PLATE 7.

Schema of the Ventricular System.

The corpus callosum (c. call) has been divided through the genu, or anterior descending portion, and through the thickened posterior fold, or splenium, exposing in the median line the fornix and its two crura.

In front the interval between the genu of the corpus callosum and the most anterior segment of the fornix, which is also reflected downward to the base of the brain, is filled by two narrow bands (the septum lucidum) containing between them the fifth ventricle (v.s).

The section through the lateral ventricle has been made at a lower level, so that the following structures are exposed: The anterior cornu (corn. a) in the frontal lobe, the posterior cornu (corn. p) in the occipital lobe, and the descending or inferior cornu (corn. i) in the substance of the temporal lobe, which is here assumed to be transparent. Between the diverging occipital lobes is seen the tentorium, which is a process of the dura mater and covers the hemispheres and vermiform process of the cerebellum.

The ascending crura of the fornix arise in the descending cornu at the free border of a thickened fold in that region, the cornu ammonis (c. Am), where they are known as the fimbria; (f. d) fascia dentata; if the crura are divided in front and the entire fornix reflected backward, the descending crura of the fornix (f.d) are seen descending into the substance of the brain. Beneath the fornix a delicate vascular process of the pia mater, the tela choroidea superior, with the two cord-like choroid plexuses, is seen. The choroid plexuses pass immediately behind the descending crura of the fornix and enter the lateral ventricles through the foramina of Monro, and thence pass into the inferior cornua as the plexus choroidei laterales.

Horizontal Section through the Brain after the Third Ventricle Has Been Exposed.

(The section is deeper on the right than on the left.)

After the choroid plexuses of the ascending crura of the fornix (ƒ) have been removed and the cornu ammonis divided, the cavity of the third ventricle, bounded on either side by the optic thalamus, is exposed.

On the left side is the free surface of the caudate nucleus (the caput is in the anterior horn, the tail extends outward toward the posterior horn). The caudate nucleus is separated from the optic thalamus by the narrow stria cornea (str. c). The anterior portion of the optic thalamus is known as the tuberculum anticum (t. a), its posterior projecting extremity is called the pulvinar (Pul).

In front of the descending crura of the fornix (f. d), at a deeper level, is the anterior commissure; behind this, in the middle of the third ventricle, the middle commissure (c. m); and in the posterior extremity of the ventricle, the posterior commissure (p). Immediately above the posterior commissure the tænia thalami (t), which forms the mesial border of the optic thalamus, unites with the corresponding structure of the other side. Immediately behind this is the pineal gland (E) (epiphysis), flanked by the ganglion habenulæ (h). Behind the optic thalamus are the anterior and posterior corpora quadrigemina (qa. qp). Behind the latter is the cerebellum (Cb), which has been divided near the surface, showing the superior vermiform process (v. 8) in the middle and the hemispheres to either side (central white matter and peripheral cortical gray matter). Isa, lsm, Isp, lobus sup. ant., med., post.; cd, corpus dentatum.

On the right side of the plate the caput and tail of the caudate nucleus (n. c) are seen in transverse section. The convex middle portion has been removed. We see also the optic thalamus in cross-section, the uppermost layer having been removed (a, anterior, m, mesial, 7, lateral nucleus of the optic thalamus). Laterally from the optic thalamus the lenticular nucleus (n. 1), which with the caudate nucleus forms the corpus striatum, makes its appearance. The area of white substance to the inner side of the lenticular nucleus is known as the internal capsule (c. i); that on the outer side, as the external capsule (c. e); the internal capsule is divided into three portions: The anterior and posterior limbs (cia and cip), between them the genu (g), and the most posterior segment, the caps. int. retrolenticularis (cirl). Convolutions. (See Plate 6.)

Op fr, Op c, frontal and central operculum; ca, anterior horn, ei, posterior horn of the ventricle; c, tail-piece of caudate nucleus; fli, inferior longitudinal bundle; spl, splenium; s, stria cornea ; tap, tapetum.