696 



MAMMALIA. 



and concealed by the hemispheres. Nevertheless the " tubercnla 

 quadrigemina (Jig. 322, rf, d) occupy the same relative position 

 as in the Tortoise, (vide Jig. 262, 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. 



(806.) The two optic nerves before passing to their final des- 

 tination 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. 



(807.) Minutely to describe the Fi^.322. 



construction of the eye-ball in the 

 Mammalia would be quite super- 

 fluous, seeing that in every essen- 

 tial particular it exactly corresponds 

 with that of Man. The disposi- 

 tion of the sclerotic and choroid 

 coats, the structure of the cornea, 

 the arrangement of the humours 

 and of the retina, the organization 

 of the iris 9 in short, the whole 

 economy of the eye is the same 

 throughout the entire class. Ne- 

 vertheless, there are a few points 

 of secondary importance deserving 

 our attention, whereby the organ 

 is adapted to peculiarities of cir- 

 cumstance in which different tribes are placed. 



In the Cetacea 9 and also in the amphibious Garni vora that 

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

 rical as in Fishes ; and the antero-posterior diameter of the eye 

 is in consequence considerably diminished by the extraordinary 

 thickness of the sclerotic at the posterior aspect of the eye-ball, 

 an arrangement approaching very nearly to that already described 

 (560). 



(808.) Instead of the dark brown paint which lines the choroid 

 of the human eye, in many Mammals the Ruyschian tunic secretes 

 a pigmentum of various brilliant hues, that shines with metallic 

 splendour. This membrane, called the " tapetum" partially lines 

 the bottom of the eye-ball, but its use has not as yet been satis- 

 factorily pointed out. 



(809.) The shape of the pupil likewise varies in different 



