OKGAN OF SIGHT IN MAMMALIA. 



247 



light. The choroid, c, is lined by the expansion of the optic nerve 

 called ' retina/ which extends to the ' ciliary processes,' and is 

 kept outstretched by the f vitreous humour ' contained in the 

 cells of the delicate membrane called ' hyaloid,' which restrains 

 its forward advance beyond the ( crystalline humour ' or lens, f. 

 The space in front of this body is occupied by the ' aqueous 

 humour,' and is divided by the iris into an f anterior ' and ' pos- 

 terior chamber.' 



The rays of light admitted by the cornea and pupil are 

 slightly refracted in traversing the aqueous humour, and are sub- 

 ject to a greater degree of convergence in passing through the 



194 



Diagram of course of luminous rays iu traversing the humours of the eye. 



denser lens, fig. 1 94 ; when, striking the retina at the back of 

 the globe, they there depict the image of the visual object, in- 

 verted. 



In crepuscular and nocturnal Mammals (Pteromys, Aye-aye, 

 Lemur) the cornea gains in size and convexity and the iris in 

 breadth ; the latter being capable of admitting many rays through 

 a very wide pupil, which also it can completely close against the 

 glare of noontide. The convexity of the lens is concomitantly 

 increased, and it approaches the spherical form most nearly, in 

 bats and nocturnal rodents. The vitreous humour is less in pro- 

 portion to the crystalline and aqueous humours in such eyes. In 

 aquatic Mammals, on the contrary, the cornea hardly projects 

 (seals, whales), and there is little aqueous humour ; here, also, the 

 convexity of the lens is in excess, fig. 195, d. In most diurnal 

 and terrestrial mammals, the eyeball is subspherical, the cornea 

 slightly projecting at the fore part, as forming part of a smaller 

 sphere than the rest of the globe. The lens retains much of the 

 proportions shown in fig. 194. 



