THE SKULL. 221 



Pass a. seeker down the aqumditctus vestibuli on one side 

 into the ear. 



ii. From the outer side of the aperture of the aqusB- 

 ductus vestibuli on each side, a ridge runs for- 

 wards and outwards across the skull-roof and 

 towards the orbit. In this lies the anterior 

 vertical semicircular canal of the ear, which 

 can usually be seen through the cartilage. 



iii. A similar ridge, running backwards and out- 

 wards from the same point, lodges the posterior 

 vertical semicircular canal. 



iv. The external or horizontal semicircular *anal 

 can usually be seen through the cartilage, 

 lying to the outer side of the other two canals, 

 and with its convexity directed outwards. 



3. The side of the skull. 



a. The rostrum and olfactory capsule have already been 



seen. 



b. The orbit is the deep oval concavity at the side of the 



skull, extending from the olfactory to the auditory 

 capsule, and bounded above and below by the 

 prominent supra-orbital and sub-orbital ridges. 

 It lodges the eyeball and its muscles, and is 

 traversed by numerous nerves ; the rest of the 

 space being occupied by the large orbital blood- 

 sinus. 

 Identify the folloiuing structures within the orbit. {See fig. 

 45.) 



i. The orbito-nasal foramen is a small roimd 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. 



