444 ANATOMICAL TECHNOLOGY. 



regarded as extending only from the middle of the chiasma to the diamond-shaped area 

 just caudad of the rhinencephalic Radices mesales. 



The extent of the metencephalon coincides nearly with that of the pyramis. 



1133. Hemisection of the Brain. The encephalic segments are more easily recog- 

 nized and examined upon a hemiencephalon than upon the entire organ because (1) all the 

 segments and most of the coelise are then exposed ; (2) the parts can be more readily bent 

 upon each other or dissected apart. 



Instruments and Materials. A brain, symmetrical and well-hardened ; distorted speci- 

 mens may serve for dissections, but for the hemisection select the most perfect. Razor or 

 large scalpel, with very keen, smooth edge ; if the scalpel is used, it should be especially 

 sharp at the heel, which is usually neglected. A small dish or vial of alcohol, in which 

 the knife may be dipped. A piece of sheet cork, or of soft wood free from knots, at least 

 6x4 cm., and preferably 16x8. Remove from the brain any pins which may cross the 

 meson. 



1134. Upon a preparation or figure (Fig. 115, 116 ; PL II, 

 Fig. 4), note the position of the callosum and the angle which it 

 forms with the dorsal and ventral surfaces of the brain. 



Place the brain upon its basis, with its long axis coincident with 

 the length of the cork or the grain of the wood, and the cephalic 

 end of the brain to the left. 



Dip the scalpel in alcohol, and grasp it bow-fashion. Introduce 

 the tip of the blade between the hemispheres at their highest part, 

 and gently push it ventro-cephalad until the point reaches the cork 

 a little in advance of the Lobi olfactorii, and the back of the blade 

 if a scalpel is used is about on a level with the dorsal margins of 

 the hemispheres. The blade then forms an angle of about 30 

 degrees with the cork, and its heel is a little cephalad of the cere- 

 bellum. 



Grasp the brain gently but firmly as follows: The pollex 

 should be on the left, against the lateral surface of the cerebellum 

 and the caudal part of the hemisphere ; on the right the tips of the 

 index, medius and annularis press respectively upon the lateral 

 aspect of the cerebellum and the temporal and frontal regions of the 

 hemispheres. The minimus may be raised out of the way. The 

 pressure upon the two sides of the brain must be equal and uniform. 



Draw the scalpel slowly caudad, keeping the convexity near the 

 tip constantly upon the cork, and the entire instrument at the same 

 angle until it emerges through the cerebellum and the medulla. 



At the beginning of the hemisection, the edge of the scalpel 

 was firmly held by the closely approximated hemispheres, and 

 its edge rested on the callosum, so that the latter was probably 

 divided accurately on the meson. But a very slight initial deflec- 



