DISSECTION OF THE BRAIN, OR ENCEPHALON 251 



will be seen to be connected to the frontal lobe of the hemisphere by 

 a lamina termed by Broca the superior olfactory root. This lamina 

 is grey on its surface and white beneath, and if it be ruptured the 

 cavity of the bulb and the communication between that cavity and 

 the anterior cornu of the lateral ventricle will be brought into view. 



External to the outer olfactory tract, each hemisphere shows 

 numerous winding worm-like ridges, termed convolutions, and internal 

 to the posterior half of the same tract there is seen a thick ridge 

 — the hippocampal convolution — which terminates behind the fissure 

 of Sylvius in a nipple-like eminence — the uncinate convolution (mastoid 

 lobule, or mammillary eminence). These will presently be more 

 particularly described. 



Directions. — The student must 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 proceeding 

 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. 34) a 

 white body — the corpus callosum — 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 surfaces of the hemispheres. 



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

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

 surface of the hemisphere is not smooth, but traversed by numerous 

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

 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 irregular 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 although, 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 



