378 



THE EYE IN EVOLUTION 



Crocodile 



populated mainly by rods, in which all the visual elements are slender 

 and more closely packed than elsewhere ; a fovea is absent. 



The fundus seen ophthalmoscopically presents a uniform yellow 

 background stippled with brownish pigment and orange dots in the 

 centre of which is the white circular optic disc with its patch of dark 

 moss-like pigment (Plate VIII). The retina is avascular and is nour- 

 ished from the choroid ; in the crocodile a small, flat pigmented glial 

 pad with one or two capillaries represents a rudimentary and function- 

 less conus ; in the alligator the disc is devoid of vessels although 

 there are a few capillaries in the optic nerve (Mann, 1929). The optic 

 nerve is slender and elementary in structure with no septal system. 



Figs. 457 and 458. 

 P 



-The Eye of the Alligator. 



Fig. 457.— (After Bland-Sutton.) 



Fig. 458.— (After Franz.) 



A'^. tendon of nictitans ; OiV, optic nerve ; P, pyramidalis muscle ; i?, retractor 



bull>i muscle. 



THE OCULAR ADNEXA. The lids are said to be peculiar in that, 

 alone among Reptiles, the upper is the more mobile, an observation, 

 however, which has been questioned (Prince, 1956). This lid usually 

 contains a tarsal plate of fibrous tissue ; it is fringed by a tough mem- 

 brane split at the margin into some 20 broad pieces giving the appear- 

 ance of a row of exceptionally thick eyelashes which had been glued 

 together and then had their tips cut off. In addition there is a well- 

 developed nictitating membrane so transparent that 'all the details of 

 the iris can be seen through it with ease ; its convex free border 

 is marked by three or four bands of brown pigment and the mem- 

 brane itself is stiffened by a cartilage. It moves obliquely backwards 

 and slightly upwards controlled directly through a long tendon by a 

 pyramidalis muscle corresponding to that in Chelonians (Figs. 457-8). 

 The membrane is often moved across the eye without the eyelids being 

 closed ; and, if the eyes are closed the nictitans is first moved across, 

 not simultaneously with the lids, as occurs in most other Reptiles. 

 Both the harderian and lacrimal glands are well developed as are the 

 conjunctival glands, the latter associated with the movable upper lid ; 



