246 THE ANATOMY OF THE HORSE. 



In the brain of man the hemisphere is primarily subdivided into five lobes, viz. , frontal, 

 parietal, occipital, temporo-sphenoidal, and central, the last being also known as the 

 Island of Eeil, or the Insula. The lines of separation between these lobes are certain 

 well-marked fissures, distinguished from the sulci in general by their greater depth and 

 constancy. In each lobe, again, the secondary sulci form the lines of separation between 

 a definite number of convolutions. 



In tho third edition of Professor Chauveau's admirable work {TraiU d'Anatomie com- 

 parSe des Animaiix domest-iques) an attempt is made to describe the cerebral convolu- 

 tions of the horse after the plan followed in human anatomy, and to establish an almost 

 complete correspondence of these parts in the two brains. It appears to me, after very 

 careful consideration, that except in a few points, an identity between convolutions 

 in tlie two brains is not clearly indicated on anatomical grounds alone. That most of 

 the convolutions of the human brain have corresponding convolutions in the brain of the 

 horse is more than probable ; and experimental, pathological, or developmental evidence 

 may yet place this correspondence beyond doubt. In the meantime, however, and pro- 

 visionally, I think it preferable to describe the cerebral convolutions of the horse accord- 

 ing to what appears the most natural plan. 



The surface of each hemisphere (excluding from present consideration its inner aspect) 

 is divided into three lobes or areas, viz., an anterior lobe, a postero-superior lobe, and a 

 postero-inferior lobe. This subdivision is effected by certain fissures (Plate 34), as 

 follows : — 



1. The Crucial Fissure. This is a short fissure which begins near the middle of each 

 hemisphere where it margins the great longitudinal fissure. Passing outwards, it joins 

 the great oblique fissure. The crucial fissure separates the anterior from the postero- 

 superior lobe. In the right hemisphere of Plate 34 these lobes are connected across the 

 fissiu'e by a small annectent or bridging convolution. 



2. The G-EBAT Oblique Fissure. This is the most iwonounoed fissure of the hemi- 

 sijhere. Beginning near the middle of the upper surface of the hemisphere, where it is 

 continuous with the crucial fissiu^e, it is directed obliquely outwards, downwards, and 

 backwards, to reach the tentorial aspect of the hemisphere (Plate 33). It separates the 

 postero-superior from the postero-inferior lobe. 



3. The Lateral Fissure. This fissure begins on the upper surface of the hemisphere, 

 at the point of junction of the crucial and great oblique fissiu-es. It curves round the 

 side of the hemisphere, with a slightly forward inclination ; and it separates the anterior 

 from the x^ostero-inferior lobe. 



4. The Fissure oy Sylvius.* This begins at the base of the brain (Plate 33), at the 

 side of the optic commissure. It passes outwards as a faint and ill-defined depression in 

 front of the uncinate convolution ; and crossing the outer olfactory tract, it divides into 

 four branches, which, however, are mere siilci. One of these is directed backwards between 

 the outer olfactory tract and the postero-inferior lobe ; another passes forwards between 

 the inner olfactory tract and the anterior lobe ; a third ascends into the postero-inferior 

 lobe ; and the fourth is directed forwards into the anterior lobe. In the first part of its 

 course the fourth branch separates the adjacent convolutions of the anterior and postero- 

 inferior lobes, being itself separated by a bridging convolution between these lobes from 

 the lower extremity of the lateral fissure. 



To the outer side of the outer olfactory tract, at the point from which these branches 

 of the Sylvian fissure radiate, there is a minute nodular convolution that is partially or 

 entirely concealed from view until the adjacent convolutions are slightly separated. 

 This seems to foreshadow the convolutions of the insula of man. 



The Anterior Lobe presents four convolutions : — 



1. The First Anterior Convolution (Plate 33, 1. A) is seen on the under surface of the 

 lobe. It lodges the olfactory peduncle in the olfactory fissure ; and when the peduncle 

 is in position it shows an inner and an outer part, the former occupying the position of 

 the gyrm rectus of human anatomy. 



2. The Second Anterior Convolution begins on the under surface of the lobe ( Plate 33, 



* The first part of the fissure of Sylvhis, as far as the outer olfactory tract, is sometimes and more 

 oorrectly called the valle^j of Sylvius, and Broca restricts the term fiisiirc of Sylvius to the third of 

 the above-described branches. 



