ORGAN OF SIGHT. 



113 



over the eye, and becomes transparent where it enters the 

 orbit ; sometimes it simply passes over the orbit, sometimes 

 it forms a circular fold. The anterior and posterior portions 

 may be especially broad and the seat of an adipose deposit 

 [adipose eyelids), as in Scomber, Caranx, Mugil, etc. In many 

 of these fishes the extent of these eyelids varies with the 

 seasons ; during the spawning season they are so much loaded 

 with fat as nearly to hide the whole eye. Many Sharks pos- 

 sess a nictitating membrane, developed from the lower part of 

 the palpebral fold, and moved by a proper set of muscles. 



The form of the hulbus (Fig. 48) is subhemispherical, the 

 cornea (co) being flat. If it were convex, as in higher Verte- 

 brates, it would be more liable to injury ; but being level with 

 the side of the head the chances of injury by friction are dim- 

 inished. The sclerotica {sc) is cartilaginous in Chondropterygians 

 and Acipensers, fibrous and of varying thickness in Teleosteans, 

 in the majority of which it is supported 

 by a pair of cartilaginous or ossified 

 hemispheroid cups (c). In a few fishes, 

 as in Ceratodus, XijjJiias, the cups are 

 confluent into one cup, which possesses 

 a foramen behind to allow the passage 

 of the optic nerve (o). The cornea of 



Fig. 48. 



AnaUeps shows an unique peculiarity. Vertical section through 



tj.- ji J ^ ^ • X ^ eye of Xiphias. (After 



It IS crossed by a dark horizontal o ) 



stripe of the conjunctiva, dividing it co, Cornea ; sc, sclerotica ; 



into an upper and lower portion; also the ''', ^®^^^ °P ^*;"^ ' ''' 



^^ ^ ' sclerotic capsule ; a, 



iris is perforated by two pupils. This fish 

 is observed to swim frequently with half 

 of its head out of the water, and it is a 

 fact that it can see out of the water as 

 well as in it. 



The membranes situated between 

 the sclerotica and retina are collectively called choroidea, and 



membrana argentea ; 

 Vjmembrana vasculosa ; 

 u, membrana uvea ; ch, 

 choroid gland; r, re- 

 tina ; /, processus fal- 

 ciformis ; h, humor 

 vitreus ; I, lens ; i, 

 iris. 



