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(4) The infra-orbital nerve is the terminal branch of the maxillary nerve, which enters the orbit through the inferior orbital fissure and traverses the infra-orbital canal to reach the face.

In the infra-orbital canal the infra-orbital nerve supplies one and sometimes two branches to the teeth-the middle and anterior superior alveolar nerves (ramus alveolaris superior medius et rami alveolares superiores anteriores). The former may be only a secondary branch of one of the latter nerves, or it may arise independently from the infra-orbital nerve. However formed, the nerves descend in bony canals in the wall of the maxillary sinus (to the lining of which branches are given), and reach the alveolar arch, where they form minute plexuses and supply the teeth (joining posteriorly with the branches of the posterior superior alveolar nerves). The anterior superior alveolar nerve supplies the incisor and canine teeth; the middle superior alveolar nerve supplies the premolar teeth.

After emerging on the face from the infra-orbital foramen, the infra-orbital nerve divides into a number of radiating branches arranged in three sets-(a)

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FIG. 654.-COURSE AND BRANCHES OF THE MAXILLARY NERVE.

inferior palpebral (rami palpebrales inferiores), for the lower eyelid; (b) external nasal (rami nasales externi), for the skin of the side of the nose; and (c) superior labial (rami labiales superiores), for the cheek and upper lip. These branches form communications with the zygomatic branches of the facial nerve, and give rise to the infra-orbital plexus (Fig. 657, p. 783).

Ganglion Spheno-palatinum.-The spheno-palatine ganglion occupies the upper part of the pterygo-palatine fossa. It is a small reddish-gray ganglion, suspended from the maxillary nerve by the two spheno-palatine branches which constitute its sensory roots. The motor and sympathetic roots of the ganglion are derived from the nerve of the pterygoid canal. This nerve is formed in the cranial cavity, upon the cartilage filling up the foramen lacerum, by the union of the greater superficial petrosal nerve from the genicular ganglion of the facial nerve (emerging from the temporal bone through the hiatus canalis facialis) with the deep petrosal nerve, a branch of the sympathetic plexus on the internal carotid artery. The nerve of the pterygoid canal passes through the pterygoid canal to the pterygo-palatine fossa, where it joins the spheno-palatine ganglion.

The branches from the ganglion are seven in number.

(a) The pharyngeal branch passes backwards through the pharyngeal canal to supply the mucous membrane of the roof of the pharynx.

(b) Nervi Palatini.—The palatine nerves, three in number, are directed downwards to the palate through the palatine canals.

The large anterior palatine nerve emerges on the under surface of the palate through the greater palatine foramen, and at once separates into numerous branches for the supply of the mucous membrane of the soft and the hard palate. Its anterior filaments communicate with branches of the naso-palatine nerve. The main nerve gives off, as it lies in the palatine canal, a small posterior inferior lateral nasal nerve (rami nasales posteriores inferiores laterales), which enters the nasal cavity and supplies the mucous membrane of the lower part of its lateral wall.

The middle palatine nerve descends through a small palatine canal, and, piercing the pyramidal process of the palate bone, is distributed to the mucous membrane of the soft palate, uvula, and palatine tonsil. It possibly conveys motor fibres to the levator veli palatini and uvular muscles. The n. palatinus posterior (posterior palatine nerve) consists of one or more small twigs which pass through lesser palatine canals, and supply branches to the mucous membrane of the tonsil, soft palate, and uvula.

(c) The branches directed medially from the spheno-palatine ganglion enter the nasal cavity through the spheno-palatine foramen. They are two in number-the posterior superior lateral nasal and the naso-palatine. The posterior superior lateral nasal branch (rami nasales posteriores superiores laterales) is a small nerve destined for the mucous membrane of the superior and posterior part of the lateral wall of the nasal cavity. The n. nasopalatinus (naso-palatine nerve), after passing through the spheno-palatine foramen, crosses the roof of the nasal cavity, and extends obliquely downwards and forwards along the nasal septum, grooving the vomer in its course, to reach the incisive foramen near the front of the hard palate. The nerves pass through the subordinate median foramina (of Scarpa), the left nerve in front of the right. In the incisive foramen the two nerves communicate together. They then turn backwards and supply the mucous membrane of the hard palate. They communicate posteriorly with terminal filaments of the anterior palatine nerves. In its course through the nasal cavity the naso-palatine nerve furnishes collateral branches to the mucous membrane of the roof and septum of the nose (posterior superior medial branches) (Fig. 649, p. 774).

(d) Rami Orbitales. The orbital branches, one or more minute branches, pass upwards to the periosteum of the orbit from the spheno-palatine ganglion.

NERVUS MANDIBULARIS.

The mandibular nerve is formed by the union of two roots; a large sensory root, from the semilunar ganglion, and the small motor root of the trigeminal nerve, which is wholly incorporated with the mandibular trunk. The two roots pass together in the dura mater of the middle fossa of the base of the skull to the foramen ovale, through which they emerge into the infra-temporal fossa Outside the skull they combine to form a single trunk, which soon separates into anterior and posterior divisions.

At its emergence from the skull the nerve is deeply placed beneath the middle of the zygomatic arch, and is concealed by the ramus of the mandible, and by the masseter, temporal, and external pterygoid muscles.

The branches of the nerve may be divided into two series-(1) those derived from the undivided nerve, and (2) those derived from its terminal divisions.

The branches of the undivided nerve are two in number. (a) A small nervus spinosus (O.T. recurrent nerve) arises just outside the skull, and accompanying the middle meningeal artery through the foramen spinosum, supplies the dura mater. (b) In the infra-temporal region a small branch arises for the supply of the internal pterygoid muscle. This nerve forms a connexion with the otic ganglion.

The terminal divisions of the nerve are a small anterior and a large posterior

trunk.

The small anterior trunk (nervus masticatorius or masticator nerve) passes

downwards and forwards medial to the external pterygoid muscle, and separates into the following branches:-(1) A branch for the external pterygoid muscle, which supplies it on its deep surface; (2) a branch to the masseter muscle (n. massetericus), which passes over the superior border of the external pterygoid and through the mandibular notch of the mandible; (3) and (4) two deep temporal branches (nn. temporales profundi), an anterior and a posterior, to the temporal muscle, which also ascend above the external pterygoid muscle; and (5) the n. buccinatorius (buccinator (O.T. buccal) nerve), which passes obliquely forwards between the two heads of the external pterygoid to reach the buccinator muscle. This nerve is sensory, and its fibres are, in part, distributed to the skin of the cheek (communicating with buccal branches of the facial nerve); they are also, in part, distributed to the mucous membrane of the inside of the mouth, to reach which they pierce

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the fibres of the buccinator muscle. The buccinator nerve usually supplies a third branch to the temporal muscle, after emerging between the two heads of the external pterygoid muscle (Fig. 655).

The large posterior trunk extends downwards a short way medial to the external pterygoid muscle. After giving off, by two roots, the auriculo-temporal nerve, it ends by dividing into two, the lingual and the inferior alveolar nerves.

N. Auriculotemporalis.-The auriculo-temporal nerve is formed by the union of two roots which embrace the middle meningeal artery. The nerve passes backwards medial to the external pterygoid muscle and between the spheno-mandibular ligament and the neck of the mandible. After passing through the parotid gland, it is directed upwards to the temple over the zygoma, in company with the superficial temporal artery. It is finally distributed as a cutaneous nerve of the temple and scalp, and reaches almost to the vertex of the skull.

The auriculo-temporal nerve gives off the following branches:-(1) A small branch to the mandibular articulation. (2) Branches to the parotid gland (rami

parotidei). (3) A twig for the supply of the skin of the external acoustic meatus and membrana tympani (n. meatus auditorii externi et ramus membranæ tympani). (4) Branches to the superior half of the auricle on its lateral aspect. (5) Terminal branches to the skin of the temple and scalp (rami temporales superficiales).

It has the following communications with other nerves :-(1) Important communications are effected by the roots of the nerve, which are separately joined by small branches from the otic ganglion. (2) The parotid branches of the nerve are connected with branches of the facial nerve in the substance of the gland (rami anastomotici c. nervo faciali). (3) The temporal branch of the nerve is in communication superficially with the temporal branches of the facial nerve.

N. Lingualis.-The lingual nerve is the smaller of the two terminal branches of the posterior division of the mandibular trunk. It proceeds downwards in front of the inferior alveolar nerve, medial to the external pterygoid muscle, to its inferior border. After passing between the internal pterygoid muscle and the ramus of the mandible, it crosses beneath the mucous membrane of the floor of the mouth in the interval between the mylo-hyoid and hyoglossus muscles and beneath the duct of the submaxillary gland. It sweeps forwards and medially to the side of the tongue, to the mucous membrane over the anterior two-thirds of which it is distributed.

Two nerves communicate with the lingual nerve in its course to the tongue :(1) The chorda tympani branch of the facial nerve joins it medial to the external pterygoid muscle, and is incorporated with it in its distribution to the tongue. (2) The hypoglossal nerve forms larger or smaller loops of communication with the lingual nerve as they course forwards together over the hyoglossus muscle (rami anastomotici cum nervo hypoglosso).

Besides supplying the aforesaid branches to the mucous membrane over the sides and dorsum of the tongue in its anterior two-thirds, the lingual nerve supplies the mucous membrane of the side wall and floor of the mouth. It also assists, along with the chorda tympani nerve, in forming the roots of the submaxillary ganglion. Ganglion Submaxillare. The submaxillary ganglion is a minute reddish ganglion placed on the hyoglossus muscle, between the lingual nerve and the duct of the submaxillary gland. It is suspended from the former by two trunks, consisting for the most part of fibres of the lingual and chorda tympani nerves, which at that point become separated from the lingual nerve and incorporated with the ganglion. The roots of the ganglion are-(1) an afferent root, derived from the lingual nerve; (2) an efferent root, derived from the chorda tympani; and (3) a sympathetic root, from the sympathetic plexus upon the external maxillary artery.

The branches from the ganglion are distributed to the submaxillary gland and duct (rami submaxillares), and by fibres which become reunited with the trunk of the lingual nerve, to the sublingual gland.

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N. Alveolaris Inferior.-The inferior alveolar nerve (O.T. inferior dental) is larger than the lingual nerve. It passes from beneath the inferior border of the external pterygoid muscle to reach the interval between the ramus of the mandible and the spheno-mandibular ligament. Entering the mandibular canal through the mandibular foramen, it traverses the substance of the ramus and body of the mandible, distributing branches to the teeth in its course. A fine plexus is formed by the dental branches before they finally supply the teeth.

Branches and Communications.-(1) N. Mylohyoideus.-The mylo-hyoid nerve is a small branch arising just before the inferior alveolar nerve passes through the mandibular foramen. Grooving the ramus in its course, it descends into the submaxillary triangle on the superficial aspect of the mylo-hyoid muscle. Concealed in this situation by the submaxillary gland and the external maxillary artery, it is distributed to the mylo-hyoid muscle and the anterior belly of the digastric muscle. (2) N. Mentalis.-The mental branch of the inferior alveolar nerve is a trunk of considerable size arising from the main nerve in the mandibular canal. It emerges from the mandible through the mental foramen, and is distributed by many branches to the chin and lower lip. It communicates, under cover of the facial muscles, with the ramus marginalis mandibule of the facial nerve (Fig. 657, p. 783). (3) The incisor

branch is the terminal part of the inferior alveolar nerve remaining after the origin of the mental branch. It supplies the canine tooth and the incisor teeth.

Ganglion Oticum.-The otic ganglion is situated deep to the mandibular nerve just below the foramen ovale. Like the other ganglia described above, it possesses three roots :-(1) A motor root, derived from the nerve to the internal pterygoid muscle; (2) a sensory root, formed by the lesser superficial petrosal nerve from the tympanic plexus (through which communications are effected with the tympanic branch of the glossopharyngeal nerve, and a branch from the genicular ganglion of the facial nerve); (3) a sympathetic root, from the plexus on the middle meningeal artery (Fig. 656).

Five branches arise from the ganglion-three communicating and two motor branches. The three communicating nerves are fine branches which join respectively the nerve of the pterygoid canal, the roots of the auriculo-temporal nerve, and the chorda tympani nerve. The two motor nerves supply the tensor tympani and tensor veli palatini muscles.

Summary. The trigeminal, the largest and most complex of the cerebral nerves, is (1) the chief sensory nerve for the face, the anterior half of the scalp, the orbit and eyeball, the nose and nasal cavity, the lips, teeth, mouth, and anterior two-thirds of the tongue; (2) the motor fibres of the nerve supply the muscles of mastication, the mylo-hyoid and anterior belly of the digastric, possibly the levator veli palatini and uvular muscle (through the spheno-palatine ganglion), and the tensor tympani and tensor veli palatini muscles (through the otic ganglion); (3) through the ganglia placed on the three divisions of the nerve, not only are important organs, areas, and muscles innervated, but communications are also effected with the sympathetic system, with the oculo-motor nerve (ciliary ganglion), facial nerve (spheno-palatine and otic ganglia), and glossopharyngeal nerve (otic ganglion).

In its distribution to the skin of the face the branches of the fifth nerve present two striking peculiarities (1) While the branches to the skin reach the surface at many points and in diverse ways, the three main divisions are severally, by their branches, responsible for the supply of three clearly demarcated cutaneous areas (Fig. 648, p. 772). (2) By numerous communications with the facial nerve, sensory fibres are given to the muscles of expression supplied by the facial nerve.

NERVUS ABDUCENS.

The sixth or abducens nerve issues from the brain at the inferior border of the pons, just above the pyramid of the medulla oblongata (for the deep origin, see p. 600). It is directed forwards, it pierces the dura mater in the posterior fossa of the base of the skull alongside the dorsum sellæ, and enters the cavernous sinus (Fig. 647, p. 771). In the sinus it is placed close to the lateral side of the internal carotid artery. After it leaves the sinus it passes through the superior orbital fissure below the oculo-motor and naso-ciliary nerves and between the two heads of the lateral rectus muscle (Fig. 652, p. 776). In the cavity of the orbit it supplies the lateral rectus muscle on its deep (ocular) surface.

Communications.-In the wall of the cavernous sinus the sixth nerve receives two communicating filaments :-(1) From the carotid plexus of the sympathetic, and (2) from the ophthalmic division of the trigeminal nerve.

NERVUS FACIALIS.

The seventh or facial nerve emerges from the brain at the inferior border of the pons, below the trigeminal nerve and medial to the acoustic nerve (for the deep origin, see p. 598). Between it and the acoustic nerve is the minute nervus intermedius (Fig. 656, p. 782). The facial nerve passes through the internal acoustic meatus, and through the canalis facialis in the petrous portion of the temporal bone, emerges at the base of the skull by the stylo-mastoid foramen, and passes forwards through the parotid gland to supply the muscles of the face. In the internal acoustic meatus the nerve is placed upon the acoustic nerve, the nervus intermedius intervening. In the canalis facialis the nerve first passes forwards and laterally to the hiatus of the canal, then backwards on the medial side of the tympanum, and finally downwards behind the tympanum, in the medial wall of the tympanic antrum. In the parotid gland it crosses the external carotid artery and the posterior facial vein superficially. On the face its branches radiate from the anterior border of the parotid gland and enter the deep surface of the facial muscles.

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