810 



MAMMALIA. 



Nevertheless the tubercula quadrigemina (fig. 408, d d) occupy the same 

 relative position as in the Tortoise (vide fig. 348, B, c, e), and in like 

 manner still give origin to the nerves appropriated to the instruments of 

 sight, of which they are the proper ganglia. 



(2373.) The two optic nerves, before passing to their final destina- 

 tion, partially decussate each other, as in the human subject ; they then 

 proceed forward into the orbit, and, penetrating the globe of the eye, 

 expand into the retinae. 4 



(2374.) Minutely to describe the construction of the eyeball in the 

 Mammalia would be quite superfluous, seeing that in every essential 

 particular it exactly corresponds with that of Man. The disposition of 

 the sclerotic and choroid coats, the structure of the cornea, the arrange- 

 ment of the humours and of the retina, the organization of the iris in 

 short, the whole economy of the eye is the same throughout the entire 

 class. Nevertheless there are a few points of secondary importance 

 deserving our attention, whereby the organ is adapted to peculiarities of 

 circumstances in which different tribes are placed. 



(2375.) In the Cetacea, and also in the amphibious Carnivora that 

 catch their prey in the water, the shape of the lens is nearly spherical 

 as in Fishes ; and the an tero -posterior dia- 

 meter of the eye is in consequence consider- Fig. 409. 

 ably diminished by the extraordinary thick- 

 ness of the sclerotic at the posterior aspect of 

 the eyeball, an arrangement approaching 

 very nearly to that already described ( 1809). 



(2376.) Instead of the dark-brown paint 

 which lines the choroid of the human eye, in 

 many Mammals the Euyschian tunic secretes 

 a pigmentum, of various brilliant hues, that 

 shines with metallic splendour. This mem- 

 brane, called the " tapetum" partially lines 

 the bottom of the eyeball; but its use has not 

 as yet been satisfactorily pointed out. 



(2377.) The shape of the pupil likewise 

 varies in different quadrupeds : for the most 

 part, indeed, the pupillary aperture is round, 

 as it is in Man ; but in Ruminants, and many 

 other Herbivora, it is transversely oblong. 

 In the Cats (Felidce), that hunt in the gloom, structure of the eye . 



and consequently require every ray of light 



that can be made available, the pupil is a long vertical fissure : but this 

 only obtains among the smaUer genera ; for in those Feline Carnivora 

 that surpass the Ocelot in size, such as the Leopard, the Lion, and the 

 Tiger, the pupil again assumes a round form. 



(2378.) The eyes of Mammalia are lodged in bony orbits, as in the 



