HEMISECTION OF THE BRAIN. 445 



tion may have brought the blade out 1-2 mm. laterad of the ventri- 

 meson. 



Ascertain the extent of the departure by apposing the two halves 

 of the brain and noting the position of the cut with respect to the 

 ventrimeson. If the cut and the meson coincide, the surfaces of the 

 two sides of the brain will be practically identical ; but according 

 to the degree of the separation there will be found a difference 

 which is sometimes so great as to be puzzling to the beginner, 

 especially since in these cases neither surface resembles that shown 

 in the figure. 



After becoming somewhat familiar with the organ, it may be 

 well to make purposely a hemisection about 1 mm. laterad of the 

 meson, so that the commissures and other mesal structures may be 

 dissected out in relief ; but at the outset it is better to secure a 

 view of the mesal surface itself. 



Select that half of the brain to which part of the other half is 

 attached, and hold it with the meson up and the base toward you ; 

 grasp the large scalpel bow-fashion, and apply the heel at the ven- 

 trimeson, upon the chiasma if it be the right half, or at the ventri- 

 mesal (" anterior median ") fissure of the metencephalon if tfce left ; 

 cut away from you with a long steady sweep, taking care not to cut 

 upon the other side of the meson. Certain parts of the surface may 

 require subsequent special treatment in order that the surfaces may 

 be fairly exposed. 



1135. Aside from detailed comparison of the surface with the 

 figures, the best test of the mesal surface is the exposure of the 

 series of mesal cceliae, extending, as in the Amphibian brain (Fig. 

 Ill), in uninterrupted continuity from the ventro-caudal angle of 

 the cerebellum to a point dorsad of the chiasma. Just ventrad of 

 the cerebellum the lateral extent of the cavity, epicoelia, is con- 

 siderable, but at other points it is no more than 1-2 mm., so as to 

 appear on the mesal surface like a shallow depression. The most 

 obscure portion of the cavity is the cephalic part of the epiccelia, 

 where the lateral extent is considerable, while its vertical diameter 

 is very slight, and the roof, valvula, is quite thin. 



Fig. 117. Mesal aspect of the left hemiencephalon ; a diagram enlarged from PI. II, 

 Fig. 4. 



Explanation. Upon this diagram are named nearly all the parts and surfaces which 

 are exposed by a mesal hemisection. 



To avoid turning the volume in referring to it, the figure has been divided. Unfor- 



