Part 33

=Internal Structures of the Cerebrum.=--The cavity of each of the cerebral hemispheres is known as a =lateral ventricle=. The two lateral ventricles constitute the _first_ and _second_ of the ventricles of the brain, whence the application of the names _third_ and _fourth_ ventricles to the cavities of the ’tween-brain and hindbrain. The lateral ventricles do not grow at the same rate as the walls of the hemispheres, so that they remain comparatively small. The cavity of each ventricle is further reduced in size by the development on its floor of a large ridge-like thickening, the =corpus striatum= (Fig. 148, _f_).

The dorsal wall or roof of the lateral ventricle joins the thin roof of the third ventricle on each side along an oblique curved line (Fig. 141, 1) which follows the cranial or lateral border of the thalamus (Fig. 141, _t_). Along this line the thin roof of the brain is folded in together with the pia mater to form the =choroid plexus= of the =lateral ventricles= (=lamina chorioidea epithelialis=) (Fig. 148, _e_). When this is pulled out there is left a fissure, the “great transverse fissure of the cerebrum” (Fig. 141, 1). Just dorsad of the groove between the thalami there runs a tract of white fibres known as the =fornix= (Fig. 148, _a_). The two halves of the fornix separate at the cranial ends of the thalami and pass ventrad, forming thus the =pillars= of the =fornix= (Fig. 148, _b_; Fig. 143, _v_). Dorsad of the cranial end of the fornix the =corpus callosum= (Fig. 143, _p_) passes from one hemisphere to the other. Caudad of the pillars of the fornix, the lateral ventricles communicate with the third ventricle by way of the =interventricular foramen= (foramen of Monroe).

The parts of the cerebrum may now be taken up in detail.

The =corpus callosum= (Fig. 147; Fig. 143, _p_; Figs. 149-152, _a_) is a broad transverse band of fibres forming a secondary connection between the medial walls of the two hemispheres, dorsad of the roof of the third ventricle. Its outer surface (Fig. 147) is exposed at the bottom of the fissure which separates the hemispheres. On each side it passes laterad, forming the roof of the lateral ventricle. Its cranial part lies dorsad of the corpus striatum, and its caudal part dorsad of the thalamus. Laterally its fibres radiate into the substance of the hemispheres. At its cranial end the corpus callosum bends ventrad and then caudad (Fig. 143). The part which turns to pass ventrad is the =genu= (Fig. 143, _q_) or knee, while the part which projects caudad is the =rostrum= (_r_). The caudal border of the corpus callosum is also thickened and turned ventrad and is called the =splenium= (Fig. 143, _s_); it lies dorsad of the cranial corpora quadrigemina (_z_). The caudal half of the ventral surface of the corpus callosum is united with the fornix (Fig. 143, _u_).

The =fornix= (Fig. 148, _a_; Fig. 143, _u_, _v_; Figs. 150-152, _b_) consists of an arched tract of longitudinal fibres near the medial border of each hemisphere, ventrad of the corpus callosum. Each tract begins in the mammillary bodies, and passes dorsad, the two converging until they run side by side, forming a cylindrical dorsoventral bundle known as the columns or pillars of the fornix (Fig. 143, _v_; Fig. 150, _e_), which cross the anterior commissure (Fig. 143, _c_; Fig. 150, _f_) caudad of the latter. Caudad of the pillars of the fornix, between these and the thalamus, lies on each side the interventricular foramen or foramen of Monroe, a small opening which connects the lateral ventricles with the third ventricle. Dorsad of the foramen the fornix turns caudad, the two fibre-tracts of each hemisphere lying side by side and closely connected (Fig. 148, _a_), forming thus another secondary union between the medial surfaces of the two hemispheres. This portion of the fornix is the =corpus= or =body= (Fig. 148, _a_); it lies dorsad of the roof of the third ventricle and passes to the splenium (Fig. 143, _s_) of the corpus callosum, and its dorsal surface unites with the ventral surface of the latter (Fig. 143). Caudad the two halves of the fornix diverge, forming the =crura= of the fornix (Fig. 148, _c_); these and the body are continuous laterally with the =hippocampus= (Fig. 148, _d_) and the =fimbria= (Fig. 148, beneath _e_).

The =anterior commissure= (Fig. 143, _c_; Fig. 150, _f_) is a transverse band of white fibres which stretches from one hemisphere to the other about half way between the interventricular foramen or foramen of Monroe and the floor of the third ventricle, and just craniad of the pillars of the fornix. This tract of fibres is developed in the original wall of the third ventricle, so that it does not form a _secondary_ connection between the halves of the cerebrum, as do the fornix and corpus callosum. It lies dorsad of the lamina terminalis (Fig. 143, _d_) and is continuous with it.

The =septum pellucidum= (Fig. 143, _t_) is a vertical partition which separates the lateral ventricles and fills the interval between the corpus callosum dorsad and the fornix ventrad. It is triangular and translucent. It is formed from the medial walls of the two hemispheres and therefore is made up of two laminæ which embrace between them a space which originally was a part of the fissure separating the hemispheres. This space has been called the fifth ventricle.

The =hippocampus= (Fig. 148, _d_; Fig. 152, _d_) is an elongated rounded elevation of the floor of the lateral ventricle. It is continuous mediad with the fornix (Fig. 148, _a_) and extends thence along the inferior horn of the lateral ventricle to its end in the temporal lobe. It is somewhat narrower at its lateral end. It is thus curved into a semicircle in conformity with the inferior horn of the ventricle. Its dorsal surface is convex and looks into the lateral ventricle; its ventral surface is concave and rests upon the thalamus and the optic tract. Along the craniolateral edge of the hippocampus is a broad, clearly marked fibre-tract, the =fimbria=; this runs parallel to the choroid plexus of the lateral ventricle and beneath it, and is continuous at its medial end with the fornix.

The =corpus striatum= (Fig. 148, _f_; Fig. 141, _y_; Fig. 150, _i_) is a fusiform or ovoid elevation of the floor of the lateral ventricle. One of its narrow ends lies about opposite the middle of the thalamus (Fig. 141, _t_) and it extends thence obliquely mediad nearly parallel with the thalamus. In the groove between it and the thalamus is the choroid plexus of the lateral ventricles (Fig. 148, _e_). Internally the corpus striatum is made up of several layers of different texture (Fig. 150, _i_), whence its name.

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_).