784 NERVOUS SYSTEM OF THE HORSE 



which the broader end is posterior, and the greatest transverse diameter is a little 

 behind the middle. The two hemispheres are separated by a deep median cleft, 

 the longitudinal fissure of the cerebrum (Fissura longitudinalis cerebri), which is 

 occupied by a sickle-shaped fold of dura mater, the falx cerebri. In front the 

 separation is complete, and it appears to be behind also, but here the two hemi- 

 spheres are attached to each other over a small area by the pia mater. When the 

 hemispheres are gently drawn apart, it is seen that the fissure is interrupted in its 

 middle part at a depth of a little more than an inch (ca. 3 cm.) by a white commis- 

 s\xral mass, the corpus callosiun ; this connects the hemispheres for about half of 

 their length. The transverse fissure (Fissura transversa cerebri) separates the 

 hemispheres from the cerebellum, and contains the tentorium cerebelli. 



The convex or dorso-lateral surface (Fades convexa cerebri) conforms closely 

 to the cranial wall. The medial surface (Facies medialis cerebri) (Fig. 639) is flat 

 and sagittal and bounds the longitudinal fissure; to a large extent it is in contact with 

 the falx cerebri, but behind the great cerebral vein the two hemispheres are in 

 contact and are attached to each other over a small area as noted above. In well- 



FiG. 63S. — Left Cerebral Hemisphere of Horse; Lateral Vip:w. The Olfactory Bulb is Cut Off. 



1, Lateral fissure (of Sylvius) ; 2, 3, 4, middle, posterior, and anterior branches of t; 6, presylvian fissure; 6, 6', sulcus 



rhinalis, anterior et posterior; 7, suprasylvian fissure; S, ectomarginal fissure; 9, 9', ectosylvian fissure. 



hardened specimens there is usually an impression for the vein in front of the area 

 of adhesion. The base or ventral surface (Basis cerebri) (Fig. 629) is irregular. 

 Its anterior two-thirds is adapted to the cerebral fossa of the cranial floor. Cross- 

 ing this area in front of the optic tract is a depression, the lateral fossa (Fossa 

 lateralis), which leads outward to the lateral fissure (Fissura lateralis),^ and lodges 

 the middle cerebral artery. In front of the fossa there is a considerable rounded 

 elevation known as the trigontun olfactoriiim. The trigonum and the medial part 

 of the fossa are pierced by numerous openings for the passage of small blood-vessels 

 and are equivalent to the substantia perforata anterior of man. Behind the lateral 

 part of the fossa is the rounded anterior end of the piriform lobe (Lobus piriformis). 

 Traced backward, the lobe curves dorso-medially over the optic tract and the 

 thalamus to the tentorial aspect of the hemisphere; its continuation, the hippo- 

 campus, forms part of the floor of the lateral ventricle, and will be examined later. 

 The tentorial surface (Fig. 640) is flattened, faces medially and backward as well 

 as ventrally, and rests largely on the tentorium cerebelli; on its anterior part there 

 is a shallow depression adapted to the corpora quadrigemina and the pineal body. 

 1 Also commonly known as the fissure of Sylvius. 



