188 COMPARATIVE ANATOMY. 



is always well developed, though not to such an extreme degree as 

 in Birds. 



In Keptiles (Lizards, for instance) a tapetum may be developed, 

 but an argentea and choroid gland are never present ; all these 

 structures are wanting in Birds. A structure which is homologous 

 with the processus f'alciformis of the eye of Fishes is, however, 

 present in Reptiles and Birds. Absent in Hatteria and Chelonia, 

 this so-called pecten is largely developed in other Reptiles and 

 in Birds (Fig. 155, P). In the latter it may extend from the point 

 of entrance of the optic nerve to the capsule of the lens, but as a 

 rule it does not reach so far. In Birds it is always more or less 

 folded, and consists mainly of a closely-felted network of capillaries, 

 and appears in both Reptiles and Birds to have an important 

 relation to the nutrition of the contents of the eyeball and of the 

 retina. It has nothing to do with accommodation. 



The iris, which is regulated by striated muscle, by means of which it is 

 able to respond very quickly to visual impressions, is often brightly coloured, 

 and this colour is due to the presence not only of pigment, but also of 

 coloured fat globules. 



The pupil is as a rule rounded, but it may have the form of a vertical slit, as 

 in many Reptiles and in Owls. In certain Fishes (Coregonus) and Amphibians 

 (Bombinator) it is angular. 



Mammals. In Mammals the eyeball is always more 

 completely enclosed within the bony orbit than in most other 

 Vertebrates, and this may partially account for the fact that the 

 sclerotic no longer shows traces of cartilage or bone, but is 

 entirely of a fibrous character. 



With the exception of aquatic Mammals, in which it is some- 

 what flattened, the cornea is moderately convex, and the whole 

 eyeball is of a more or less rounded form. 



A tapetum (tapetum cellulosum vel fibrosum), consisting either of 

 cells or fibres, exists in the choroid of numerous Mammals, and gives rise by 

 interference to a glistening appearance when seen in the dark (Garni vora, 

 Sirenia, Ruminants, Perissodactyla, &c.). 



Certain structures homologous to the processus falciformis and pecten are 

 present in Mammals in the embryo only, but details of these structures cannot 

 be described here. 



The ciliary muscle consists of smooth elements only, and serves to 

 accommodate the eye for seeing near objects (compare the eye of Fishes). The 

 lens of Mammals in its position of rest is accommodated for distance. 



The external surface of the lens is less convex than the internal, which 

 latter lies in the so-called fossa patellaris of the vitreous humour. 



The pupil is not always round, but may be transversely oval (Ungulates, 

 Kangaroos, Cetacea), or have the form of a vertical slit (e.g. Cat). 



