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 (ea. 3 em.) by a white commis- 
sural mass, the corpus callosum; 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 (Iacies 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- 
Fic. 638.—Lerr Cereprart HemispHere or Horse; Laterat View. THe Otractrory Burs is Cut Orr. 
1, Lateral fissure (of Sylvius); 2, 3, 4, middle, posterior, and anterior branches of 7; 5, presylvian fissure; 6, 6’, suleus 
rhinalis, anterior et posterior; 7, suprasylvian fissure; 8, 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 (Iissura lateralis) ,! and lodges 
the middle cerebral artery. In front of the fossa there is a considerable rounded 
elevation known as the trigonum olfactorium. 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 optie 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 (lig. 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. 
