DISSECTION OF THE BRAIN, OR ENCEPHALON. 245 



campal convolution — which terminates behind the fissure of Sylvius 

 in a nipple-like eminence — the unci7iat€ convohdion (mastoid lobule, or 

 mammillary eminence). These will presently be more particularly 

 described. 



Directions. — The student nuist now reverse the position of the brain, 

 laying it with its base downwards, while he proceeds to examine its 

 upper aspect. 



The Great Longitudinal Fissure is now seen in its entirety. It 

 extends from the anterior to the posterior end of the cerebrum, and 

 appears to completely separate the right and left hemispheres. In the 

 natural state the fissure is occupied by the falx cerebri. Gently 

 separate the contiguous margins of the hemispheres, so as to widen out 

 the fissure. Except towards the hinder end of the fissure, this proceed- 

 ing requires no dissection, but at that point the hemispheres are united 

 on the middle. It is, however, a mere adhesion through the medium of 

 pia mater. Separate the hemispheres here by traction, or by cutting 

 carefully in the mesial plane. There will now be exposed (Fig. 31) a 

 white body — the corpus callostim — which connects the hemispheres at 

 the bottom of the great longitudinal fissure. At the same time there 

 will be brought into view the opposed inner s\irfaces of the liemispheres. 

 The Cerebral Convolutions. In his examination of the base of the 

 brain, and more clearly now, the student will have observed that the 

 siu'face of the hemisphere is not smooth, but traversed by numeroiis 

 winding worm-like elevations. These are termed the cerebral convohi- 

 tions or gyri; and the intermediate grooves or fissures are technically 

 termed sulci. 



At first sight it might be supposed, as indeed was believed until a 

 comparatively recent date, that the disposition of these convolutions is 

 quite iiTCgular and hap-hazard. Observation has shown, however, that 

 such is far from being the case, and that the convolutions have a nearly, 

 if not altogether constant, arrangement. In the human subject, indeed, 

 the surface of the hemisphere has been accurately mapped, and each 

 convolution named. In the brain of the horse the plan of these convo- 

 lutions appears to be as uniform as in man ; and althoiigh, perhaps, the 

 convolutions are not absolutely identical in any two brains, or even in 

 the two hemispheres of the same brain, still the irregularities are so 

 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 vitility from the discovery that 

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

 when these areas are destroyed or injm-ed there follows total loss or dis- 

 turbance of these functions, and that in some cases the exercise of par- 

 ticular functions can be brought about by applying stimuli to particular 

 spots of the cerebral cortex. 



