PHYSIOLOGY. 



land stales, that in one experiment, a copper cylinder, con- 

 taining atmospheric air, was crushed flat under a pressure 

 of 300 fathoms, and bottles filled with air were crushed in- 

 stantly. This shows the necessity of a strong sclerotic coat 

 in animals that dive far beneath the surface. . 

 Fig. 2. 



Front and side view of the ball of the eye. 



5. The Choraid Coat*-4This constitutes the second in- 

 vesting membrane of the eye, which is of a dark brown 

 colour, soft, cellular, and vascular. This coat is situated 

 on the inner surface of the sclerotic, to which it is slightly 

 attached by means of blood-vessels and nerves, of which in- 

 deed it seems to be chiefly composed J for, on injecting the 

 eye carefully with coloured wax, it assumes a uniform red 

 colour. The inner as well as outer surface of this mem- 

 brane is covered by the pigmentum nigrum, or black paint ; 

 which seems to absorb the rays of light immediately after 

 they have struck the sensible surface of the retina. This 

 pigment, sometimes called tapetum\ in animals that see best 

 at night, is wanting in albinos, as I have already stated ; 

 and it is owing to this cause that the iris and pupil appear 

 of a red colour,, and that their vision is so imperfect, that 

 they cannot view objects in a strong light ; and indeed can 

 scarcely see enough to go about in the day-time. In ani- 

 mals that prowl by night, this pigment is wanting, or of a 



* Choroides, fleecy, like a lamb skin, 

 t Tapetwn, like cloth, or tapestry. 



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