THE BRAIN. 397 



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. 



ii. 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 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 

 obvious 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 thalamen- 

 cephalon with each other. Of the three the 

 middle one alone really crosses the cavity of 

 the ventricle. 



a. The anterior commissure is a small band of 

 transverse nerve-fibres in the substance of 

 the anterior wall of the ventricle, and 

 immediately in front of the fornix. 



/?. The middle commissure is a very large band 

 of fibres connecting the two optic thalami 

 across the ventricle. It is nearly circular 

 in outline, and crosses the ventricle about 

 its middle, filling up the greater part of it. 



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