674 



NERVOUS SYSTEM OF THE HORSE 



give off fibers to form the alveus or white matter which covers the ventricular face 

 of the hippocampi, and between them are transverse fibers which constitute the 

 hippocampal commissure. 



The septum pellucidum is the median partition between the two lateral ven- 

 tricles. Its convex upper border blends Avith the corpus callosum and its concave 

 lower 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 (Laminae septi pellucidi) which are in direct contact with 

 each other. They consist of nerve-fibers and gray matter. The latter exists in consiuerable 

 amount in the thicker part of the septum adjacent to the anterior pillars 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. 



Interventricular foramen 

 Anterior commissure 



Chiasma 

 opticum 



Optic tract 



Fig. 512. — Cro.ss-skction of Brain of Horse, about N.\tural Size. 

 Section passes through chiasma opticum, and is viewed from in front. 1, Longitudinal fissure; S, septum 

 pellucidum; S, columns (anterior pillars) of fornix; 4, lateral ventricle; 5, chorioid plexus; 6, corpus medullare 

 (central white matter) of hemisphere; 7, caudate nucleus; S, lenticular nucleus; 9, internal capsule; 10, external 

 capsule; //, insula. 



The lateral ventricle (V'entriculus lateralis) is the irregular cavity in the 

 interior of each cerebral hemisphere.^ Each communicates with the third ventricle 

 through the interventricular foramen (of Monro), 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 inferior horns. The 

 anterior horn (Cornu nasale) is the part in front of the interventricular foramen; 

 it communicates ventrally with the olfactory cavity. The central part (Pars 

 centralis) extends liack to the splenium of the corpus callosum. It opens into the 

 third v(Mitricle through the interventricular foramen, which is situated between the 

 fornix and the anterior part of the thalamus. The inferior horn (Cornu ventrale) 



' It is hardly p()ssil)l(> to get an accurate idea of the shape of the ventricle except by studying 

 a cast of it. The size of the ventricles varies in (UfT(>rent subjects, and it is not rare to find more 

 or less disparity between the two \-entricles of the same brain. 



