Part 32

The =choroid plexus= of the lateral ventricles (=lamina chorioidea epithelialis=) (Fig. 148, _e_).--The line of junction of the roof of the third ventricle (i.e., the choroid plexus of the third ventricle) and the cerebral hemispheres is along the lateral (or cranial) border of the thalamus (Fig. 141, _t_). Along this line (Fig. 141, 1) the brain-wall remains very thin and becomes closely associated with the pia mater, so that on the removal of the pia there is left the “transverse fissure” which leads into the lateral ventricle. In an early stage, before the cerebrum has grown far caudad, the fissure is exposed in dorsal view, but after the hemispheres have covered the thalami the fissure appears to be in the lower face of the hemisphere. Through this fissure the pia mater extends upward into the lateral ventricles as a fold separated from the ventricle by the thin brain-wall. This fold is vascular and is known as the choroid plexus of the lateral ventricle (Fig. 148, _e_). It is a fringe-like fold lying on the fimbria and extending from the foramen of Monroe (at _h_) nearly to the end of the inferior horn of the lateral ventricle.

The =lateral ventricles= are the extensions into the hemispheres of the originally unpaired cavity of the forebrain. Each consists of a portion parallel to the basis cranii and lying at the side of the septum pellucidum between the corpus callosum dorsad and the corpus striatum, hippocampus, and fornix ventrad, and of two horns, an =anterior= and an =inferior= horn. The inferior horn (Fig. 152, _c_) is a narrow cleft, crescent-shaped in cross-section, which follows the dorsal surface of the hippocampus (Fig. 148, _d_; Fig. 152, _d_) along the temporal lobe to its end almost against the tuber cinereum. It thus passes first caudad and then ventrad and finally mediad. The choroid plexus and hippocampus project into it from its floor. The =anterior= horn extends ventrad and then slightly caudad in the frontal lobe, following the corpus striatum (Fig. 148, _f_), on which it lies.

The =interventricular foramen= (or =foramen of Monroe=).--If the choroid plexus of the lateral ventricle (Fig. 148, _e_) is followed to its medial end, it is found to pass through a foramen (at _h_) in which it becomes continuous with the opposite plexus or roof of the third ventricle. This foramen leads from one lateral ventricle to the other and is connected ventrally by a median opening with the third ventricle. It is thus Y-shaped or T-shaped and is the interventricular foramen (or foramen of Monroe). The foramen lies opposite the middle of the corpus striatum and caudad of the pillars of the fornix.

=Membranes of the Brain.=--The membranes of the brain are the =dura mater=, the =arachnoid=, and the =pia mater=.

The dura mater is a strong fibrous membrane lining the cranial cavity and covering the brain. It is strongly attached to the projections of the base of the skull and to the tentorium. It dips between the cerebral hemispheres and olfactory bulbs, forming the =falx cerebri=. It likewise dips between the cerebrum and the cerebellum, covering both surfaces of the bony tentorium. The dura mater does not pass into the fissures or sulci of the surface of the brain. It becomes continuous with the sheath of the cranial nerves as they leave the skull. It is continuous with the dura mater of the spinal cord.

The arachnoid and pia mater are essentially similar to the same structures on the spinal cord. The pia mater dips into the fissures and sulci of the brain.

II. THE PERIPHERAL NERVOUS SYSTEM.

1. =Cranial Nerves. Nervi cerebrales.=--I. N. OLFACTORIUS.--The fasciculi of the olfactory nerves arise from the olfactory bulb (Fig. 144, _a_) and pass through the foramina of the cribriform plate, upon which the bulb lies, to be distributed to the olfactory mucous membrane of the nose.

II. N. OPTICUS.--The second nerve or optic (Fig. 138, _II_) arises from the optic chiasma (Fig. 138, _c_), passes through the optic foramen, and extends in an S-shaped curve to the eyeball. Its course is craniodorsad. It pierces the sclerotic and choroid coats of the eye and spreads out into the retina.

III. N. OCULOMOTORIUS.--The third or oculomotor nerve arises (Fig. 138, _III_) from the pedunculus cerebri and passes into the orbit through the orbital fissure. It passes between the lateral and superior recti, sends a large branch to the superior rectus, supplies the medial rectus and the retractor oculi, passes laterad of the optic nerve and supplies the inferior rectus, and sends a long branch to the inferior oblique (Fig. 166, _f_). At the point where the branch is given off to the inferior oblique muscle is the triangular reddish =ciliary= (or ophthalmic) =ganglion=, about two millimeters in diameter. It is directly attached to the inferior oblique branch of the third nerve and receives a delicate sensory filament from the ophthalmic nerve. From the ciliary ganglion proceed two =short ciliary nerves= which pass along each side of the optic nerve to the eyeball. These nerves penetrate the sclerotic at the sides of the eyeball to be distributed to the ball.

IV. N. TROCHLEARIS.--The fourth nerve, the trochlearis (or patheticus), arises from the lateral border of the velum medullare anterius, as already described (Fig. 141, _n_). It passes craniad, emerges through the orbital fissure in company with the oculomotor, abducens, and ophthalmic, passes dorsad of the superior rectus muscle, and reaches (Fig. 154, _l_) the caudal border of the belly of the superior oblique muscle near its middle. It supplies only the superior oblique.

V. N. TRIGEMINUS.--The fifth nerve, the trigeminus (or trifacial) arises (Fig. 138, _V_) by two roots, a large sensory and a small motor root (4), as already described (p. 347). One of these, the motor (4), is smaller and more ventral; the other, the sensory root, is larger and dorsal. The dorsal root soon enlarges to form a large ganglion, the =semilunar= (or Gasserian) =ganglion= (Fig. 138, _k_), from which three branches diverge. One branch is joined by the ventral root (4), which passes over the ventral surface of the semilunar ganglion; and the nerve thus formed is the =mandibular= division (1) of the fifth nerve. Of the other two branches from the ganglion, the middle and longest is the =maxillary= nerve (2), and the smallest is the =ophthalmic= (3). The mandibular nerve is thus mixed, motor and sensory, while the others are sensory.

1. =N. ophthalmicus.=--The ophthalmic or first division of the fifth nerve arises from the semilunar (or Gasserian) ganglion. It passes out of the cranial cavity and into the orbit by way of the orbital fissure, in company with the third, fourth, and sixth nerves and with the extension of the carotid (arterial) plexus.

It passes between the superior and medial recti along with the third nerve, crosses dorsad of the optic nerve, and divides into =infratrochlear= and =ethmoidal= branches. In the orbital fissure it gives off the =frontal= nerve, and while crossing the optic it gives off the =long ciliary= nerve.

_a._ =N. frontalis.=--The frontal nerve passes along the lateral border of the superior oblique muscle and then laterad of the pulley to near the middle of the supraorbital crest of the frontal bone. Here it passes out of the orbit and is distributed to the integument of the upper eyelid and the adjacent region at the side of the nose.

_b._ =N. infratrochlearis.=--The infratrochlear nerve passes between the superior rectus and the superior oblique in the first part of its course. It then passes ventrad of the superior oblique and ventrad of the pulley to be distributed to the integument of the upper eyelid near the inner angle.

_c._ =N. ethmoidalis.=--The ethmoidal nerve passes along with the ethmoidal artery through the ethmoidal foramen (or foramina) in the orbital plate of the frontal bone. It is finally distributed to the mucosa of the nose and to the cartilage and integument of the snout.

_d._ =N. ciliaris longus.=--The long ciliary nerve arises from the ophthalmic and passes along the optic nerve to be distributed to the eyeball. It divides into several branches before penetrating the sclerotic.

One or two small communicating branches to the =ciliary ganglion= are given off at about the same point as the long ciliary nerve. (For a description of this ganglion see the account of the oculomotor nerve, page 369.)

2. =N. maxillaris.=--The maxillary nerve, the second division of N. trigeminus, rises from the semilunar (Gasserian) ganglion and leaves the skull by the foramen rotundum. It is the sensory nerve of the palate, upper teeth and upper lip, and of part of the forehead and cheek.

On leaving the foramen the maxillary nerve divides into three branches, the two =infraorbital= nerves (Fig. 154, _f_) and the =sphenopalatine= (Fig. 154, _g_). Two smaller branches are likewise given off either within or just outside of the foramen, the =lachrymal= nerve (Fig. 154, _j_) and the =zygomatic= (subcutaneus malæ) (Fig. 154, _i_).

_a._ =N. lachrymalis= (Fig. 154, _j_; Fig. 155, _m_).--The lachrymal nerve passes along the periorbita to the lachrymal gland (Fig. 154, 11), to which it gives branches. It then continues caudad of the zygomatic process of the temporal to the integument; here it turns caudad and is distributed to the integument in the region between the eye and the external ear (Fig. 155, _m_). It anastomoses with the zygomatic branch of the seventh nerve (Fig. 155, _h_).

_b._ =N. zygomaticus= (subcutaneus malæ) (Fig. 154, _i_).--This arises with the preceding and follows it for some distance. It passes through a foramen in the frontal process of the malar bone and is distributed to the lower eyelid and adjacent integument.

_c._ =Nn. infraorbitales= (Fig. 154, _f_; Fig. 155, _l_).--The infraorbital nerves are two of nearly equal size. They pass through the orbit ventrad of the eyeball to the infraorbital canal. On their course each divides once or twice and each divides again in the infraorbital foramen, so that about eight branches emerge from the infraorbital foramen and diverge to the integument and whiskers of the upper lip and to the side and wing of the nose (Fig. 155, _l_). In the infraorbital canal, and before reaching it, branches are given to the molar teeth, and a branch continues in the bone to the canine, incisor, and premolar teeth.

_d._ =N. sphenopalatinus= (Fig. 154, _g_).--The sphenopalatine nerve turns mediad from the infraorbitals, directing its course toward the sphenopalatine foramen. Before reaching this it gives off the =greater palatine= nerve (=N. palatinus major=), which enters the posterior palatine canal and passes to the hard palate. The sphenopalatine then usually divides into two branches which pass along side by side to enter the =sphenopalatine ganglion=. This is a large elongated triangular ganglion lying on the dorsal surface of the external pterygoid muscle, just laterad of the sphenopalatine foramen.

The following nerves are connected with the sphenopalatine ganglion:

_a._ =N. palatinus minor.=--This leaves the craniolateral angle of the ganglion and passes to the soft palate.

_b._ =N. nasalis posterior.=--The posterior nasal nerve enters the nasal cavity by the sphenopalatine foramen and is distributed to the mucosa of the ventral and middle parts of the nasal cavity.

_c._ =N. canalis pterygoidii= (Vidian Nerve).--This is a large nerve which leaves the caudal angle of the sphenopalatine ganglion and passes caudad. It enters the orbital fissure, lying in a groove on its ventral wall (the cranial end of this groove is sometimes converted into a canal). The groove ends caudally in a foramen which pierces the sphenoid bone between the wing and the body and lies just mediad of the foramen rotundum. The groove and foramen constitute the pterygoid canal (from which the nerve is named). After emerging from the pterygoid canal onto the ventral surface of the basisphenoid the nerve enters the tympanic bulla along with the Eustachian tube, lying on the medial side of the latter. Just after entering, on reaching the internal carotid artery, it divides into two. One of these, =N. petrosus superficialis major= (p. 375), passes into the hiatus facialis of the petrous bone and joins the facial nerve. The other, =N. petrosus profundus=, accompanies the internal carotid artery caudad, turning therefore out of the bulla and passing caudad along its medial side; it finally joins the superior cervical ganglion of the sympathetic system.

3. =N. mandibularis.=--The third division of the fifth, the mandibular nerve, takes origin by a strong root from the semilunar (or Gasserian) ganglion, and receives also after separating from the ganglion the smaller ventral root (portio minor) of the fifth nerve. The root from the ganglion is sensory; the smaller root is motor, so that the mandibular nerve is both sensory and motor. The nerve passes through the foramen ovale, and sends off at once the following branches:

_a._ =N. auriculotemporalis= (Figs. 154 and 155, _n_).--This passes dorsad between the cartilaginous auditory meatus and the zygomatic process of the temporal bone (Fig. 154, _n_), emerges at the caudal border of the masseter muscle (Fig. 155, _n_), and divides into two chief branches. One, the =auricular= branch, passes along the cranial side of the external ear and is distributed to its integument. The other, the =temporal= branch, passes along the zygomatic arch, gives branches to the temporal muscle and to the skin, follows the zygomatic arch almost to the angle of the mouth, and anastomoses with the superior buccal branch of the facial (Fig. 155, _k_).

_b._ =N. temporalis profundus.=--One or more large branches passing mediad of the zygomatic arch to the temporal muscle.

_c._ =N. massetericus.=--The masseteric nerve passes dorsocraniad to the masseter muscle.

_d._ =N. pterygoideus.=--One or more pterygoid branches pass to the pterygoid muscles. A small twig from the nerve to the internal pterygoid passes into the tympanic cavity and supplies the tensor tympani muscle.

_e._ =N. buccinatorius= (Fig. 154, _e_).--This passes craniad along the dorsolateral surface of the pterygoid muscles (Fig. 154, 8) to the angle of the mouth. Here it divides into branches to the masseter muscle, the mucosa of the mouth, and to the lips.

[Illustration: FIG. 154.--DISSECTION SHOWING A NUMBER OF THE CRANIAL NERVES.

The mandible and zygomatic arch have been cut and removed; the temporal and internal pterygoid muscles are also removed. _a_, N. hypoglossus; _b_, N. lingualis; _c_, N. chorda tympani; _d_, N. alveolaris inferior (cut); _e_, N. buccinatorius; _f_, Nn. infraorbitales; _g_, N. sphenopalatinus; _h_, branch of N. oculomotorius; _i_, N. zygomaticus; _j_, N. lachrymalis; _k_, N. abducens; _l_, N. trochlearis; _m_, N. frontalis; _n_, N. auriculotemporalis. 1, cut end of zygomatic process of temporal; 2, tympanic bulla; 3, M. digastricus; 4, M. constrictor pharyngis medius; 5, M. hyoglossus; 6, M. genioglossus; 7, M. geniohyoideus; 8, M. pterygoideus externus (cut); 9, cut cranial end of zygomatic arch; 10, mandible (cut); 11, lachrymal gland.]

After giving off the above branches the mandibular nerve passes three or four millimeters laterad along the root of the zygomatic process of the temporal bone, and divides into two large branches, the =inferior alveolar= (or dental) nerve (_d_) and the =lingual= nerve (_b_).

_f._ =N. alveolaris inferior= (Fig. 154, _d_).--This passes toward the mandibular foramen, but before reaching it gives off a branch to the mylohyoid and digastric muscles. It then enters the foramen, passes through the mandibular canal, and gives numerous =inferior dental= branches to the teeth. At the mental foramen (or foramina) it passes out of the canal and divides into several =mental= nerves, to the chin, lower lip, and mucosa of the lower jaw.

_g._ =N. lingualis= (Fig. 154, _b_).--The lingual nerve passes between the internal and external pterygoid muscles, and two or three millimeters from its origin it receives the =chorda tympani= nerve (_c_), a communicating branch from the facial (see page 376). It passes onto the ventral surface of the external pterygoid, thence onto the side of the tongue, beneath the mylohyoid. On the side of the tongue it divides into branches which enter the tongue and are distributed to its mucous membrane. One branch passes to the mucosa of the pharynx. Just before entering the tongue a branch is given off to the sublingual and submaxillary glands.

VI. N. ABDUCENS.--The sixth nerve, the abducens, arises from the medulla, as already described (Fig. 138, _VI_, and page 347). It passes into the orbit through the orbital fissure, then extends obliquely cranioventrad along the medial surface of the lateral rectus muscle (Fig. 154, _k_). At about the middle of the length of the muscle it divides into two or three branches which enter at once into the lateral rectus and supply it.

VII. N. FACIALIS (Fig. 155).--The seventh or facial nerve arises from the trapezium at the caudal margin of the pons, craniad of the origin of the eighth nerve, as described in the account of the brain (Fig. 138, _VII_). It passes into the internal auditory meatus, traverses the facial canal through the petrous bone, and emerges at the stylomastoid foramen.

Within the facial canal the nerve bears an enlargement, the =ganglion geniculi=. It gives off within the canal a branch to the stapedius muscle, and the =superficial petrosal= and =chorda tympani= nerves.

The =superficial petrosal= (N. petrosus superficialis major) nerve passes into the hiatus facialis, through the canal which forms its continuation, and joins the sphenopalatine ganglion (p. 373).

[Illustration: FIG. 155.--SUPERFICIAL NERVES OF THE FACE.

_a_, N. facialis; _b_, branch to digastric; _c_, N. auricularis posterior; _d_, branch to inside of ear; _e_, ventral ramus; _f_, dorsal ramus; _g_, temporal nerve; _h_, zygomatic nerve; _i_, nerve to stylohyoid muscle; _j_, inferior buccal nerve; _k_, superior buccal nerve; _l_, infraorbital branches of fifth nerve; _m_, lachrymal branches of fifth nerve; _n_, auriculotemporal branches of fifth nerve. 1, M. digastricus; 2, M. stylohyoideus; 3, M. masseter.]

The =chorda tympani= is given off two or three millimeters before the emergence of the facial at the stylomastoid foramen. It passes into the tympanic cavity, extends across it between the malleus and incus, passing close against the tensor tympani muscle, and leaves it (by a small cleft, the canal of Huguier) between the bulla and the squamous portion of the temporal. It then passes craniad (Fig. 154, _c_), ventrad of the root of the zygomatic process of the temporal, and joins the lingual nerve (Fig. 154, _b_) (branch of the third division of the fifth nerve).

On emerging from the stylomastoid foramen the facial nerve (Fig. 155, _a_) gives off at once a small branch (_b_) to the digastric muscle (1), and a larger branch, =N. auricularis posterior= (_c_), which passes dorsocaudad about the base of the ear, supplying some of its muscles. Another small branch (_d_) pierces the cartilaginous external ear and is distributed to its inner surface. The facial nerve then curves craniad about the proximal part of the cartilaginous external ear, and divides five or six millimeters from its emergence into two main branches, which may be designated as the =dorsal= (_f_) and =ventral= (_e_) rami. A third small branch may also rise from the point of union of the two; it is usually, however, a twig from the dorsal ramus, and passes to M. submentalis.

The dorsal ramus (_f_) sends two or three small branches to the cheek (including the one just mentioned), and divides into the =temporal= (_g_) and the =zygomatic= (_h_) branches. The former (_g_) passes along the cranial margin of the external ear, supplying the superficial muscles, and extends into the temporal region, where it lies deeper than the terminal branches of the lachrymal nerve (_m_) (from the fifth cranial). The =zygomatic= branch (_h_) passes across the malar bone to the caudal angle of the eye, sends branches into both eyelids, anastomoses with twigs from the lachrymal branch (_m_) of the fifth nerve, and passes along the medial side of the eye to the lateral surface of the nose, where it ramifies.

The ventral ramus (_e_) gives off a minute branch (_i_) to the stylohyoid muscle (2), then proceeds toward the angle of the mouth and divides into =superior= (_k_) and =inferior= (_j_) buccal branches, the former to the muscles of the upper lip and the contiguous regions, the latter to those of the lower lip and chin.

The seventh nerve thus supplies most of the muscles of the head except those of mastication, and of these it supplies the digastric.

VIII. N. ACUSTICUS.--The eighth or auditory nerve takes origin (Fig. 138, _VIII_) from the floor of the fourth ventricle, as before described (page 347). It passes into the internal auditory meatus and is distributed to the internal ear within the petrous bone.

IX. N. GLOSSOPHARYNGEUS (Fig. 156, _a_).--The ninth or glossopharyngeal nerve takes origin (Fig. 138, _IX_) from the side of the medulla, as described in the account of the brain. Close to its origin its trunk bears a small enlargement, the =ganglion superius=. It passes along with the tenth and eleventh nerves through the jugular foramen. At its exit it has a ganglionic enlargement, the =ganglion petrosum=; this is connected by fine fibres with the ganglion nodosum (Fig. 156, _d_) of the vagus. The glossopharyngeal (Fig. 156, _a_) then passes craniad over the surface of the tympanic bulla (15) and mediad of the digastric muscle. It continues mediad of the carotid artery, and as it approaches the cranial cornu of the hyoid divides into two portions, one of which passes to the muscles and mucosa of the pharynx, and the other to the tongue, where it is the special nerve of taste.

X. N. VAGUS.--The vagus nerve arises from the side of the medulla in the manner described under the Brain (page 346 and Fig. 138, _X_). It passes through the jugular foramen along with the glossopharyngeal and accessory nerves.

=Cervical Portion of the Vagus= (Fig. 156, _d_, _d′_).--In the foramen, or just before entering it, it presents a ganglionic enlargement, the =ganglion jugulare= (or “ganglion of the root”), and a short distance beyond the foramen it forms a second ganglion, the =ganglion nodosum= (_d_) (or “ganglion of the trunk”), which lies dorsocaudad of the superior cervical sympathetic ganglion (_e_). The ganglia of the vagus and sympathetic are closely bound together by connective tissue, and that portion of the vagus craniad of the ganglion nodosum is interconnected by a network of nervous fibres with the sympathetic (_e_), hypoglossal (_b_), and accessory (_c_) nerves. From the ganglion nodosum the vagus (_d′_) passes caudad, closely bound up with the sympathetic; the two lie at the side of the common carotid artery. Just before entering the thorax the two separate, the vagus being the larger and lying ventrad of the sympathetic. (In rare cases the two are distinctly separated throughout their length.)

[Illustration: FIG. 156.--GLOSSOPHARYNGEAL, HYPOGLOSSAL, VAGUS, SYMPATHETIC, AND FIRST CERVICAL NERVES IN THE NECK.

_a_, N. glossopharyngeus; _b_, N. hypoglossus; _c_, N. accessorius; _d_, ganglion nodosum of vagus nerve; _d′_, N. vagus; _e_, ganglion cervicale superius of sympathetic; _f_, ramus descendens of N. hypoglossus (united with first cervical); _g_, branch to M. thyrohyoid from ramus descendens of N. hypoglossus; _h_, N. laryngeus superior of N. vagus; _i_, united vagus and sympathetic nerves; _j_, first cervical nerve; _k_, second cervical nerve; _l_, N. pharyngeus from vagus. 1, M. masseter; 2, outline of external ear; 3, M. splenius; 4, M. levator scapulæ ventralis; 5, M. longus capitis; 6, trachea; 7, M. sternothyreoideus; 8, M. sternohyoideus; 9, M. thyreohyoideus; 10, M. constrictor pharyngis inferior; 11, M. constrictor pharyngis medius; 12, cut end of M. stylohyoideus; 13, M. hyoglossus; 14, M. stylopharyngeus; 15, bulla tympani; 16, M. jugulohyoideus; 17, M. styloglossus; 18, M. mylohyoideus.]

Branches of the vagus in the cervical region (Fig. 156).

_a._ =N. auricularis.=--This leaves the ganglion jugulare, passes into the petrous bone to the facial canal, leaves the skull with the facial nerve by the stylomastoid foramen, and is distributed to the external ear.