THE SKULL. 229 



Pass a seeker down the aquceductus vestibuli on one side 

 into the ear. 



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

 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 canal 



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 following structures within the orbit (see 

 Jig. 47). ' 



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. 



