668 NERVOUS SYSTEM OF THE HORSE 



by numerous openings for the passage of small blood-vessels and are equivalent 

 to the locus perforatus anticus of man. Behind the outer part of the fossa is the 

 rounded anterior end of the pyTiform lobe. Traced backward the lobe curves up- 

 ward and inward over the optic tract and the thalamus to the tentorial aspect of the 

 hemisphere; its continuation, the hippocampus, forms part of the floor of the 

 lateral ventricle, and will l)e examined later. 



The posterior part or tentorial area is flattened, faces inward and backward as 

 well as downward, and rests largely on the tentorium cerebelli; on its anterior part 

 there is a shallow depression adapted to the corpora ciuadrigemina and the pineal 

 body. 



The frontal pole or anterior extremity (exclusive of the olfactory bulb) is 

 compressed laterally, and the occipital pole or posterior extremity forms a blunt 

 ])oint. 



The hemisphere comprises: (1) the pallium or mantle, which consists of an 

 outer layer of gray matter, the cortex (Substantia corticalis) , covering a large mass 



Fig. 607. — Left Cerebral Hemisphere of Horse, Lateral View. The Olfactory Bulb is Cut Off. 

 /, Lateral fissure (of Sylvius); 2, S, 4, middle, posterior, and anterior branches of 1; 5, presylvian fissure; 

 6, 6,' sulcus rhinalis, anterior et posterior; 7, suprasylvian fissure; 8, eetomarginal fis.sure; 9, 9' , ectosylvian 

 fissure. 



of white matter (Centrum semiovale); (2) the rhinencephalon or olfactory portion 

 of the brain; (3) the corpus callosum and fornix, the great commissural white 

 masses; (4) the lateral ventricle and certain important structures associated there- 

 with. 



The pallium is thrown into numerous folds, the gyri cerebri, which are sep- 

 arated by sulci or fissures of varying depth. The general pattern of the gyri and 

 sulci is similar in normal brains of the same species, but the details are very variable 

 and are never alike on the two hemispheres of the same brain. In the horse the 

 arrangement is complicated by the existence of numerous short accessory fissures 

 which cut into the gyri at right angles and tend to confuse the observer. The 

 principal fissures and sulci of the convex surface (Figs. 497, 507) are as follows: 



1. The lateral fissure (Fissura lateralis Sylvii) ascends on the lateral surface 

 of the hemisphere as the continuation of the fossa transversa in front of the pyriform 

 lobe. After crossing the external olfactory tract it divides into three branches; 

 of these one passes upward, one runs obliquely forward and upward, and the third 

 is directed upward and backward. 



2. The suprasylvian fissure (F. suprasylvia)' is long and divides a large part 

 ' Termed by Lesbre the parietal fis.sure and by M'Fadyean the great oblique fissure. 



