THE EYE. 461 



run upwards and backwards, converging to 

 form a long thread-like tendon, which, passing 

 through the tubular sheath of the quadratus, 

 runs down on the inner surface of the eye- 

 ball, and then under it, to be inserted into 

 the ventral border of the nictitating mem- 

 brane. 



By the combined action of these two 

 muscles the rapid movement of the nictitating 

 membrane is effected. 



3. The structure of the eye. 



Place the eye under water, and divide it by an equatorial 

 incision into two halves. 



The general structure of the eye is the same as 

 that of other vertebrates ; but the following points 

 are of special interest. 



a. The pecten is a vascular pigmented fold, which pro- 



jects into the cavity of the eye from its orbital 

 surface, ventral to the entrance of the optic nerve. 

 It is folded longitudinally like a fan ; and should 

 be compared with the falciform process of the 

 dog-fish (p. 288). 



b. The lens is, as in terrestrial animals generally, 



much flatter than that of the dog-fish. 



c. The sclerotic plates are a ring of small flat bones 



overlapping each other, and surrounding the 

 eye, just behind the junction of the cornea and 

 the sclerotic. 



B. The Ear. 

 1. The tympano-Eustachian passage. 



This is the modified hyo-mandibu'ar cleft of the 

 embryo, corresponding to the spiracle of the dog-fish. 

 Its connection with the ear is of a purely secondary 

 character. It serves to keep the air in the tympanic 

 cavity at the same pressure as outside. 



II 



