THE CEREBRAL HEMISPHERES 785 
The frontal pole or anterior extremity (exclusive of the olfactory bulb) is 
compressed laterally, and the occipital pole or posterior extremity forms a blunt 
point. 
The hemisphere comprises: (1) The pallium, which consists of an outer layer 
of gray matter, the cortex (Substantia corticalis), covering a large mass 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 therewith. 
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. 631, 632, 638) are as follows: 
1. The lateral fissure (Fissura lateralis Sylvii) ascends on the lateral surface 
of the hemisphere as the continuation of the fossa lateralis in front of the piriform 
lobe. After crossing the lateral olfactory stria it divides into three branches; 
of these one passes dorsally, one runs obliquely forward and dorsally, and the third 
is directed dorsally and backward. It contains the middle cerebral artery. 
2. The suprasylvian fissure (F. suprasylvia)? is long and divides a large part 
of the convex surface of the hemisphere into dorsal and lateral portions. It begins 
on the dorso-medial border near its anterior end, and, inclining gradually downward, 
passes back to end on reaching the tentorial surface. It is usually continuous 
medially with the transverse fissure and in front with the presylvian fissure. 
3. The presylvian fissure (F. preesylvia) is on the anterior part of the hemi- 
sphere, passes forward and ventro-laterally almost to the frontal pole, and then 
inclines backward to end at the groove which marks the dorsal limit of the rhimen- 
cephalon (Sulcus rhinalis). 
4. The marginal fissure (F. marginalis) extends along the dorso-medial border. 
Tt begins a little in front of the middle of the border and turns around the occipital 
pole to end on its tentorial aspect. 
5. The entomarginal fissure (F. entomarginalis) lies medial to the dorso- 
medial border. It does not extend quite as far forward as the marginal fissure, 
from which it is separated by a narrow gyrus. 
6. The ectomarginal fissure (F. ectomarginalis) lies about midway between the 
marginal fissure and the posterior part of the suprasylvian fissure. 
7. The sulcus rhinalis is a very distinct furrow on the ventral part of the 
lateral surface which marks off the olfactory part of the brain (rhinencephalon) from 
the rest of the hemisphere. It is undulating and is highest where it is crossed by 
the lateral fissure. 
Just above this point is a lobe which is homologous with the insula of man. When the 
overhanging gyri which partly conceal it—forming the operculum—are removed, there are dis- 
closed several short, deeply placed gyri (Gyri breves). 
On the medial surface (Fig. 639) the main fissures and sulci are as follows: 
1. The calloso-marginal fissure (F. callosomarginalis) is extensive and well 
defined. It is approximately parallel to the dorso-medial border of the hemisphere, 
from which it is about half an inch distant. It begins in front a short distance 
below and in front of the genu of the corpus callosum, and forms a C-shaped curve, 
its posterior part extending on the tentorial surface to a point behind the depression 
* The homologies and terminology of many fissures of the brain in the domesticated animals 
are still in a chaotic state. Only a few of the most important and constant ones are given here. 
2 Boned by Lesbre the parietal fissure and by M’Fadyean the great oblique fissure. 
