72 ANATOMY OF A CHELONIAN. 



exit nearer the median line than any of the 

 above, so that the brain must be turned farther 

 over in order to see them. The sixth arises 

 from the ventral aspect of the medulla near its 

 anterior end, and, after a short course, enters a 

 canal in the basisphenoid bone ; farther for- 

 ward it reappears in the cranial cavity beneath 

 the anterior clinoid process and passes on to 

 the region where it has been already described 

 (174). The twelfth nerve arises from the ventral 

 aspect of the medulla near its posterior end, 

 and passes out through one of the condyloid 

 foramina. 



181. Remove the brain (dividing the nerves close to 

 their origins with sharp scissors) and place it aside in 

 alcohol for future examination ; then trace out the 

 branches proceeding from the left Gasserian ganglion. 

 On cleaning away the dura mater, which covers the 

 inner surface of the descending plate of the parietal 

 bone and of the alisphenoid, the ophthalmic branch of the 

 fifth nerve will be found. Trace it forward to the orbit ; 

 it enters this a little external and posterior to the optic 

 nerve, which will be recognized passing in beneath the 

 superior rectus muscle. In the orbit the ophthalmic 

 crosses under the muscle and appears again on its 

 inner side, and here has the fourth nerve lying close 

 along it. 



182. Follow the fourth nerve forward till it enters 

 the superior oblique muscle, and then trace out the origin 

 and insertion of the latter ; it arises from the frontal 

 bone immediately in front of the membranous inter- 



