THE SKULL 205 



traversed by numerous nerves ; the rest of the 

 space being occupied by the large orbital blood- 

 sinus.' 

 Identify the following structures within the orbit. 



i. The orbito-nasal foramen is a small round hole 

 at the anterior inferior angle of the orbit, 

 through which the veins of the anterior part of 

 the head communicate with the orbital sinus. 



ii. The optic foramen is a large hole about the 

 middle of the length of the orbit and near its 

 ventral border. Through it the optic nerve 

 enters the orbit. 



iii. The foramen for the third nerve is a small hole 

 in the inner wall of the orbit, about a quarter 

 of an inch behind the optic foramen and a 

 little above it. 



iv. The foramen for the fourth nerve is a very 

 small hole near the dorsal border of the inner 

 wall of the orbit, vertically above or very 

 slightly behind the optic foramen. 



v. The foramen for the fifth and seventh nerves is 

 a large hole at the posterior and ventral angle 

 of the orbit, immediately in front of the audi- 

 tory capsule. Through it the main branches 

 of the fifth and seventh nerves, and also the 

 sixth nerve, enter the orbit. 



vi. The aperture "of the interorbital canal is a 

 small round hole just in front of, and a little 

 below the foramen for the fifth and seventh 

 nerves, and below and behind' the foramen 

 for the third nerve. The canal traverses the 

 base of the skull, and places the orbital sinuses 

 of the two sides in communication with each 

 other. 



vii. The foramina for the ophthalmic branches of the 

 fifth and seventh nerves. These nerves enter 

 the orbit through two holes at its hinder end, 

 just in front of the auditory capsule, and 



