252 THE ANATOMY OF 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 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 Reil, 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 the third edition of Professor Chauveau's admirable work (Traite d' Anatoviie 

 comparee dts Animaux domcstiques) an attempt is made to describe the cerebral con- 

 volutions 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 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 provisionally, I think it preferable to describe the cerebral 

 convolutions of the horse according 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 fissure by a small annectent or bridging convolution. 



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

 sphere. Beginning near the middle of the upper 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 Sylvius. 1 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 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- 



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

 times and more correctly called the valley of Sylvius, and Broca restricts the term 

 fissure of Sylvius to the third of the above-described branches. 



