Part 79

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

The dorsal portion of the hemispheres has been dissected away, then sliced off, showing the plate of transverse fibres forming the corpus callosum. a, splenium; b, genu; c, line marking the medial edge of the hemispheres; d, line marking the lateral boundary of the supracallosal sulcus; laterad of this line the corpus callosum lies in the substance of the hemispheres, which have been dissected away; e, line marking medial limit of cut surface.]

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 dorsal portion of the hemispheres has been dissected away and the corpus callosum removed. a, fornix; b, columns or pillars of the fornix; c, crura of the fornix; d, hippocampus; e, choroid plexus of the lateral ventricles overlying the fimbria (the choroid plexus shows an artery); f, corpus striatum; g, corpora quadrigemina; h, position of the interventricular foramen (foramen of Monroe).]

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.

FIG. 150.--CAUDAL SURFACE OF TRANSVERSE SECTION OF BRAIN THROUGH THE OPTIC CHIASMA AND THE REGION OF THE INTERVENTRICULAR FORAMEN (FORAMEN OF MONROE).

Fig. 149.--Gray matter dotted; white matter with a few lines. 1, right hemisphere; 2, left hemisphere. a, corpus callosum, section through genu and rostrum; b, cavity of lateral ventricles.

Fig. 150.--1, right hemisphere; 2, left hemisphere. a, corpus callosum; b, fornix (cut); c, cavity of lateral ventricles; d, fimbria (d′, cut surface; d, caudal uncut surface of the part that curves ventrad); e, pillars of fornix; f, anterior commissure; g, parts of third ventricle; h, optic chiasma; i, parts of corpus striatum.]

a, corpus callosum; b, fornix (continuous at the sides with the hippocampus); c, cavity of lateral ventricles; d, hippocampus; e, choroid plexus of the third ventricle (roof of the diencephalon); f, dorsal part of cavity of the third ventricle; g, section of thalamus; h, massa intermedia of the thalami, dividing the third ventricle into a dorsal (f) and a ventral (i) portion; i, ventral part of third ventricle; j, part of optic tracts; k, thin wall of infundibulum, with part of cavity of third ventricle; l, line separating roof of thalamus (diencephalon) from floor of hemispheres (telencephalon).]

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.

FIG. 153.--CAUDAL SURFACE OF TRANSVERSE SECTION OF THE BRAIN THROUGH THE CRANIAL PAIR OF CORPORA QUADRIGEMINA.

Fig. 152.--a, corpus callosum; b, fornix; c, inferior horn of lateral ventricles; d, hippocampus; e, thalamus; f, third ventricle; g, pineal body; h, space between the floor of the hemisphere (hippocampus) and roof of the ’tween-brain (thalamus).

Fig. 153.--a, cranial corpora quadrigemina; b, hippocampus; c, part of inferior horn of lateral ventricle; d, aqueductus cerebri (aqueduct of Sylvius); e, space between the outer surface of the midbrain and the lower surface of the hemisphere; f, cross-section of midbrain; g, pedunculi cerebri.]

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.