26 GUIDES FOR VERTEBRATE DISSECTION 



inner cerebral artery, which meets its fellow of the opposite side 

 and extends forwards and upwards between the hemispheres. 

 The posterior branch forms a posterior communicating artery 

 which extends back to meet its fellow beneath the medulla and 

 there to form a basilar artery extending back beneath the medulla 

 oblongata. It will thus be seen that the inner cerebrals and the 

 posterior communicating arteries form a closed ring, the circle 

 of Willis. 



Bisect the brain and cut away one half of the cerebrum until 

 the cavity it contains is exposed. Draw the section, noting the 

 varying differences in the thickness of the walls and making out 

 the lateral cavities (ventricles) in the cerebrum; a third ventricle 

 in the thalamencephalon, extending into the infundibulum; and 

 a fourth ventricle in the medulla oblongata, beneath the cerebel- 

 lum. Are optic lobes, cerebellum, or corpora restiformia hollow? 



Make a transverse cut through the snout passing through 

 the nostrils, laying open the olfactory sac. See that this is en- 

 closed in a cartilaginous nasal capsule and that the lining olfactory 

 membrane is thrown into numerous leaves or folds, thus greatly 

 increasing the surface. Draw the section X 3. 



Lay open the spiracle and notice the reduced gill or pseudo- 

 branch. Would this be an anterior or a posterior demibranch? 

 Draw. 



