66 ANATOMY OF A CHELONIAN. 



e. Ventral to the cerebellum, and reaching an- 

 teriorly to the mid-brain, is the medulla oblon- 

 gata or viyeloncephalon, which is seen to be 

 continuous posteriorly with the spinal cord or 

 myelon. 



f. The medulla and the ventral parts of the mid- 

 brain are white ; the other portions of the 

 brain now exposed are gray. 



-171. Carefully raise the cerebral hemisphere so as to 

 expose beneath its posterior portion the thalamencepha- 

 lon, and under this note the large, white optic tract 

 running forward from the mid-brain to join its fellow in 

 the optic chiasma ; from the latter, trace the right optic 

 nerve forward to the point where it has been divided. It 

 will be seen to turn out into the orbital fossa, just be- 

 hind the inter-orbital septum ; and, as soon as it enters 

 the fossa, to cross out beneath the ophthalmic nerve, 

 which runs between it and the superior rectus muscle 

 near the origin of the latter. 



172. About the point where cerebellum, medulla ob- 

 longata, and mid-brain meet, the posterior end of the 

 ^u\2X\. fourth nerve will be seen ; trace it forward. Next, 

 gently raise the mid-brain from the base of the skull, 

 and note the third cranial nerve (motor oculi) running from 

 it to the orbit, which it enters somewhat posterior and 

 external to the optic trunk. 



173. On the small right hand portion of the skull 

 (167) will be seen : 



a. A much larger portion of the nasal chamber 

 than that which was found on the left section. 



