglion into the nasal cavity through the sphenopalatine foramen. It runs across the roof of the nasal cavity to reach the back of the nasal septum. It then passes downwards and forwards, lying in a groove on the vomer, to supply the posteroinferior part of the septum. The floor of the nose is supplied anteriorly by the nasal branch of the anterior superior alveolar nerve and posteriorly by the nasal branches of the greater (anterior) palatine and by the nasopalatine nerves. Autonomic fibres to glands and vessels in the nose are distributed with the above mentioned branches of the maxillary nerve via the pterygopalatine ganglion. In addition, autonomic fibres are presumed to be distributed with the anterior ethmoidal nerve via the ciliary ganglion.

Both motor and sensory nerves are found in the orbit (see Figure 6.4). The motor nerves are the oculomotor, trochlear, and abducent nerves. They supply the extraocular muscles. There are also motor nerves derived from the autonomic nervous system. Parasympathetic fibres from the oculomotor nerve (via the ciliary ganglion) supply the sphincter pupillae and ciliary muscles. Parasympathetic fibres from the facial nerve (via the pterygopalatine ganglion) supply the lacrimal gland. Sympathetic fibres supply the dilator pupillae muscle. The sensory nerves within the orbit are the optic, ophthalmic, and maxillary nerves. The ophthalmic and maxillary nerves are essentially only passing through the orbit to supply the face and jaws.

This is the third cranial nerve. It is the main source of innervation of the extraocular muscles. The oculomotor nerve also contains parasympathetic fibres which relay in the ciliary ganglion.

The oculomotor nerve emerges at the midbrain, on the medial side of the crus of the cerebral peduncle. It passes along the lateral dural wall of the cavernous sinus. The oculomotor nerve then divides into superior and inferior divisions and runs beneath the trochlear and ophthalmic nerves. The two divisions of the oculomotor nerve enter the orbit through the superior orbital fissure, within the common tendinous ring of the recti muscles. Here, the nasociliary branch of the ophthalmic nerve lies between the divisions of the oculomotor nerve.

The superior division of the oculomotor nerve passes above the optic nerve to enter the inferior surface of the superior rectus muscle. It supplies this muscle and provides a branch which runs to the levator palpebrae superioris muscle.

The inferior division of the oculomotor nerve divides into three branches: medial, central, and lateral. The medial branch passes beneath the optic nerve to enter the lateral surface of the medial rectus muscle. The central branch runs downwards and forwards to enter the superior surface of the inferior rectus muscle. The lateral branch travels forwards on the lateral side of the inferior rectus muscle to enter the superior surface of the inferior oblique muscle. The lateral branch also communicates with the ciliary ganglion to distribute parasympathetic fibres to the sphincter pupillae and ciliary muscles.

This is the fourth cranial nerve. It is the only cranial nerve which emerges from the dorsal surface of the brain. The trochlear nerve passes from the midbrain onto the lateral surface of the crus of the cerebral peduncle. It runs through the lateral dural wall of the cavernous sinus. The nerve then crosses the oculomotor nerve and enters the orbit through the superior orbital fissure, above the common tendinous ring of the recti muscles. Here, it lies above the levator palpebrae superioris muscle and medial to the frontal and lacrimal nerves. The trochlear nerve travels but a short distance to enter the superior surface of the superior oblique muscle. Indeed, the innervation of the superior oblique muscle is the sole function of the trochlear nerve.

The abducent nerve is the sixth cranial nerve. It emerges from the brain stem, between the pons and the medulla oblongata. The abducent nerve is related to the cavernous sinus but, unlike the oculomotor, trochlear, ophthalmic, and maxillary nerves which merely invaginate the lateral dural wall, it passes through the sinus itself. The abducent nerve enters the orbit through the superior orbital fissure. It is here situated within the common tendinous ring of the recti muscles, first below and then between the two divisions of the oculomotor nerve and lateral to the nasociliary nerve. The abducent nerve passes forwards to enter the medial surface of the lateral rectus muscle. The innervation of this muscle is the sole function of the abducent nerve.

The ophthalmic nerve and its branches are described elsewhere in this chapter.

The maxillary nerve gives rise directly to two nerves that pass into the orbit (the zygomatic and infraorbital nerves) and indirectly to an orbital branch from the pterygopalatine ganglion. All three nerves enter the orbit through the inferior orbital fissure.

The optic nerve is the second cranial nerve. It arises from the optic chiasma on the floor of the diencephalon. It enters the orbit through the optic canal, accompanied by the ophthalmic artery. The shape of the optic nerve changes from being flattened at the chiasma to being rounded as it passes through the optic canal. The optic nerve in the orbit passes forwards, laterally, and downwards. It pierces the sclera at the lamina cribrosa, slightly medial to the posterior pole. The optic nerve has a slightly wavy course which allows for movements of the eye.

Within the orbit, the optic nerve is surrounded by extensions of the three meninges. This reflects the fact that the nerve is really an ‘outgrowth’ of the brain.

The optic nerve has important relationships with other orbital structures. As the nerve leaves the optic canal, it lies superomedial to the ophthalmic artery. The oculomotor, nasociliary, and abducent nerves (and sometimes the ophthalmic veins) are situated between the optic nerve and the lateral rectus muscle. The optic nerve is also closely related to the origins of the four recti muscles. More anteriorly, however, the muscles diverge and the nerve becomes separated from them by a substantial amount of orbital fat. Just beyond the optic canal, the ophthalmic artery and the nasociliary nerve cross the optic nerve to reach the medial wall of the orbit. The central artery of the retina enters the substance of the optic nerve about halfway along its length. Near the back of the eye, the optic nerve becomes surrounded by long and short ciliary nerves and vessels.

The ciliary ganglion is a parasympathetic ganglion which is located near the apex of the orbit. It lies in front of the optic canal, between the lateral rectus muscle and the optic nerve, and close to the ophthalmic artery. The ganglion appears as a small swelling connected to the nasociliary nerve. Short ciliary nerves pass from the ganglion to the eyeball. Functionally, the ciliary ganglion is related to the eye, in particular the motor supply of intraocular muscles.

The parasympathetic fibres to the ciliary ganglion arise from the Edinger–Westphal nucleus of the oculomotor nerve. The preganglionic fibres run with the oculomotor nerve into the orbit, leaving in the branch to the inferior oblique muscle. The fibres then pass to the ciliary ganglion where they synapse. Postganglionic fibres travel to the back of the eye in the short ciliary nerves.

The sympathetic fibres to the ciliary ganglion arise from the plexus around the internal carotid artery within the cavernous sinus. These postganglionic fibres form a fine branch which enters the orbit through the superior orbital fissure, inside the common tendinous ring of the recti muscles. This branch then travels through the ganglion (without synapsing) and into the short ciliary nerves.

The sensory fibres to the ciliary ganglion are derived from the nasociliary nerve. They also pass through the ganglion to the short ciliary nerves without synapsing.

The short ciliary nerves convey parasympathetic, sympathetic and sensory fibres between the eyeball and the ciliary ganglion. The nerves pierce the sclera at the back of the eye and run forwards between the sclera and the choroid. The parasympathetic fibres are distributed to the sphincter pupillae and ciliary muscles. Contraction of the ciliary muscles is associated with the accommodation reflex. The sympathetic fibres supply arteries within the eye. (The sympathetic fibres supplying the dilator pupillae muscle are thought to run in the long ciliary nerves.) The sensory fibres carry sensation from the cornea, the ciliary body, and the iris.

The neck, being an ‘intermediate zone’ between the trunk and the head, is innervated from both spinal nerves (anterior and posterior primary rami) and cranial nerves (mainly Cr IX, X, XI, XII). In contrast, the head is primarily innervated by cranial nerves (the back of the head is innervated by posterior primary rami of spinal nerves) and the trunk by spinal nerves (not ignoring the important contributions of Cr X).

The skin of the neck is innervated by branches of primary cervical spinal nerves, via both posterior and anterior rami (Figure 6.7). The posterior rami supply skin over the back of the neck and scalp. The anterior rami supply skin covering the lateral and anterior portions of the neck and even extend onto the face over the angle of the mandible.

The posterior primary rami of the first, sixth, seventh, and eighth cervical nerves have no cutaneous distribution. From the medial branch of the posterior ramus of the second cervical nerve comes the greater occipital nerve. This pierces the trapezius muscle close to its attachment onto the superior nuchal line of the occiput and then ascends to supply the skin over the occipital part of the scalp up to the vertex of the skull. The medial branches of the posterior rami of the third, fourth, and fifth cervical nerves also pierce trapezius to supply skin over the back of the neck in a serial manner.

The anterior primary rami of the second, third, and fourth cervical nerves supply cutaneous branches via the cervical plexus. This plexus is located deep to the sternocleidomastoid muscle and supplies both motor and sensory branches to structures in the neck. The cutaneous branches from the plexus are the lesser occipital, the great auricular, the transverse cervical, and the supraclavicular nerves. All four nerves appear from beneath the sternocleidomastoid muscle at its posterior margin.

The lesser occipital nerve takes fibres mainly from the second cervical nerve, although fibres from the third cervical nerve may contribute. It ascends along the posterior margin of the sternocleidomastoid muscle to supply the scalp above and behind the ear and a small area on the cranial surface of the auricle.

The great auricular nerve receives fibres from the second and third cervical nerves. It runs up the superficial surface of the sternocleidomastoid muscle towards the ear. It supplies the skin overlying the mastoid process (the mastoid branch), much of the auricle (auricular branches), and the parotid region, and the angle of the mandible (facial branches).

The transverse cervical nerve also takes fibres from the second and third cervical nerves. It crosses the sternocleidomastoid muscle horizontally to supply skin overlying the anterior part of the neck from the mandible to the sternum.

The supraclavicular nerves receive fibres from the third and fourth cervical nerves. Initially, it is a single nerve. This passes downwards towards the clavicle where it divides into three branches (medial, intermediate, and lateral supraclavicular nerves). These nerves supply skin at the root of the neck and over the upper part of the thorax.

Within the carotid sheath runs the vagus nerve. Related to the carotid sheath near the base of the skull are the glossopharyngeal, accessory, and hypoglossal nerves. Lying behind the carotid sheath, and in front of the prevertebral fascia, is the cervical sympathetic trunk. Deep to the internal jugular vein, and in front of the scalenus medius and levator scapulae muscles (at the level of the first four cervical vertebrae), lies the cervical plexus of nerves. Associated with the cervical plexus is the ansa cervicalis. The brachial plexus for the arm lies in the deep part of the posterior triangle in the root of the neck. Both the cervical and brachial plexuses are derived from the anterior primary rami of cervical spinal nerves. The cutaneous contributions of the posterior rami have already been described.

This is the ninth cranial nerve. It emerges from the medulla oblongata of the brain stem as three or four rootlets. These rootlets are found in a groove between the olive and the inferior cerebellar peduncle. At this site, the glossopharyngeal nerve lies above the rootlets of the vagus nerve. The glossopharyngeal nerve has sensory, motor, and parasympathetic fibres.

The glossopharyngeal nerve leaves the skull through the central part of the jugular foramen. Within the foramen, the nerve shows the superior and inferior ganglia. Below the foramen, the glossopharyngeal nerve is located anterior to the vagus and accessory nerves, passing between the internal jugular vein and the internal carotid artery. It then runs anteriorly between the internal and external carotid arteries and onto the stylopharyngeus muscle. Winding around this muscle, it passes between the superior and middle constrictor muscles of the pharynx to be distributed to the tonsil, pharynx, and tongue. Branches include:

The tympanic nerve arises from the inferior ganglion. It passes upwards through the tympanic canaliculus to reach the middle ear cavity. Here, it contributes to the tympanic plexus, which is found on the promontory of the medial wall of the tympanic cavity. This plexus provides sensory fibres to the mucosa of the tympanic cavity, the auditory tube, and the mastoid air cells. From the plexus arises the lesser petrosal nerve.

The lesser petrosal nerve contains preganglionic parasympathetic fibres which relay through the otic ganglion to the parotid salivary gland. The nerve passes from the tympanic plexus, through the anterior wall of the tympanic cavity and onto the floor of the middle cranial fossa. It then emerges through the foramen ovale to join the otic ganglion in the infratemporal fossa.

The carotid branch(es) arises just below the skull, as the glossopharyngeal crosses the internal carotid artery. It then passes between the internal and the external carotid arteries to the carotid sinus and the carotid body. During its course it is joined by the carotid branch of the vagus nerve.

The pharyngeal branches contribute to the pharyngeal plexus on the middle constrictor muscle (the other components of this plexus being from the sympathetic trunk and the pharyngeal branch of the vagus). The glossopharyngeal contribution to the plexus is sensory to the pharynx.

The stylopharyngeus branch supplies the stylopharyngeus muscle (the nerve and muscle being associated embryologically with the third pharyngeal arch).

The tonsillar branches supply the palatine tonsil. They form a plexus with the lesser palatine nerve. Branches from the plexus are distributed to the soft palate.

There are two lingual branches of the glossopharyngeal nerve. One branch supplies the region around the sulcus terminalis of the tongue, including the circumvallate papillae. The other branch supplies the posterior third of the tongue. The lingual branches are concerned with both taste perception and general sensation.

The vagus nerve is the tenth cranial nerve. It has the most extensive distribution of any of the cranial nerves and contains sensory, motor, and parasympathetic fibres.

The vagus emerges from the brain stem at the medulla oblongata, between the olive and the inferior cerebellar peduncle. It exits the cranium through the jugular foramen with the glossopharyngeal and accessory nerves.

The vagus nerve has two ganglia, the superior and inferior ganglia. The superior ganglion lies within the jugular foramen. The inferior ganglion is situated just below.

Just below the inferior ganglion, the vagus is joined by the cranial part of the accessory nerve. The vagus then passes downwards within the carotid sheath and enters the thorax at the root of the neck.

The vagus nerves in the neck differ in one important respect, namely the origins of the recurrent laryngeal nerves. Branches include:

The meningeal branch(es) arises from the superior ganglion in the jugular fossa. It supplies dura in the posterior cranial fossa. There is some evidence that this nerve is not truly a branch of the vagus but is derived from upper cervical nerves and/or the superior cervical sympathetic ganglion.

The auricular branch also arises from the superior ganglion. It enters the temporal bone via the mastoid canaliculus on the lateral wall of the jugular fossa. It then passes out through the tympanomastoid fissure and divides into two branches. One branch joins the posterior auricular branch of the facial nerve, the other contributes to the innervation of the skin of the auricle, external acoustic meatus, and tympanic membrane.

The pharyngeal branch is, in fact, derived from the cranial part of the accessory nerve. It runs from the inferior ganglion of the vagus, between the internal and external carotid arteries, and towards the middle constrictor of the pharynx. There it forms the pharyngeal plexus with branches from the sympathetic trunk, and the glossopharyngeal and external laryngeal nerves. The pharyngeal nerve is the main motor nerve to the muscles of the pharynx and palate.

Although the carotid body is supplied mainly by the glossopharyngeal nerve, the vagus nerve can also contribute.

The superior laryngeal nerve also arises from the inferior ganglion. It then passes deep to both the internal and external carotid arteries on its way to the larynx. It divides into internal and external branches. The internal branch passes between the middle and inferior constrictor muscles to supply sensation to the larynx. The external branch runs down on the inferior constrictor muscle (with the superior thyroid artery) to supply the cricothyroid muscle of the larynx.

The right recurrent laryngeal nerve arises in the root of the neck. It leaves the vagus in front of the subclavian artery, loops below, and behind the artery, and then ascends towards the larynx.

The left recurrent laryngeal nerve arises in the thorax, as the vagus passes across the arch of the aorta. Both recurrent laryngeal nerves reach the larynx by passing upwards in grooves between the trachea and the oesophagus and they are closely related to the inferior thyroid arteries. They pass beneath the inferior borders of the inferior constrictor muscles to supply the mucosa of the larynx and most of the intrinsic muscles.

Usually two or three cardiac branches emanate from the vagus nerve in the neck. They run downwards and medially into the thorax, terminating at the deep part of the cardiac plexus.

This is the eleventh cranial nerve. It consists of two distinct parts, the cranial accessory and the spinal accessory nerves.

The cranial part of the accessory nerve is a motor nerve which emerges from the medulla oblongata between the olive and the inferior cerebellar peduncle. It joins the spinal part of the accessory at the jugular foramen. Once through the jugular foramen, the cranial and spinal parts again separate. The cranial part then joins the vagus nerve, eventually to be distributed in the pharyngeal branch of the vagus to the pharyngeal and palatine musculature. Some of its fibres also run with the recurrent laryngeal nerve and the cardiac branches of the vagus. Because of its close association with the vagus, some anatomists consider the cranial part of the accessory nerve to be a part of the vagus and not a separate cranial nerve.

The spinal part of the accessory nerve is also a motor nerve, although there may be some sensory fibres. It is derived from the upper five segments of the cervical spinal cord. A series of rootlets emerge from the cord between the dorsal and ventral roots of the upper cervical nerves. They join to form the main nerve trunk which passes intracranially through the foramen magnum. At the jugular foramen, the spinal and cranial parts of the accessory nerve unite but soon separate on exiting the cranium. The spinal part of the accessory nerve then crosses the internal jugular vein (usually on its lateral surface) and runs obliquely downwards and backwards to reach the upper part of the sternocleidomastoid muscle. It passes into the substance of this muscle and subsequently enters the posterior triangle of the neck. It crosses the posterior triangle on the levator scapulae muscle before entering the trapezius muscle. The spinal part of the accessory nerve provides the motor supply of the sternocleidomastoid and trapezius muscles.

This is the twelfth cranial nerve and is a motor nerve supplying the musculature of the tongue. It originates as a series of rootlets on the medulla oblongata, between the pyramid and the olive.

The hypoglossal nerve runs through the hypoglossal canal of the occipital bone and emerges deep to the carotid sheath. It then passes downwards and, under cover of the posterior belly of the digastric muscle, outwards between the internal jugular vein and the internal carotid artery. Subsequently, it runs forwards across the vagus nerve and the external and internal carotid arteries. Indeed, it loops around the occipital artery near its origin (at its sternocleidomastoid branch). Continuing forwards, it passes below the submandibular salivary gland, onto the hyoglossus muscle to be distributed to the muscles of the tongue.

Like most cranial nerves, the hypoglossal nerve has connecting branches with other cranial and cervical spinal nerves and with the sympathetic system. An important connection is with the anterior primary ramus of the first cervical nerve.

Branches include:

The meningeal branch is probably derived from the upper cervical and sympathetic fibres which communicate with the hypoglossal. It appears as the hypoglossal nerve emerges through its canal in the occipital bone. It mainly supplies the dura in the posterior cranial fossa.

The upper root of the ansa cervicalis is also derived from the anterior ramus of the first cervical nerve. This branch first appears as the hypoglossal nerve loops around the occipital artery. It passes down on the carotid sheath covering the carotid arteries and is joined by the lower root of the ansa cervicalis from the cervical plexus to form the ansa cervicalis. The upper root of the ansa cervicalis gives a branch to the superior belly of the omohyoid muscle.

The muscular branches supplying the thyrohyoid and geniohyoid muscles are also derived from the first cervical spinal nerve. The nerve to thyrohyoid arises as the hypoglossal nerve reaches the hyoglossus muscle. The nerve to geniohyoid is given off in the floor of the mouth, above the mylohyoid muscle.

The branches to the tongue musculature are the only true branches of the hypoglossal nerve. They are distributed to the intrinsic muscles of the tongue and to the styloglossus, hyoglossus, and genioglossus muscles.

The sympathetic outflow for all parts of the body is derived principally from the thoracic spinal cord (segments T1 to L2). These preganglionic fibres then pass into the sympathetic trunk via the spinal nerves as white rami communicantes. Here, they may synapse at a ganglion or they pass up or down the sympathetic trunk to a ganglion at a different level. In this manner, the cervical part of the sympathetic trunk receives its preganglionic fibres from the upper thoracic nerves.

The cervical sympathetic trunk exhibits a variable number of ganglia (usually between two and four). The ganglia are designated according to their position (superior, middle, inferior).

The superior cervical ganglion lies at the level of the second and third cervical vertebrae. It is situated behind the carotid sheath on the longus capitis muscle (a prevertebral muscle). It is the largest of the cervical sympathetic ganglia and is believed to represent the coalescence of four ganglia which correspond with the upper four cervical spinal nerves.

The branches from the superior cervical ganglion are variable, but can be broadly classified into lateral, medial, and anterior groups.

The lateral branches include the grey rami communicantes to the upper four cervical spinal nerves. In addition, there are branches which communicate with some of the cranial nerves: to the inferior ganglion of the glossopharyngeal nerve, to both ganglia of the vagus nerve, and to the hypoglossal nerve. The nerve which joins the glossopharyngeal and vagus nerves is termed the jugular nerve. The lateral branches of the superior cervical ganglion also include nerves to the superior jugular bulb and to the meninges of the posterior cranial fossa.

There are two medial branches of the superior cervical sympathetic ganglion. There is a laryngopharyngeal branch (which supplies the carotid body and the pharyngeal plexus), and a cardiac branch.

The anterior branches pass onto the common and external carotid arteries to form plexuses. In addition to supplying the blood vessels, the plexus around the facial branch of the external carotid provides the sympathetic supply to the submandibular parasympathetic ganglion. The plexus around the middle meningeal artery (a branch of the maxillary artery from the external carotid) serves the otic parasympathetic ganglion.

Emerging above the superior ganglion is the internal carotid nerve. This nerve may be thought of as the cranial part of the sympathetic system. It passes with the internal carotid artery into the carotid canal. Within the canal, it forms the internal carotid plexus around the internal carotid artery.

The internal carotid plexus can be divided into two parts, lateral and medial. The lateral part gives branches which communicate with the trigeminal and abducent cranial nerves. Superior and inferior caroticotympanic nerves traverse the posterior wall of the carotid canal to communicate with the tympanic branch of the glossopharyngeal nerve. An important branch is the deep petrosal nerve. This nerve is destined for the pterygopalatine ganglion. It passes through the foramen lacerum and, joining the greater petrosal branch of the facial nerve, becomes the nerve of the pterygoid canal.

The medial part of the internal carotid plexus supplies the internal carotid artery itself and communicates with the oculomotor, trochlear, ophthalmic division of the trigeminal, and the abducent cranial nerves. Branches also pass through the superior orbital fissure to the ciliary ganglion in the orbit. These branches subsequently run with the short ciliary nerves to be distributed to the blood vessels of the eyeball. The fibres to the dilator pupillae travel by a different course (via the ophthalmic, nasociliary, and then the long ciliary nerves). The terminal branches of the internal carotid plexus form plexuses around the ophthalmic artery and the anterior and middle cerebral arteries of the brain, passing eventually to the pia mater.

The middle cervical ganglion is usually situated at the level of the sixth cervical vertebra. It is the smallest cervical ganglion and is occasionally absent. It may fuse with the superior cervical ganglion. The middle ganglion lies close to the inferior thyroid artery just before it enters the gland. Some claim that it represents the coalescence of two ganglia which correspond with the fifth and sixth cervical segments.

Branches from the middle cervical ganglion communicate with the fifth and sixth cervical spinal nerves (also sometimes the fourth and seventh). Two distinct cords pass down to the inferior cervical/cervicothoracic sympathetic ganglion. The anterior cord loops in front and below the subclavian artery as the ansa subclavia. The posterior cord encloses the vertebral artery. The middle cervical ganglion also sends branches to the thyroid gland (along the inferior thyroid artery), to the heart via its cardiac branches, and to the trachea and oesophagus.

An occasional ganglion known as the vertebral ganglion may be found on the front of the vertebral artery. It can be considered as either a low middle cervical ganglion or as a detached part of the inferior ganglion. When present, it gives rise to the ansa subclavia.

The inferior cervical often combines with the first thoracic ganglion to form the cervicothoracic ganglion (stellate ganglion). The inferior cervical ganglion (or upper end of the cervicothoracic ganglion) is situated just posterior to the vertebral artery. The lower end of a cervicothoracic ganglion lies behind the subclavian artery on the first thoracic vertebra.

Branches from the inferior cervical ganglion pass to the seventh and eighth cervical nerves and to the first thoracic nerve. There are also cardiac branches and fibres which form plexuses around the subclavian artery and its derivatives. Around the vertebral artery is a plexus which continues up into the skull. This plexus eventually meets the plexus around the internal carotid artery. Some anatomists believe this to be the main intracranial extension of the sympathetic system.

This plexus lies deep to the sternocleidomastoid muscle and the internal jugular vein, and in front of the scalenus medius and levator scapulae muscles. It is formed by the anterior primary rami of the upper four cervical spinal nerves (i.e. Cl, C2, C3, C4). The cervical plexus contains both sensory and motor fibres. In addition, grey rami communicantes near the origins of the anterior rami of the cervical nerves supply sympathetic fibres.

Branches from the cervical plexus are distributed to some of the muscles of the neck, to the diaphragm, and to much of the skin of the back of the head, the neck, and the chest around the thoracic inlet. The cutaneous nerves are superficial, the muscular branches deep.

The cutaneous nerves from the cervical plexus are the lesser occipital (C2), great auricular (C2, C3), transverse cervical (C2, C3), and supraclavicular (C3, C4) nerves.

The deep (mainly motor) branches can be subdivided into those which pass medially and those which pass laterally.

The medial branches supply the following muscles:

Other medial branches are the inferior root of the ansa cervicalis (C2, C3) and the phrenic nerve (C3, C4, C5). Some branches also communicate with the hypoglossal and vagus nerves, and the sympathetic trunk.

The lateral branches supply the following muscles:

There is also a communicating branch to the accessory nerve (C2, C3, C4).

The phrenic nerve arises from the cervical plexus and usually takes fibres from the third, fourth, and fifth cervical nerves (mainly from the fourth). It provides the motor nerve supply to the diaphragm.

The phrenic nerve in the neck passes downwards and medially on the superficial surface of the scalenus anterior muscle. Here, it lies under cover of a layer of the prevertebral fascia. As it passes through the thoracic inlet, it runs behind the subclavian vein and in front of the subclavian artery and its internal thoracic branch.

Some of the roots may not join the main nerve trunk until just before leaving the neck. Such roots are called accessory phrenic nerves.

The phrenic nerve contains not only motor fibres but also proprioceptive fibres to the diaphragm and sensory fibres to the pleura and pericardium. Sympathetic fibres may join the phrenic nerve from cervical sympathetic ganglia.

The branches to the hypoglossal nerve from the cervical plexus arise mainly from the first cervical spinal nerve. These fibres leave the hypoglossal nerve as four distinct nerves: the meningeal branch of the hypoglossal nerve; the superior root of the ansa cervicalis; and the motor nerves to the thyrohyoid and the geniohyoid muscles. Some Cl fibres also travel with the vagus nerve, and indeed may form its meningeal branch.

The ansa cervicalis is a nerve plexus located in front of the common carotid artery. It is formed by the union of two nerve trunks, the superior root of the ansa cervicalis from the hypoglossal nerve (conveying Cl fibres) and the inferior root of the ansa cervicalis from the cervical plexus (conveying C2 and C3 fibres). The superior root is also called the descendens hypoglossi, indicating its path from the hypoglossal nerve as it crosses the external carotid artery. The inferior root may also be termed the descendens cervicalis. It usually appears lateral to the internal jugular vein, crossing the vein to join the superior root. Occasionally, the inferior root may run medial to the internal jugular vein.

The ansa cervicalis supplies all the infrahyoid muscles with the exception of the thyrohyoid muscle. The innervation of the superior belly of the omohyoid muscle is often given off from the superior root just before it reaches the ansa cervicalis.

This plexus lies in the deep part of the posterior triangle of the neck, between the clavicle and the lower part of the posterior border of the sternocleidomastoid muscle. It emerges between the scalenus anterior and scalenus medius muscles to pass between the clavicle and first rib, around the axillary artery, and into the upper limb.

The brachial plexus is formed by the anterior primary rami of the fourth to the eighth cervical nerves and by most of the anterior ramus of the first thoracic nerve. The plexus provides the innervation for structures in the upper limb.

The branches arising from the brachial plexus above the clavicle in the neck are:

Thus, the branches are mainly motor.

Most of the pharynx derives its sensory nerve supply from the glossopharyngeal nerve through its pharyngeal and tonsillar branches. The pharyngeal branch arises just before the glossopharyngeal nerve passes onto the posterior surface of the stylopharyngeus muscle. This branch then joins the pharyngeal branch of the vagus to reach the pharyngeal plexus. The tonsillar branch of the glossopharyngeal nerve supplies the region around the oropharyngeal isthmus.

The anterior part of the nasopharynx is not supplied by the glossopharyngeal nerve but by the pharyngeal branch of the maxillary nerve. Furthermore, the soft palate is innervated by the lesser palatine branch of the maxillary nerve. Both the pharyngeal and lesser palatine nerves are branches of the maxillary division of the trigeminal nerve via the pterygopalatine ganglion.

The lower part of the pharynx is innervated by the superior laryngeal branch of the vagus nerve.

The muscles of the pharynx derive their innervation from the nucleus ambiguus in the brain stem. Fibres pass from this nucleus within the glossopharyngeal, vagus, and cranial accessory nerves. The cranial accessory nerve, however, joins the vagus nerve shortly after emerging through the jugular foramen of the skull. These fibres, together with those already in the vagus, reach the pharyngeal plexus via the pharyngeal branch of the vagus to supply most of the muscles of the pharynx. The stylopharyngeus muscle, however, is supplied by fibres from the nucleus ambiguus that run with the glossopharyngeal nerve.

The pharyngeal plexus lies on the external surface of the middle constrictor muscle. It is formed by the pharyngeal branches of the glossopharyngeal and vagus nerves, with contributions from the superior cervical sympathetic ganglion. The glossopharyngeal nerve supplies only sensory fibres to the plexus. The vagus contains motor fibres associated with the cranial part of the accessory nerve which, in addition to supplying the muscles of the pharynx, also supply the muscles of the soft palate.

As can be appreciated from the above description, there is considerable anatomical variation in the innervation of the head and neck. Therefore, the ‘standard’ anatomy that is outlined in this chapter is inevitably a ‘consensus’ view of human anatomy and the perceptive, and skilled, clinician will be aware that his, or her, patients may have different anatomies, minor or major.