THE BRAIN 351 



c, The mid-brain, or mesencephalon. 



i. The corpora quadrigemina, or optic lobes, form 

 the roof 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 to the optic 

 thalami. 



2. Longitudinal section of the brain (fig. 43, p. 432). 

 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 callosum is seen cut across, and its 

 boundaries can easily be determined. 



iL The body of the fornix is a thin median band 

 lying below the corpus callosum. It is con- 

 tinuous with this latter behind, but in front 

 diverges from it considerably, the space 

 between 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 

 obvious by adding a small quantity of some 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 its 

 sides together. Of the three the middle one 

 alone really crosses the cavity of the ventricle. 



