260 THE DOG-FISH 



The oKactory organ is a sac, the walls of which are thrown 

 into a large number of transverse folds attached in pairs to 

 a median septum. Each fold is covered by a soft mucous 

 membrane, the surface of which is increased by secondary 

 foldings. The olfactory nerves enter the organ from the 

 dorsal surface, and spread out in the folds. 



The great size of the olfactory organ, and of the nerves it 

 receives, is very characteristic of Elasmobranch fishes. 



B. The Eye. 



Bemove one of the eyes by cutting across the eye-muscles 

 and optic nerve. Sn/ip off the remains of the muscles, and 

 divide the eye into an inner and an outer half by cutting all 

 round it equatorially. Place the two halves under water in a 

 dissecting dish. 



, The eye is hollow ; its walls consist of three concentric 

 layers, and within its cavity, near the outer side, is the 

 spherical lens, about the size of a pea. 



1. The selerotic, or outermost coat of the eye, is thin 



and cartilaginous. It invests the whole of the eye 

 except the part covered by the cornea. 



2. The cornea is an oval patch on the outer surface of the 



eye, through which the light enters. It is about half 

 an inch long and a quarter of an inch broad. It 

 consists of transparent connective tissue, and is con- 

 tinuous at its edges with the sclerotic. 



3. The choroid is a black opaque membrane, lying within 



the sclerotic. It adheres closely to the sclerotic, and 

 is firmly attached to the line of union of sclerotic and 

 cornea. Beyond this line it is continued into the 

 iris, which Hes free behind the cornea, and is per- 

 forated in its centre by an elongated oval slit, the 

 pupil, through which alone light can get to the in- 

 terior of the eye. 



The inner surface of the choroid, especially in 



the inner half of the eye, is covered by a glistening 

 silvery membrane, the tapetum. 



