Part 76

The masseter and superficial muscles and the parotid gland are in great part removed. _a_, _a_, Levator labii superioris proprius; _b_, _b_, levator nasolabialis; _c_, dilatator naris inferior; _d_, buccinator; _e_, common mass of buccinator and depressor labii inferioris; _f_, depressor labii inferioris; _g_, masseter; _h_, orbicularis oculi; _i_, temporalis; _k_, occipito-hyoideus; _k′_, stylo-maxillaris; _l_, sterno-cephalicus; _l′_, tendon of same; _m_, omo-hyoideus; _n_, crico-pharyngeus; _o_, mastoid tendon of mastoido-humeralis; _p_, splenius; _q_, inferior buccal and labial glands; _r_, superior buccal glands; _s_, remnant of parotid gland; _t_, submaxillary gland; _u_, anterior cervical (subparotid) lymph glands; _v_, probe passed into diverticulum nasi; _w_, cornu of alar cartilage; _x_, internal palpebral ligament; _y_, wing of atlas; _z_, scutiform cartilage of ear; _1_, external nasal nerve; _2_, anterior nasal nerve; _3_, superior labial nerve; _4_, anterior part of superior buccal nerve; _5_, buccinator nerve; _6_, _6′_, inferior alveolar nerve; _6″_, mental nerve-continuation of _6_; _7_, masseteric nerve; _8_, facial nerve (cut); _9_, superficial temporal nerve; _10_, anastomosis between _9_ and _8_; _11_, internal auricular nerve; _12_, posterior auricular nerve; _13_, digastric nerve; _14_, cervical branch of facial nerve (cut); _15_, auriculo-palpebral nerve; _16_, lacrimal nerve; _17_, frontal nerve; _18_, infratrochlear nerve; _19_, n. zygomaticus s. subcutaneus malæ; _20_, spinal accessory nerve; _21_, ventral branch of spinal accessory nerve (to sterno-cephalicus); _22_, ventral end branch of first cervical nerve; _23_, thyro-laryngeal artery; _24_, internal maxillary artery; _25_, masseteric artery; _26_, great (posterior) auricular artery; _27_, external branch of _26_; _28_, deep auricular artery; _29_, superficial temporal artery; _30_, transverse facial artery; _31_, facial artery; _32_, inferior labial artery; _33_, superior labial artery; _34_, lateral nasal artery; _35_, dorsal nasal artery; _36_, angular artery of eye; _37_, labial twigs of palato-labial artery—left uncolored by oversight; _38_, branch of buccinator artery; _39_, buccinator vein; _40_, vena reflexa; _41_, _42_, jugular vein; _43_, external maxillary vein; _44_, inferior cerebral vein; _45_, stump of great auricular vein; _46_, ramus of mandible; _47_, facial crest; _48_, zygomatic arch; _49_, parotid duct. (After Ellenberger-Baum, Top. Anat. d. Pferdes.) ]

(3) The =lesser= or =posterior palatine nerve= (N. palatinus minor s. posterior), also termed the staphyline, is much the smallest of the three branches (Figs. 438, 439). It passes downward and forward with the palatine vein in the groove at the inner side of the tuber maxillare and ramifies in the soft palate.

The sphenopalatine ganglia and plexus (Fig. 515) lie on the perpendicular part of the palate bone and the pterygoid process under cover of the maxillary nerve. The afferent fibers of the plexus and ganglia come chiefly from the branches of the sphenopalatine nerve and the nerve of the pterygoid canal. Interspersed in these are several minute ganglia and one or more larger ones. The =nerve of the pterygoid canal= or =Vidian nerve= (N. canalis pterygoidei) is formed by the union of sympathetic fibers with the superficial petrosal branch of the facial nerve. It passes forward at first between the Eustachian tube and the sphenoid bone, enters the canal between the pterygoid bone and process, and joins the posterior part of the plexus. It is probable that it furnishes the motor fibers to the levator palati and palatinus muscles. Efferent filaments go to the periorbita and the ophthalmic vessels, and others accompany the branches of the maxillary nerve, around which they have a plexiform arrangement.

3. The =infraorbital nerve= (N. infraorbitalis) is the continuation of the maxillary trunk. It traverses the infraorbital canal, emerges through the infraorbital foramen, and divides into nasal and superior labial branches. Along its course it gives off =superior alveolar= or =dental branches= (Rami alveolares maxillæ), which supply the teeth, alveolar periosteum, and gums.

The delicate posterior alveolar branches are given off in the pterygo-palatine fossa, pass through small foramina in the tuber maxillare, and supply the posterior molar teeth and the maxillary sinus. The middle alveolar branches are given off in the infraorbital canal, and constitute the chief nerve-supply to the cheek teeth and the maxillary sinus. The anterior or incisor branch runs forward in the anterior alveolar or incisor canal and supplies branches to the canine and incisor teeth. The foregoing unite with each other to form the superior dental plexus, from which the dental and gingival branches are given off.

The =external nasal branches= (Rami nasales externi), two or three in number, accompany the levator labii superioris proprius and ramify in the dorsum nasi and the nasal diverticulum.

The large =anterior nasal branch= (Ramus nasalis anterior) passes over the nasal process of the premaxilla under cover of the dilatator naris inferior, gives branches to the nasal mucous membrane, and terminates in the skin of the upper lip.

The =superior labial branch= (Ramus labialis dorsalis) is the largest of the terminals of the infraorbital nerve. It passes downward and forward under the levator nasolabialis and, after supplying the skin of the anterior part of the cheek, forms a rich terminal ramification in the skin and mucous membrane of the upper lip. It anastomoses with the superior buccal branch of the facial nerve.

III. The =mandibular nerve= (N. mandibularis), also termed the inferior maxillary branch, is formed by the union of two roots; of these the large sensory root comes from the semilunar ganglion, and the small motor root is the pars minor of the trigeminus. It emerges from the cranium through the oval notch of the foramen lacerum, between the temporal wing of the sphenoid bone and the muscular process of the petrous temporal, and gives off the following branches:

1. The =masseteric nerve= (N. massetericus) (Figs. 437, 515) passes outward through the sigmoid notch of the mandible and enters the deep face of the masseter muscle, in which it ramifies.

2. The =deep temporal nerves= (Nn. temporales profundi) (Fig. 437), two or three in number, arise by a common trunk with the masseteric. They supply the temporal muscle.

3. The =buccinator nerve= (N. buccinatorius) passes obliquely forward through the anterior part of the external pterygoid muscle, then between the internal pterygoid and the tuber maxillare (Fig. 514). It continues forward in the submucous tissue of the cheek along the lower border of the buccinator and divides into branches which ramify in the mucous membrane and glands of the lips in the vicinity of the commissure. It supplies small branches to the external pterygoid and temporal muscles and detaches numerous collateral twigs to the mucous membrane of the cheek and to the buccal glands. It also communicates with the inferior buccal branch of the facial nerve.

4. The =pterygoid nerve= (N. pterygoideus) arises in common with the preceding, passes forward on the guttural pouch, and divides into branches for the pterygoid muscles (Fig. 515).

The =otic ganglion= (G. oticum) is situated near the origin of the pterygoid and buccinator nerves, and is related internally to the tensor palati and the Eustachian tube. It receives motor fibers from the pterygoid nerve and sensory fibers by the small superficial petrosal nerve from the tympanic plexus, through which communications are made with the facial and glosso-pharyngeal nerves. Sympathetic fibers are derived from the plexus on the internal maxillary artery. Efferent filaments go to the tensor palati, tensor tympani, and pterygoid muscles, and to the Eustachian tube.

The ganglion is small and somewhat difficult to demonstrate. In many cases it is replaced by a number of minute ganglia interspersed in a fine plexus.

5. The =superficial temporal nerve= (N. temporalis superficialis) (Figs. 437, 514, 515) runs outward across the pterygoideus externus, passes between the parotid gland and the neck of the ramus of the mandible, turns around the latter, and divides into two branches. The upper branch (Ramus transversus faciei) accompanies the transverse facial vessels and ramifies in the skin of the cheek. The larger inferior branch unites with the inferior buccal division of the facial nerve.

[Illustration:

FIG. 515.—DEEP DISSECTION OF BASE OF CRANIUM, VIEWED FROM THE RIGHT AND BELOW.

The tympanic cavity is opened and the paramastoid process is sawn off. _a_, Occipital condyle; _b_, body of sphenoid; _c_, external auditory meatus (part removed); _d_, malleus; _e_, incus; _1_, stump of ophthalmic nerve; _2_, maxillary nerve (cut off); _3_, sphenopalatine plexus; _4_, Vidian nerve; _4′_, deep petrosal nerve; _4″_, branch to tympanic plexus; _4‴_, superficial petrosal nerve; _5_, superficial temporal nerve (cut off); _6_, masseteric nerve (cut off); _7_, mandibular nerve (raised); _8_, lingual nerve (cut off); _9_, pterygoid nerve (cut); _10_, nerve to tensor tympani; _11_, otic ganglion; _12_, facial nerve; _13_, stapedial nerve; _14_, chorda tympani; _15_, glosso-pharyngeal nerve (cut); _16_, tympanic nerve; _17_, vagus (cut off); _18_, auricular branch of vagus; _19_, spinal accessory nerve (cut off); _20_, hypoglossal nerve (cut off); _21_, sympathetic nerve (cut off); _22_, internal carotid artery. (After Ellenberger, in Leisering’s Atlas.) ]

Before its division the nerve gives off twigs to the guttural pouch, the parotid gland, the external ear, and the skin of the external auditory meatus and the membrana tympani. Branches from it concur with filaments from the cervical branch of the facial nerve in the formation of the auricular plexus.

6. The =inferior alveolar= or =dental nerve= (N. alveolaris mandibulæ) (Figs. 436, 437, 514) arises with the lingual by a common trunk which passes forward at first on the external pterygoid muscle, then inclines ventrally between the internal pterygoid and the ramus of the mandible. The lingual and alveolar separate at an acute angle, and the latter enters the mandibular foramen and courses in the canal within the ramus (Fig. 516). Emerging at the mental foramen, it terminates by dividing into six to eight inferior labial and mental branches, which ramify in the lower lip and chin. Before entering the bone, the nerve detaches the =mylo-hyoid nerve= (N. mylo-hyoideus), which runs downward and forward between the ramus and the mylo-hyoid muscle; it supplies that muscle, the anterior belly of the digastricus, and the skin of the anterior part of the submaxillary space. The dental and gingival branches detached from the nerve within the mandible are arranged like the corresponding nerves of the upper jaw.

7. The =lingual nerve= (N. lingualis) arises by a common trunk with the inferior alveolar or dental nerve (Figs. 436, 437). After separating from the latter it runs downward and forward, lying at first between the ramus of the mandible and the internal pterygoid muscle, then on the inner face of the mylo-hyoid. On reaching the root of the tongue it divides into superficial and deep branches. The =superficial branch= (Ramus superficialis) runs forward on the stylo-glossus and accompanies the submaxillary duct on the deep face of the sublingual gland. It supplies the mucous membrane of the tongue and the floor of the mouth. At the root of the tongue it gives off a recurrent branch to the isthmus faucium, which communicates with the lingual branch of the glosso-pharyngeal nerve. The larger =deep branch= (Ramus profundus) turns around the lower edge of the hyo-glossus, passes upward and forward between that muscle and the genio-glossus, and continues forward on the latter to the tip of the tongue. It gives branches to the mucous membrane and the fungiform papillæ of the tongue, and anastomoses with branches of the hypoglossal nerve and with the superficial branch. Minute ganglia occur on the finer branches of the lingual nerve. The =chorda tympani= branch of the facial joins the lingual nerve at the origin of the latter and is incorporated with it in its distribution to the tongue.

[Illustration:

FIG. 516.—PART OF BRANCH OF LOWER JAW OF HORSE, INTERNAL VIEW.

The bone has been removed to show the vessels and nerves. _1_, _1′_, Inferior alveolar or dental nerve; _2_, _2_, branches to cheek teeth and gums; _2′_, branch to canine and incisor teeth; _3_, _3′_, inferior alveolar or dental artery; _4_, satellite vein. (After Leisering’s Atlas.) ]

THE SIXTH OR ABDUCENT NERVE

The =abducent nerve= (N. abducens) emerges from the brain behind the pons and just external to the pyramid (Fig. 499). It passes forward across the pons, pierces the dura mater, and accompanies the third and ophthalmic nerves, below which it emerges through the foramen lacerum orbitale. In the orbit it divides into two branches; the larger of these enters the external rectus, the smaller supplies the superior and external parts of the retractor muscle of the eyeball. Within the cranium it receives filaments from the carotid plexus of the sympathetic.

The fibers of the abducent nerve are axones of the large multipolar cells of the abducent nucleus which is situated beneath the eminentia teres of the floor of the fourth ventricle. The nucleus lies within the loop formed by the fibers of origin of the facial nerve. It is connected with (_a_) the anterior olive; (_b_) the oculomotor nucleus of the opposite side; (_c_) the motor area of the cerebral cortex by means of the pyramidal tract of the opposite side.

THE SEVENTH OR FACIAL NERVE

The =facial nerve= (N. facialis) has its superficial origin at the lateral part of the corpus trapezoideum, immediately behind the pons (Fig. 499). It passes outward in front of the eighth nerve and enters the internal auditory meatus. At the bottom of the meatus the two nerves part company, the facial coursing in the facial canal of the petrous temporal bone. The canal and nerve are at first directed outward between the vestibule and the cochlea, then curve backward and downward in the posterior wall of the tympanum to end at the stylo-mastoid foramen. The bend formed by the nerve is called the =knee= (Geniculum n. facialis) and bears at its highest point the round =geniculate ganglion= (G. geniculi).

The nerve consists of two parts, motor and sensory. The =motor part= constitutes the bulk of the nerve. Its deep origin is from the cells of the =facial nucleus=, which is situated in the medulla above the facial tubercle. On leaving the nucleus the root-fibers pass upward and inward, incline forward close to the median plane, and then bend sharply downward to the point of emergence. The highest point of the bend is subjacent to the gray matter of the eminentia teres in the floor of the fourth ventricle, and the abducent nucleus lies in the concavity of the curve. The small =sensory part= (N. intermedius) consists of axones of cells of the geniculate ganglion, which is interposed on the facial nerve as it bends downward in the facial canal. The fibers of this part after entering the medulla, pass to the nucleus of termination which it shares with the ninth and tenth nerves. The peripheral fibers from the geniculate ganglion constitute the chorda tympani.

After its emergence through the stylo-mastoid foramen (Fig. 514) the nerve passes downward, forward, and outward on the guttural pouch under cover of the parotid gland, and crosses between the origin of the superficial temporal and internal maxillary arteries internally and the superficial temporal vein externally. It then crosses the posterior border of the ramus of the mandible ventral to the transverse facial artery and about an inch and a half (ca. 3.5 to 4 cm.) below the articulation of the jaw. Emerging from beneath the parotid gland upon the masseter muscle, it receives the lower branch of the superficial temporal nerve, and divides into superior and inferior buccal branches. The following collateral branches are given off, the first five being detached within the facial canal, and the others between the stylo-mastoid foramen and the border of the jaw.

1. The =great superficial petrosal nerve= (N. petrosus superficialis major) arises from the geniculate ganglion.[203] It passes through the petrosal canal, contributes a filament to the tympanic plexus, receives the great deep petrosal nerve from the carotid plexus of the sympathetic, emerges through the foramen lacerum, and is continued as the Vidian nerve to the sphenopalatine plexus and ganglia (Fig. 515).

2. A delicate branch (R. anastomoticus cum plexu tympanico) emerges from the geniculate ganglion and unites with a filament issuing from the tympanic plexus to form the =small superficial petrosal nerve= (N. petrosus superficialis minor); this ends in the otic ganglion.

3. The =stapedial nerve= (N. stapedius) (Fig. 515) is a short filament detached from the facial nerve as it turns down in the facial canal. It innervates the stapedius muscle.

4. The =chorda tympani= (Fig. 515) is a small nerve which arises a little below the preceding and pursues a recurrent course in a small canal in the mastoid part of the temporal bone to reach the tympanic cavity. It traverses the latter, passing between the handle of the malleus and the long branch of the incus. Emerging through the petro-tympanic fissure, the nerve passes downward and forward, crosses beneath the internal maxillary artery, and joins the lingual nerve. It sends twigs to the submaxillary and sublingual glands, and through its incorporation with the lingual nerve furnishes fibers to the mucous membrane of the anterior two-thirds of the tongue which are believed to mediate the sense of taste.

5. Anastomotic filaments unite with the auricular branch of the vagus near the stylo-mastoid foramen.

6. The =posterior auricular nerve= (N. auricularis posterior) arises from the facial at its emergence from the facial canal (Fig. 514). It runs upward and backward with the posterior auricular artery under cover of the parotid gland and supplies the posterior auricular muscles and the skin of the convex surface of the external ear. It anastomoses with branches of the first and second cervical nerves.

7. The =internal auricular nerve= (N. auricularis internus) springs from the facial close to or in common with the preceding (Fig. 514). It ascends in the parotid gland just behind the styloid process of the conchal cartilage, passes through an opening in the cartilage, and ramifies in the skin of the concave surface of the ear.

8. The =digastric branch= (R. digastricus) (Fig. 514) arises from the facial below the auricular nerves. Its branches innervate the posterior belly of the digastricus, the stylo-hyoideus, and the occipito-hyoideus. At its origin it gives off a small branch which forms a loop around the great auricular artery or its posterior branch and rejoins the trunk.

9. The =auriculo-palpebral nerve= (N. auriculo-palpebralis) (Fig. 514) arises from the upper edge of the facial near the posterior border of the ramus. It ascends in the parotid gland behind the superficial temporal artery, and terminates in anterior auricular and temporal branches. The =anterior auricular branches= form with branches of the trigeminus the =anterior auricular plexus=. They innervate the anterior auricular and parotido-auricularis muscles. The =temporal branch= runs forward and inward over the temporal muscle to the inner canthus of the eye, forms a plexus with the terminal branches of the ophthalmic nerve, and is distributed to the orbicularis oculi, corrugator supercilii, and levator nasolabialis.

10. The =cervical branch= (R. colli) (Fig. 435) arises from the ventral border of the facial opposite to the preceding nerve. It emerges obliquely through the parotid gland, passes downward and backward on or near the jugular vein, and anastomoses with the cutaneous branches of the cervical nerves. It gives branches to the parotido-auricularis and the cervical panniculus. In its course along the neck the nerve is reinforced by twigs from the cutaneous branches of the second to the sixth cervical nerves.

11. Small branches are detached to the guttural pouch and the parotid gland. The latter (Rami parotidei) concur with branches of the superficial temporal nerve in forming the =parotid plexus=.

The facial nerve usually terminates after a short course on the surface of the masseter by dividing into two buccal branches (Figs. 435, 461).

1. The =superior buccal nerve= (N. buccalis dorsalis) passes forward on the upper part of the masseter, dips under the zygomaticus, and continues along the lower border of the dilatator naris lateralis. It then runs under the last-named muscle and anastomoses with branches of the infraorbital nerve, and is distributed to the muscles of the cheek, upper lip, and nostril.

2. The =inferior buccal nerve= (N. buccalis ventralis) crosses the masseter obliquely and continues forward along the depressor labii inferioris. It is connected by variable anastomotic branches with the superior nerve. It gives collateral branches to the panniculus, buccinator, and depressor labii inferioris, and ramifies with the terminal branches of the inferior alveolar nerve in the lower lip.

The buccal nerves are subject to much variation in regard to their course, anastomoses, and relations to the sensory components derived from the superficial temporal nerve. Their distribution is constant. The point at which the branch of the superficial temporal nerve joins the facial is variable.

THE EIGHTH OR AUDITORY NERVE

The =auditory nerve= (N. acusticus) is connected with the lateral aspect of the medulla just behind and external to the facial (Fig. 499). It has two roots, vestibular and cochlear (Radix vestibularis et cochlearis).

The auditory nerve consists of two distinct parts which might well be regarded as separate nerves. The cochlear part mediates the sense of hearing, while the vestibular part is not auditory in function, but is concerned in the sense of the position of the body and the mechanism of equilibration.

The nerve passes outward to the internal auditory meatus, which it enters behind the facial nerve. In the meatus it divides into two nerves, of which the upper is the vestibular and the lower is the cochlear nerve.

1. The =vestibular nerve= (N. vestibuli) is distributed to the utriculus, the sacculus, and to the ampullæ of the semicircular canals, of the internal ear. In the internal auditory meatus the nerve is connected by filaments with the geniculate ganglion of the facial nerve. At the bottom of the meatus it bears the =vestibular ganglion= (G. vestibulare), from the cells of which the fibers of the nerve arise.

2. The =cochlear nerve= (N. cochleæ) detaches a filament to the sacculus, passes through the lamina cribrosa to the labyrinth, and is distributed to the organ of Corti in the cochlea.

The fibers of the vestibular nerve arise from the vestibular ganglion as central processes (axones) of the bipolar cells of the ganglion. The peripheral processes (dendrites) of the cells form arborizations about the deep ends of the hair cells of the maculæ and cristæ acusticæ of the utriculus, sacculus, and semicircular canals. The fibers enter the medulla, pass between the restiform body and the spinal tract of the trigeminus, and spread out to end in the vestibular nucleus in the floor of the fourth ventricle. Among the central connections of the vestibular nerve are: (1) fibers which connect its nucleus with centers in the cerebellum (chiefly of the opposite side); (2) the vestibulo-spinal tract, which conveys impulses to the motor cells of the ventral columns of the spinal cord; (3) fibers which connect the nucleus with those of the abducent nerve of the same side, the third and fourth nerves, and the motor part of the trigeminus of both sides.

The fibers of the cochlear nerve are the central processes of the bipolar cells of the spiral ganglion of the cochlea. The peripheral processes of these cells end in relation to the hair cells of the organ of Corti. Some of the nerve-fibers enter the ventral cochlear nucleus in the medulla close to the superficial origin of the nerve; others end in the dorsal nucleus of the tuberculum acusticum at the lateral angle of the floor of the fourth ventricle. From the ventral nucleus fibers pass in the corpus trapezoideum to the anterior olivary nucleus of the same and of the opposite side. Thence tracts pass to the nuclei of the motor nerves of the eye, and through the lateral fillet to the posterior quadrigeminal body and the internal geniculate body. The axones of the cells of the dorsal nucleus pass largely (as the striæ acusticæ) over the restiform body and across the floor of the fourth ventricle toward the median plane. They then turn ventrally, cross to the opposite side, and are continued by the lateral fillet. From the mid-brain a tract proceeds to the cortex of the temporal lobe of the hemisphere.

THE NINTH OR GLOSSO-PHARYNGEAL NERVE