XXX.] THE BRAIN. 321 



band of white fibres, the brachium of the pos- 

 terior quadrigeminum. 



22. Note the anterior cerebral commissure, a 

 small compact bundle of fibres running trans- 

 versely in front of the anterior pillars of the 

 fornix. 



Cut through the anterior pillars of the fornix to 

 expose the commissure. 



23. The posterior cerebral commissure under- 

 neath the pineal gland. 



24. Cut through the anterior and middle commis- 

 sures and trace the cavity of the third ventricle 

 into the infundibulum ; cut through the posterior 

 commissure and the corpora quadrigemina and 

 trace out the aqueduct of Sylvius. 



B. PARTS OF THE BRAIN SEEN IN SECTIONS. 



1. Divide a brain in half by a longitudinal sec- 

 tion carried carefully through the median line. 

 Observe the relative positions of the structures 

 visible on the cut surface, noting 

 The obliquely cut fibres in the decussation of 

 the pyramids ; the transversely cut fibres of the 

 ventral part of the pons; the valve of Vieussens; 

 the corpora quadrigemina; the aqueduct of Syl- 

 vius; the posterior commissure, this will be con- 

 tinuous with transversely cut fibres forming a 

 layer in the anterior corpus quadrigeminum ; the 

 pineal gland, a small piece of its peduncle running 

 from it will be seen; the large middle commissure 

 occupying a considerable portion of the third 

 L. 21 



