THE CEREBRAL HEMISPHERES 791 
the posterior angles of the body. Each curves outward and backward over the 
thalamus (from which it is separated by the tela chorioidea), and is chiefly continued 
as the fimbria along the concave border of the hippocampus. The crura give off 
fibers to form the alveus, a layer of white matter which covers the ventricular face 
of the hippocampi, and between them are transverse fibers which constitute the 
hippocampal commissure (Commissura hippocampi). 
The septum pellucidum is the median partition between the two lateral ven- 
tricles. Its convex dorsal border blends with the corpus callosum and its concave 
ventral border joins the fornix. Its anterior part is received into the genu of the 
corpus callosum. Traced backward, it diminishes in height and the two edges meet 
at an acute angle at the splenium. 
The septum consists of two layers (Laminz septi pellucidi) which are in contact with each 
other. They consist of medullated nerve-fibers and gray matter. The latter exists in considerable 
amount in the thicker part of the septum adjacent to the columns of the fornix. Many of the 
fibers of the septum pass up through the corpus callosum to the gyrus fornicatus. Others are 
connected with the subcallosal gyrus and the parolfactory area. 
Fra. 644.—Saarrtat SecTION oF BRAIN oF Horse. 
Section is cut about 1.5 em. to the right of the median plane. M, Medulla oblongata; P, pons; P.c. (above M), 
chorioid plexus of fourth ventricle; C.m., central white matter (corpus medullare) of cerebellum and of cerebrum; 
P.c. (in front of P), cerebral peduncle; H, hippocampus; V, lateral ventricle; T, thalamus; N.c., caudate nucleus; 
C.i., internal capsule; N.l., lenticular nucleus; B.o., olfactory bulb. 
The lateral ventricle (Ventriculus lateralis cerebri) is the irregular cavity in the 
interior of each cerebral hemisphere.! Each communicates with the third ventricle 
through the interventricular foramen (Foramen interventriculare),? and by a small 
canal with the cavity of the olfactory bulb. It is usual to describe the ventricle 
as consisting of three parts, viz., the central part or body, and anterior and ventral 
horns. The central part (Pars centralis) extends back to the splenium of the corpus 
callosum. It opens into the third ventricle through the interventricular foramen, 
which is situated between the fornix and the anterior part of the thalamus. The 
anterior horn (Cornu orale) is the part in front of the interventricular foramen; 
it communicates ventrally with the olfactory cavity. The ventral horn (Cornu 
ventrale) curves downward and forward into the piriform lobe. The roof of the 
ventricle is formed by the corpus callosum, and the medial wall is the septum pellu- 
cidum. After removal of the roof the floor is seen to be formed chiefly by two bodies. 
1Tt is hardly possible to get an accurate idea of the shape of the ventricle except by studying 
a cast of it (Fig. 641). The-size of the ventricles varies in different subjects, and it is common to 
find more or less disparity between the two ventricles of the same brain. 
* Formerly termed the foramen of Monro. 
