70 ANATOMY OF A CHELONIAN. 



head, break away the bone on the roof of the skull, tak- 

 ing care not to injure the auditory capsule, and note 

 from before back, on the dorsal aspect of the brain : 

 . a. The olfactory lobes. 



b. The cerebral hemispheres, or prosencephalic 

 lobes, with the pineal gland lying in the angle 

 where they diverge behind. 



c. The cofpora bigeniina, forming the dorsal part 

 of the mid-brain. 



d. The unpaired cerebellum ; and, behind it — 



e. The medulla oblongata, covered by a highly vas- 

 cular plug of pia 7nater, forming a choroid 

 plexus. On removing this, the fourth cerebral 

 ventricle will be partly exposed on the dorsal 

 aspect of the medulla. Raise the posterior 

 free border of the cerebellum, and note that 

 the fourth ventricle is continued forward be- 

 neath it. 



180. Carefully tilt the brain over toward the right, 

 and study the origin of the various Cranial Nerves on 

 the left side. 



a. The olfactory {first cranial 7ierve), proceeding 

 from the end of the olfactory lobes. 



b. The optic or second cranial nerve (171). 



c. The third nerve {oculo-motor) arising from the 

 ventral aspect [crus cerebri) of the mid-brain. 



d. The fourth nerve (patheticus) will be first found 

 on the dorsal side of the crus cerebri, close 

 to the cerebellum ; thence it passes forward and 

 downward till it reaches the base of the skull, 

 on which it runs forward parallel and exter- 

 nal to the third nerve. The three preceding 



