THE CEEEBEAL HEMISPHEBES 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 paUium, 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 i"^ 



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 dorsalh', 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. prsesylvia) 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 rhinen- 

 cephalon (Sulcus rhinalis). 



4. The marginal fissure (F. marginalis) extends along the dorso-medial border. 

 It 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 fissxire (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 



1 The homologies and terminology of many fissures of the brain in the domesticated animals 

 ai-e still in a chaotic state. Only a few of the most important and constant ones are given here. 



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



50' 



