The trigeminus is the largest of the cranial nerves with the exception of the optic. It is usually described as the fifth cranial nerve and as possessing both a sensory and a motor root.
For reasons already given, the "motor root" is here described separately and given the separate name, masticator nerve. The fibers of the trigeminus, which are all sensory, spring from the cells of the semi- lunar (Gasserian) ganglion, which corresponds with the ganglion of the dorsal root of a spinal nerve, and they enter the brain stem through the side of the anterior third of the pons.
The semilunar (Gasserian) ganglion is a semilunar mass which lies in Meckel's cave, a cleft in the dura mater above a depression in the medial part of the upper surface of the petrous portion of the temporal bone. The convexity of the ganglion is turned forward, and from it three large nerves, the ophthalmic, the maxillary, and the mandibular, are given off. From the concavity, which is directed backward, springs the root of the nerve. The medial end of the ganglion is in close relation with the cavernous sinus and the internal carotid artery at the foramen lacerum, and the lateral end lies to the medial side of the foramen ovale. The surfaces of the ganglion are striated, due to bundles of fibers traversing them. The upper surface is separated by the dura mater from the temporal lobe of the brain, and the lower rests upon the masticator nerve and the outer layer of dura mater upon the petrous portion of the temporal bone.
The fibers of the trigeminus root as they leave the semilunar (Gasserian) ganglion, form from thirty to forty fasciculi which are bound together into a flat band, from six to seven millimetres broad, which passes backward over the upper border of the petrous portion of the temporal bone and below the superior petrosal sinus into the posterior fossa of the cranium. In the posterior fossa it runs backward, medialward, and downward, and passes into the pons through its continuation into the middle peduncle of the cerebellum. In the tegmentum of the pons region, the fibers bifurcate into ascending and descending branches which terminate about the cells of the nucleus of termination of the trigeminus. This nucleus, large at the level of the entrance of the root, has tapering superior and inferior e.xtremities. The inferior ex- tremity of the nucleus, which is much the longer, descends as low as the upper portion of the spinal cord and the fibers of the root terminating about the cells of this extremity are known as the spinal tract of the trigeminus.
The nuclei of termination of the trigeminus send impulses to the somaesthetic area of the cortex of the opposite side by the fibers of the medial lemniscus (fillet) and, for reflex actions, to the motor nuclei of other cranial nerves by the medial longitudinal fasciculus and by fascicuU propri in the reticular formation of the same, and opposite sides.
The branches of the Trigeminus
The main branches of the trigeminus, given off by the front side of the semi- lunar ganglion, are three in number (ophthalmic, maxillary, and mandibular) , each of which is referred to as a nerve and each of which is purely sensory, though the third branch, or mandibular nerve, is joined by the fibers of the masticator nerve which is motor.
The Ophthalmic Nerve or First Division
The ophthalmic nerve, the first division of the trigeminus, is the smallest of the three branches which arise from the semilunar (Gasserian) ganglion. It springs from the medial part of the front of the ganglion and passes forward, in the lateral wall of the cavernous sinus, where it lies below the trochlear nerve and lateral to the abducens nerve and the internal carotid artery. A short distance behind the superior orbital (sphenoidal) fissure the nerve divides into three ter- minal branches - the frontal, lacrimal, and naso-ciliary (nasal) nerves. They pierce the dura mater, which closes the fissure, and pass forward into the orbit. Before its division the ophthalmic nerve receives filaments from the cavernous plexus of the sympathetic and it gives off, soon after its origin, a tentorial (recur- rent meningeal) branch which runs backward, in close association with the troch- lear nerve, and ramifies between the layers of the tentorium cerebelli. Further forward three branches spring from the ophthalmic nerve which contribute sensory fibers to the oculo-motor, trochlear, and abducens nerves.
The terminal branches
The frontal nerve
The frontal nerve is the largest terminal branch. It pierces the dura mater and passes into the orbit through the superior orbital (sphenoidal) fissure, above the rectus lateralis and a little below and to the lateral side of the trochlear nerve. In the orbit it runs forward, between the levator palpebrse superioris and the periosteum, and breaks up into three branches, the supra-orbital, frontal proper, and supratrochlear.
The supra-orbital nerve, the largest of the three branches, leaves the orbit at the supra- orbital notch. As it passes thi-ough the notch it gives off a small branch which enters the bone and supphes the diploe and the mucous membrane of the frontal sinus. Its terminal branches give twigs to the pericranium and to the skin of the scalp, the upper ej-eUd, the frontal region, and the parietal region almost as far as the lambdoid suture. One branch running at the upper margin of the orbital cavity unites with a branch of the facial nerve.
The frontal branch, given off at a variable point, lies medial to the supra-orbital, passes through the frontal foramen, and is distributed to the skin of the forehead and upper eyelid.
The supratrochlear branch runs forward and medialward toward the upper and medial angle of the orbit, where it passes above the pulley of the superior obhque muscle, pierces the palpebral fascia, and ascends to the lower and middle part of the forehead, accompanied by the frontal artery. Before it leaves the orbit it sends a branch downward behind or in front of the pulley of the obliquus superior which joins with the infratrochlear nerve, and as it leaves the orbit it gives off filaments to supply the skin and conjunctiva of the medial third of the upper eyelid. Its terminal branches pierce the orbicularis and frontalis, and, as they pass to the skin of the forehead, they communicate with branches of the facial nerve.
The lacrimal nerve
Nerves op the Orbit from Above and Behind. (Schematic.)
The lacrimal nerve [n. lacrimalis] is the smallest of the three branches of the ophthalmic division. It passes through the superior orbital (sphenoidal) fissure lateral to and slightly below the frontal nerve, and is directed forward and lateralward, along the upper border of the rectus lateralis to the lacrimal gland. On the lateral wall of the orbit it receives a small branch from the zygomatic nerve (the orbital branch of the maxillary nerve). This branch brings to the lacrimal nerve secretory fibers for the lacrimal gland. A small twig passes beyond the gland, pierces the palpebral fascia, supplies filaments to the conjunctiva, and is then distributed to the integument at the lateral angle of the eye and to the skin over the zygomatic process of the frontal bone.
The naso-ciliary nerve
The naso-ciliary (nasal) nerve enters the orbit between the two heads of the rectus lateralis and between the superior and inferior branches of the oculo-motor nerve. In the orbit it lies at first lateral to the optic nerve, but, as it runs obliquely forward and medialward to the medial wall of the orbital cavity, it crosses above the optic nerve and between it and the rectus superior, and near the border of the rectus medialis it divides into its terminal branches, the chief of which are the infratrochlear and anterior ethmoidal nerves. In addition to those received from the cavernous plexus before the division of the ophthalmic nerve, the naso-ciliary nerve itself receives numerous sympathetic (secretory and vaso- motor) fibers.
Its several branches are:
- The long root of the ciliary ganglion which is given off at the superior orbital (sphenoidal) fissure. It is a slender filament which runs forward on the lateral side of the optic nerve to the superior and posterior part of the ciliary ganglion.
- The long ciliary nerves, usually two in number, which arise from the naso-cihary nerve as the latter is crossing above the optic nerve. They run forward, on the medial side of the optic nerve, pierce the sclerotic, and are distributed with the lower set of short ciliary nerves. The long root of the cihary ganglion and the long ciUary nerves carry sensory fibers which belong to the naso-ciliary nerve proper, most of which merely pass through the ganglion, and it carries sympathetic fibers, added to it, most of which may terminate about the cell-bodies of the ganglion.
- The posterior ethmoidal (spheno-ethmoidal) branch springs from the posterior border of the naso-ciUary nerve near the upper border of the rectus medialis. It passes through the posterior ethmoidal canal and is distributed to the mucous membrane of the posterior ethmoidal cells and the sphenoidal sinus.
- The infratrochlear nerve passes forward between the obhquus superior and the rectus medialis, and under the pulley of the former muscle divides into two branches: - The superior palpebral branch helps to supply the eyehds with sensory fibers and usually anastomoses with the supratrochlear nerve. The inferior palpebral branch is distributed to the lacrimal sac, the conjunctiva and skin of the medial part of the upper eyehd, the caruncle, and the skin of the upper part of the side of the nose.
- The anterior ethmoidal (distal part of the nasal) nerve, passing forward and medial- ward between the obhquus superior and the rectus medialis, leaves the orbit through the anterior ethmoidal foramen, accompanied by the anterior ethmoidal vessels, and enters into the anterior fossa of the cranium. It then crosses the lamina cribrosa of the ethmoid, lying outside the dura mater, which separates it from the olfactory bulb, and descends into the nasal fossa through the ethmoidal fissure, a slit-like aperture at the side of the crista galli. In the sub- mucosa of the nasal fossa it terminates by dividing into two sets of anterior nasal branches: the internal nasal branches and the external nasal branch.
The internal nasal branches divide into the medial nasal branches (the septal branches of the nasal nerve), which run downward and forward on the upper and front part of the septum, and the lateral nasal branches (the external terminal branch of the nasal nerve), which give twigs to the anterior extremities of the superior and middle nasal conchae (turbinated bones), and to the mucous membrane of the lateral wall of the nose.
The external nasal branch (the anterior terminal branch of the nasal nerve) runs downward in a groove on the inner surface of the nasal bone. It pierces the wall of the nose between the nasal bone and the upper lateral cartilage, and supphes the integument of the lower part of the dorsum of the nose as far as the tip.
The Maxillary Nerve or Second Division of the Trigeminus
The maxillary nerve is entirely sensory in function and it is intermediate in size between the ophthalmic and mandibular nerves.
It springs from the middle of the anterior border of the semilunar (Gasserian) ganglion and runs forward in the lower and outer part of the lateral wall of the cavernous sinus. Leaving the middle fossa of the cranium, by passing through the foramen rotundum, it enters the pterygo-palatine (spheno-maxillary) fossa, where it is joined by twigs with the spheno-palatine ganglion; then, changing its name, it passes forward, as the infra-orbital nerve, through the inferior orbital (spheno-maxillary) fissure into the infra-orbital sulcus in the floor of the orbit; continuing forward it traverses the infra-orbital canal accompanied by the infra-orbital artery, and appears in the face, beneath the levator labii superioris (quadratus) and above the levator anguli oris (caninus) where it divides into four sets of terminal branches which anastomose more or less freely with branches of the facial nerve to form the infra-orbital plexus.
The branches of the maxillary nerve are:
- branches given off in the middle fossa of the cranium;
- branches given off in the pterygo-palatine (spheno-maxillary) fossa;
- branches given off in the infra-orbital sulcus and canal;
- and terminal branches.
The middle (recurrent) meningeal branch, given off in the middle fossa of the cranium, breaks up into numerous branches which supply the dura mater with sensory fibers, reinforce the sympathetic plexus on the middle meningeal artery, and anastomose with the spinous nerve (the recurrent branch of the man- dibular nerve).
The branches given off in the pterygo-palatine (spheno-maxillar) fossa are the spheno-palatine nerves, the zygomatic branch of the maxillary nerve, and the posterior superior alveolar nerves.
The spheno -palatine nerve has two or three branches which descend in the pterygo- palatine fossa and give a small part of their fibers to the spheno-palatine (Meckel's) ganghon, the larger part of their fibers passing tlirough the ganghon into its orbital, nasal, and palatine branches. (See Spheno-palatine Ganglion)
The zygomatic (orbital or temporo-malar) branch, given off from the upper surface of the maxiUary nerve, passes forward and lateralward, and, at the end of the inferior orbital (spheno- maxillary) fissure, passes through it into the orbit and divides into two branches, facial and temporal.
The zygomatico-facial (malar) branch runs forward, passes through a zygomatico-orbital foramen, then thi-ough the zygomatico-facial (malar) foramen, pierces the orbicularis palpe- brarum, communicates with the zygomatic (malar) branch of the facial nerve, and supplies the skin of the prominence of the cheek. The zygomatico -temporal (temporal) branch runs upward in a groove in the lateral wall of the orbit, passes through a zygomatico-orbital foramen, then through the zygomatico-temporal (spheno-malar) foramen, and enters the temporal fossa. It turns around the anterior border of the temporal muscle, pierces the deep layer of the temporal fascia, and runs backward for a short distance in the fat between the superficial and deep lam- ellae, then, turning lateralward, it pierces the superficial lamellae about an inch above the zygoma, anastomoses with the temporal branch of the facial nerve, and supphes the skin of the anterior part of the temporal region.
The infra-orbital nerve, that part of the maxiUary nerve lying distal to the spheno- palatine ganghon, enters the orbit through the inferior orbital (spheno-maxiUary) fissure, accompanied by the infra-orbital artery, and with it passes through the infra-orbital canal to the face, where it divides into four sets of terminal branches, some of which, by anastomoses with the branches of the facial nerve, form the infra-orbital plexus.
Three sets of superior alveolar nerves arise from the maxillary and the infra-orbital nerves, namely, the posterior superior alveolar branches, the middle superior alveolar branch, and the anterior superior alveolar branches.
Lateral View of the Maxillary Nerve.
The posterior superior alveolar (dental) nerves are usually two in number, but sometimes arise by a single trunk. They pass downward and lateralward through the pterygo-maxillary fissure into the zygomatic fossa, where they give branches to the mucous membrane of the gums and the posterior part of the mouth; then they enter the posterior alveolar (dental) canals and unite with the other alveolar branches to form the superior dental plexus, through which they give branches to the roots and pulp cavities of the molar teeth and to the mucous membrane of the maxillary sinus.
The branches given off in the infra-orbital sulcus and canal are the middle and anterior superior alveolar (dental) nerves.
The middle superior alveolar (dental) nerve leaves the infra-orbital nerve in the pos- terior part of the inlVa-orliital sulcus, and, pa.ssing downward and forward in a canal in the max- illa, it divides into terminal branches that anastomose with the other alveolar branches to form the superior dental plexus. Through the plexus it supplies the bicuspid teeth and gives branches to the mucous membrane of the maxillary sinus and also to the gums.
The anterior superior alveolar (dental) nerve is the largest of the superior alveolar nerves. It is given off by the infra-orbital nerve in the anterior part of the infra-orbital canal, and passes downward in a bony canal in the anterior wall of the maxilla. After uniting with the other alveolar nerves to form the superior dental ple.xus, it supplies the canines and the incisors and gives branches to the mucous membrane of the maxillary sinus and the gums. It also gives off a nasal branch which enters the nasal fossa through a small foramen, and supphes the mucous membrane of the anterior part of the inferior meatus and the adjacent part of the floor of the nasal cavity.
The superior dental plexus is formed in the bony alveolar canals by the three superior alveolar nerves. It is convex downward and anastomoses across the mid-line with the corre- sponding plexus of the other side. From it arise the superior dental branches supply- ing the superior canines and incisors, superior gingival branches supplying the gums, and also branches to the mucous membrane of the maxiUary sinus and to the bone. On the plexus are two gangliform enlargements, one, called the ganglion of Valentine, situated at the junction of the middle and the posterior branches, and the other, called the ganglion of Bochdalek, at the junction of the middle and anterior branches.
The terminal branches of the maxillary nerve are the inferior palpebral, the external and internal nasal (nasal), and the superior labial.
The inferior palpebral branches, usually two, pass upward and supply sensory fibers to all the skin and conjunctiva of the lower eyelid.
The external nasal branches pass medialward under cover of the levator labii superioris (quadratus), and supply the skin of the posterior part of the lateral aspect of the nose.
The internal nasal branches pass downward and medialward under the lateral wall of the aose, and then turn ujnvard to supply the skin of the vestibule of the nose.
The superior labial branches, three or four in number, as a rule are larger than the palpebral and nasal branches. They pass downward to supply the skin and mucous membrane of the upper Up and the neighbouring part of the cheek.
The Mandtbular Nerve or Third Division of the Trigeminus
The mandibular division is the largest of the three divisions of the trigeminus. As a nerve, it is usually described as formed by the union of two distinct nerves, namely, the entire masticator nerve and the large bundle of sensory fibers derived from the semilunar (Gasserian) ganglion which pass peripherally as the third division of the trigeminus. These two nerves remain separate until they pass through the foramen ovale and then unite immediately outside the skull to form a large trunk which almost directly after its formation divides into a small anterior and a larger posterior portion. The trunk is situated between the pterygoideus externus, laterally, and the otic ganglion and the tensor palati medially. In front of it is the posterior border of the pterygoideus internus, and behind it, the middle meningeal artery. Two branches arise from the trunk of the nerve before its division, namely, the spinous (recurrent) nerve and the nerve to the pterygoideus internus.
The spinous (recurrent) nerve, after receiving a vasomotor filament from the otic ganglion, enters the cranium through the foramen spinosum, accompanying the middle meningeal artery, and divides into an anterior and a posterior branch. The anterior branch communicates with the meningeal branch of the maxillary division of the trigeminus, furnishes filaments to the dura mater, and ends in the osseous substance of the great wing of the sphenoid. The posterior branch traverses the petrosquamous suture and ends in the Uning membrane of the mastoid ceUs.
The fibers going to form the neriie to the internal pterygoid muscle are almost wholly motor fibers and therefore comprise a branch of the masticator nerve and are described as such under the description of the masticator.
The anterior portion of the mendibular nerve is smaller than the posterior and is chiefly composed of motor fibers which form branches of the masticator nerve and supply the muscles of mastication, the temporalis, masseter, and pteryg- oideus externus. Practically all of the sensory fibers of the anterior portion (fibers of the mandibular nerve proper) form the buccinator (long buccal) nerve. The latter is accompanied, in the first part of its course, by a small strand of motor or masticator fibers which leaves it to end in the anterior part of the temporal muscle.
The buccinator (long buccal) nerve, entirely sensory, passes between the two heads of the external pterygoid muscle and runs do\\Tiward and forward under cover of or through the ante- rior fibers of the temporahs to the cheek. As it passes forward it emerges from under cover of the anterior border of the masseter and lies on the superficial surface of the buccinator, where it interlaces with the buccal branches of the facial nerve and gives off filaments to supply the superjacent skin; finally it pierces the buccinator and supphes the mucous membrane on its inner surface as far forward as the angle of the mouth. The fibers of the anterior deep temporal nerve, a branch of the masticator, are frequently associated with the buccinator until the latter has passed between the heads of the external pterygoid; then the anterior deep temporal nerve separates from the buccinator and passes upward on the lateral surface of the upper head of the external pterygoid.
The posterior portion of the mandibular nerve divides into three large branches. Two of these, the lingual and the auriculo-temporal nerves, are exclusively- sensory; the third, the inferior alveolar (dental) nerve, contains a strand of motor fibers, the mylo-hyoid nerve, which comprise a branch of the masticator nerve.
The lingual nerve is the most anterior branch of the mandibular nerve. It lies in front and to the medial side of the inferior alveolar (dental) nerve and descends at first on the medial side of the pterygoideus externus, then between the pterygoideus internus and the ramus of the mandible to the posterior part of the mylohyoid ridge, where it passes off the anterior border of the ptery- goideus internus; at this point it is situated a short distance behind the last
Distribution of the Mandibular Division op the Trigeminus combined with Branches op the Masticator Nerve. (Henle.)
molar tooth and is covered in front by the mucous membrane of the posterior part of the mouth cavity. After leaving the pterygoideus internus it crosses the fibers of the superior constrictor, which are attached to the mandible, and turns forward toward the tip of the tongue, crossing the lateral surfaces of the stylo- glossus, hyoglossus, and genioglossus. In its com-se across the hyoglossus it lies first above, then to the lateral side of, and finally below Wharton's duct, and as it ascends on the genioglossus it lies on the medial side of the duct.
Communications and branches. - While it is on the medial side of the pterygoideus externus the lingual nerve is joined, at an acute angle, by the chorda tympani, a branch of the glosso-palatine nerve, and as it hes between the ramus of the mandible and the pterygoid- eus internus it is connected by a branch with the inferior alveolar (dental) nerve, and gives off one or two small branches, the rami isthmi faucium, which are ditributed as sensory fibers to the tonsil and the mucous membrane of the posterior part of the mouth.
While it is above the duct it gives a branch, which contains many sensory and visceral motor chorda tympani fibers, to the submaxillary ganghon (.seep. 963), and it receives branches, chiefly sympathetic, from that ganglion. A little further forward it is connected by one or two branches, which run along the anterior border of the hyoglossus, with the hypoglossal nerve. It then gives off the sublingual nerve, which runs forward to supply the subHngual gland and the neighbouring mucous membrane. Its terminal (lingual) branches are derived chiefly from the glosso-palatine nerve. They pierce the muscular substance of the tongue and are distributed to the mucous membrane of its anterior two-thu-ds. They interlace with similar branches of the other side and with branches of the glosso-pharyngeal nerve.
The inferior alveolar (dental) nerve is the largest branch of the posterior portion of the mandibular nerve. It commences on the medial side of the ex- ternal pterygoid muscle and descends to the interval between the spheno-man- dibular ligament and the ramus of the mandible, where it receives one or two communicating branches from the lingual nerve. Opposite the middle of the medial surface of the ramus it enters the mandibular (inferior dental) canal, ac- companied by the inferior alveolar (dental) artery, which lies in front of the nerve, and it runs downward and forward through the ramus and the body of the mandible. At the mental foramen it divides into two parts, one of which, the mental nerve, passes out through the mental foramen, the other, com- monly called the incisive branch, continues forward in the canal, and supplies, through the inferior dental plexus, the inferior canine and incisor teeth and the corresponding regions of the gums.
The branches of the inferior alveolar (dental) nerve are branches forming the inferior dental plexus, and the mental branch. A bundle of motor fibers, the mylohyoid nerve, a branch of the masticator nerve, is given off just before the inferior alevolar nerve enters the mandibular canal.
The inferior dental plexus is formed by a series of branches which communicate with one another within the bone, giving rise to a fine network. From this plexus two sets of branches are given off: - the inferior dental branches, corresponding in number to the roots of the teeth, enter the minute foramina of the apices of the roots and end in the pulp; the second set, the inferior gingival branches, supply the gums.
The mental nerve is a nerve of considerable size which emerges through the mental foramen. It communicates, near its exit from the bone, with branches of the facial nerve, and then divides into three branches. The smallest branch, turning downward, divides into several twigs, the menial branches, which supply the integument of the chin. _ The other two, inferior labial branches, pass upward, diverging as they ascend, and divide into a number of twigs. The stoutest twigs ramify to the mucous membrane which lines the lower hp. Other twigs are distributed to the integument and fascia of the hp and chin.
The auriculo-temporal nerve usually arises from the posterior portion of the mandibular nerve by two roots which embrace the middle meningeal artery and unite behind it to form the trunk of the nerve. The trunk passes backward on the medial aspect of the pterygoideus externus, and between the spheno-man- dibular ligament and the temporo-mandibular articulation, lying in close relation with the capsule of the joint. Behind the joint it enters the upper part of the parotid gland, through which it turns upward and lateralward. It emerges from the upper end of the gland, crosses the root of the zygoma close to the posterior border of the superficial temporal artery, and divides into auricular and temporal terminal branches at the level of the tragus of the pinna.
Each of the two roots of the nerve receives a communication from the otic ganglion containing fibers derived from the glosso-pharyngeal nerve. These fibers have passed from the glosso-pharyngeal through the tympanic plexus and the small superficial petrosal nerve and through the otic ganglion.
(b) Sensory filaments pass from the auriculo-temporal nerve to the temporo-facial branch of the facial nerve.
(c) Filaments of connection with the sympathetic plexus on the internal maxiUary artery.
(d) A communication to the inferior alveolar (dental) nerve.
Branches of the auriculo-temporal nerve.
- An articular branch to the temporo-man- dibular joint, given off as the nerve lies on the medial side of the capsule.
- Branches to the external auditory meatus. Two branches, as a rule, are given off in the parotid gland. They enter the meatus by passing between the cartilage and the bone and supply the upper part of the meatus, the membrana tympani by a fine branch, and occasionally the lower branch gives twigs to the skin of the lobule of the pinna.
- Parotid branches are distributed to the substance of the parotid gland. Sensory or trigeminal fibers for the gland spring either directly from the nerve or from the communicating branches previously given by it to the glosso-palatine nerve. The parotid branches also con- tain filDres derived from the glosso-pharyngeal nerve which pass successively through its tym- panic branch, the tympanic plexus, the small superficial petrosal nerve, the otic ganglion, and the communicating twigs from the otic ganglion to the roots of the auriculo-temporal nerve. The parotid branches are later again mentioned as concerned chiefiy with the ganglialed cephalic plexus.
- The anterior auricular branches, usually two in number, are distributed to the skin of the tragus and the upper and outer part of the pinna.
- The superficial temporal branches supply the integument of the greater part of the tem- poral region, and anastomose with the temporal branch of the facial nerve.
The masticator nerve (motor root or portio minor of trigeminus) . The fibers of the masticator nerve spring from two nuclei, a slender upper or mesencephalic nucleus and a clustered lower or chief nucleus. The fibers arising in the mesen- cephalic nucleus descend along the lateral aspect of the nucleus to the pons as the descending or mesencephalic root;* here they join the fibers from the chief motor nucleus and issue with them from the side of the pons in from six to ten root filaments. These blend to form the nerve, which is from one and a half to two millimetres broad. At the point where it emerges from the pons the nerve is in front of and ventral to the root of the trigeminus and it is separated from the latter by a few of the transverse fibers of the pons which constitute the lingula oj Wrisberg. From its superficial exit from the pons, the masticator nerve passes upward, lateralward, and forward in the posterior fossa of the cranum, and along the medial and anterior aspect of the trigeminus, to the mouth of Meckel's cave. In this cavity it runs lateralward below the semilunar (Gasserian) ganglion to the foramen ovale, through which it passes to join the mandibular division of the trigeminus immediately outside and below the base of the skull. The nerve is purely motor and its fibers are devoted almost wholly to the muscles having to do with mastication.
Schematic Representation of the Masticator Nerve and its Branches (in Black). Lateral view. Modified from Spalteholz. Gasserian ganglion Masticator nerve / External pterygoid nerve
The nuclei of origin of the masticator nerve are connected with the lower part of the somaesthetic area of the cerebral cortex of the opposite side by the p}Tamidal fibers descending in the genu of the internal capsule, and they are associated with the sensory nuclei of other cranial nerves through the reticular formation and by the medial longitudinal fasciculus.
Almost immediately after joining the trunk of the mandibular nerve, most of the fibers of the masticator leave it to form the greater part of the so-called anterior portion of the mandibular. However, one branch of masticator fibers, the nerve to the internal pterygoid muscle, is given off from the mandibular just before its division into anterior and posterior portions. The masticator
* Recent investigations indicate that the mesenephalic root is not wholly motor but at least in part sensory in character, and thus belongs partly to the trigeminal nerve.
branches derived from the anterior portion are the deep tem-poral nerves, the masseteric nerve, and the nerve to the external -pterygoid. One branch, the mylo-hyoid nerve, is carried in the posterior portion of the mandibular and is given off from its inferior alveolar branch.
The nerve to the internal pterygoid passes under cover of a dense layer of fascia derived from an expansion of the ligamentum pterygo-spinosum, and enters the deep surface of the muscle. Near its commencement this nerve furnishes a visceral motor root to the otic ganglion, and small twigs to the tensor tympani and tensor palati.
The deep temporal nerves, usually two in number, posterior and anterior, pass between the bone and the upper border of the external pterygoid muscle, and turn upward around the infra- temporal crest of the sphenoid bone to end in the deep surface of the temporalis. The posterior of the two often arises in common with the masseteric nerve. The anterior is frequently associated with the buccinator (long buccal) nerve till the latter has passed between the two heads of the pterygoideus externus. There is frequently a third branch, the medius, which passes lateralward above the pterygoideus externus, and turns upward close to the bone to enter the deep surface of that muscle. A small strand of masticator fibers accompanies the buccinator nerve to enter and end in the anterior part of the temporal muscle.
The masseteric nerve, which frequently arises in common with the posterior deep temporal nerve, passes between the bone and the pterygoideus externus, and accompanies the masseteric artery through the notch of the mandible to be distributed to the masseter. It is easily traced through the deeper fibers nearly to the anterior border of the masseter. As it emerges above the pterygoideus externus it gives off a twig to the temporo-mandibular articulation.
The nerve to the external pterygoid, after a course of about 3 mm. (an eighth of an inch), divides into twigs which enter the deep surface of the two heads of the muscle. It is usually adherent at its origin to the buccinator nerve.
The mylo-hyoid branch, carried in the posterior portion of the mandibular nerve, is given off immediately before the inferior alveolar (dental) nerve enters the mandibular (inferior dental) canal. It pierces the lower and back part of the spheno-mandibular ligament and runs down- ward and forward in the mylo-hyoid groove between the mandible on the lateral side, and the internal pterygoid muscle and the lateral surface of the submaxillary gland on the medial side. In the anterior part of the digastric triangle it is continued forward between the anterior part of the submaxillary gland and the mylo-hyoideus, and it breaks up into branches which supply the mylo-hyoideus and the anterior belly of the digastric.