252 THE ANATOMY OP THE HORSE. 



slight as to permit one to describe with considerable minuteness what 

 might be termed a common plan. The mapping of the surface of the 

 hemispheres derives its chief interest and utility from the discovery that 

 definite areas are associated with particular functions, in such a way 

 that when these areas are destroyed or injured there follows total loss 

 or disturbance of these functions, and that in some cases the exercise 

 of particular functions can be brought about by applying stimuli to 

 particular spots of the cerebral cortex. 



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

 parietal, occipital, temporo-sphencidal, 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 l)y their greater depth and 

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

 a definite number of convolutions. 



In the third edition of Professor Chauveau's admirable work {I'raite d' Anatomic com- 

 ■paree des Aniinaux domestiqiies) 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 ijoints, an identity between convolutions hi 

 the 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 wliat 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 i^ostero-superior lobe, and a 

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

 follows : — 



1. The Crul'Ial 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- 

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

 fissure by a small annectcnt or bridging convolution. 



2. The Gkeat Oblique Fissure. This is the most pronounced fissure of the hemi- 

 sphere. Beginning near the middle of the upi^er surface of the hemisphere, where it is 

 continuous with the crucial fissure, 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 fissures. It curves round the 

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

 from the postero-inferior lobe. 



4. The Fissure of Hvlvius.i This beguis 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 con\olution ; and, crossing the outer olfactory tract, it divides into 

 four branches, which, however, are mere sulci. One of these is directed backwards be- 

 tween 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- 



I The first part of the fissure of Sylvius, as far as the outer olfactory tract, is some- 

 times and more correctly called the vaUcy of Siilrius, and Broca restricts the ierm fissure 

 of Sitlviiis to the third of the above-described branches. 



