THE BEAIN. 391 



the dorsal portion of the mid-brain, and are 

 now well seen. 



ii. The crura cerebri form the base of the mid- 

 brain : after removal of the optic tracts they 

 can be traced forwards and outwards from 

 the anterior border of the pons Varolii to the 

 optic thalami. 



2. Longitudinal section of the brain. 



Divide the brain into right and left halves by a median 

 longitudinal section. Place the left half in a small dish of 

 spirit, and examine its inner surface. 



If the dissection has been properly performed, the left half 

 of the brain will be uninjured. If it has been damaged, a 

 second brain should be taken, and bisected as directed above. 



a. The cerebral hemisphere. 



i. The corpus eallosum is seen cut across, and its 

 boundaries can easily be determined. 



ii. The body of the fornix is a thin median band 

 lying below the corpus eallosum. It is con- 

 tinuous with this latter behind, but in front 

 diverges from it considerably, the space be- 

 tween the two being the fifth ventricle. 



b. The fore-brain, or thalamencephalon. 



i. The third ventricle, though very narrow from 



side to side, is of considerable depth from 



the dorsal to the ventral surface. Owing to 



its narrowness, its boundaries are difficult 



to determine. 



The outline of the third ventricle can be rendered more 



obviotis by adding a little ink or other colouring matter to the 



spirit in which the brain is examined. 



ii. The commissures of the third ventricle are 

 three bands of transverse fibres, connecting the 

 right and left halves of the thalamencephalon 

 with each other. Of the three the middle 



