204 THE DOG-FISH 



ophthalmic branches of the fifth and seventh 

 nerves pass from the orbit to the dorsal surface of 

 the olfactory capsules. 



f. The auditory capsules are large lateral projections 

 at the hinder end of the skull. They lodge the 

 auditory organs, certain parts of .which can be 

 recognised without dissection. 



i. Between the two auditory capsules there is a 

 median depression, at the bottom of which are 

 a pair of holes. Each hole leads downwards 

 and outwards into a canal, the aqueductus 

 vestibuli, which opens below into the internal 

 ear. 



Pass a seeker down the aqueductus vestibuli on one side 

 into the ear. 



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

 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 



