26 MAMMALIA. 



blue ; in the Carnivora and Whales a silvery or mother-of-pearl 

 tint. The form of the pupil varies frequently in one and the same 

 genus ; thus the Wolf and Dog have a round, the Fox a perpendic- 

 ular slit-shaped pupil. In the Solidungula and Ruminantia there 

 project from the uvea upon the border of the pupil, tuft-shaped flakes 

 of pigment, called the racemiform or sponge-like bodies. 



The lens is, in some Rodentia, but particularly in aquatic animals, 

 such as the Cetacea and Seals, very convex and spherical. The 

 macula lutea of the retina appears, with the exception of the Apes, 

 to occur only in the human subject. The number and mode of 

 attachment of the muscles of the eye are the same as in Man. The 

 tendon, however, of the superior oblique appears to be wanting to 

 the Cetacea. With the exception of the Apes, all the Mammalia 

 appear also to have an additional muscle, the suspensory or retractor. 

 This is a quadrifid muscle embracing the optic nerve, the portions 

 of which sometimes coalesce, as in the Ruminantia, into a single 

 infundibuliform muscle. It is fixed to the sclerotic behind the 

 cornea. 



The eyebrows and eyelashes occur only in a few Mammalia, 

 the latter being wanting in the smaller kinds. The eyelids have 

 the usual cartilages and muscles, and the lower lid is moveable. 

 The third eyelid, called also the haw or nictitating membrane, 

 has a single triangular cartilage, and is met with in nearly all 

 the Mammalia, with the exception of the true Cetacea ; it contains 

 muscular fibres, and is drawn like a curtain over the front of the 

 eye, so soon as the retractor muscle acts upon the latter. The Apes 

 have, like Man, no haw, but only a rudiment of it, the plica semi- 

 lunaris, in the inner angle of the eye. In the Ornithorynchus, and 

 Echidna, the eye is closed by a single circular eyelid, with a small 

 round opening in it. Meibomian glands and a caruncula lacrymalis 

 are frequently present ; the last, however, is wanting where the 

 haw is much developed. The lacrymal gland with its apparatus 

 appears to be wanting only in the Cetacea ; it is often very large, 

 and in addition to it, there is found in all animals provided with 

 a third eyelid, as the Hare, the Harderian gland (which occurs 

 generally in Birds) well developed, two or three ducts from it opening 

 beneath a fold of the inner surface of that lid. The mechanism for 

 moving the nictitating membrane is not the same as in Birds. It 

 seems to be drawn forward the more the retractor muscle acts, when 

 the eye by being pulled back presses within the orbit against the 

 posterior termination of the cartilage of the nictitating membrane, 



