THE DOGFISH 15 



Exercise 14. Draw a diagram showing the position of the muscles 

 of the eyeball as they appear in the orbit. 



Exercise 15. Draw a diagram showing the structure of the eye. 



The Ear. An internal ear alone is present in fishes, there being 

 no outer ear and no ear opening. The internal ear is a mem- 

 branous labyrinth, which is inclosed in a cartilaginous capsule 

 situated immediately back of the posterior margin of the orbit on 

 each side. It is made up of a spherical sac, the vestibule, from 

 the dorsal surface of which spring three semicircular canals,— an 

 anterior vertical, a posterior vertical, and a horizontal canal. Each 

 of these canals has, at one end, a small swelling called an ampulla. 

 From the dorsal surface of the vestibule there also arises a deli- 

 cate tube, the endolymphatic duct, that passes through the roof 

 of the skull to the outer skin, which is here pierced by a small 

 hole. The ear comes into existence in the embryo as an infold- 

 ing of the outer wall of the head, and this duct is a remnant of 

 this infolding. 



The membranous labyrinth is extremely delicate and is difficult 

 to dissect. With care and patience, however, this may be done ; 

 the semicircular canals, at least, should be found. Remove the 

 skin and muscles completely from the top of the head back of the 

 eye on the right side. A ridge will be seen in the roof of the skull 

 extending backward from the orbit. Within this ridge and imme- 

 diately beneath its cartilaginous wall is the anterior vertical semi- 

 circular canal. Very carefully shave off the roof of the ridge and 

 find it ; it will be seen to have the diameter of a fine needle. Just 

 back of it is the posterior vertical canal. Lying between the hinder 

 end of this canal and the orbit in the cartilage is the horizontal 

 canal. Note the ampulla of each of these canals. Beneath them 

 is the vestibule. 



In the dogfish and many other fishes the ear is an organ of 

 equilibrium and has only a slight auditory function. 



Exercise 16. Draw the membranous labyrinth so far as observed. 



The Central Nervous System. Remove the skin and muscles 

 from the dorsal surface of the head. Very carefully shave off 



