Mammalia: Nervous System 



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the eyeball is more nearly hemispherical than spherical. An externally 

 bulging eye would be inconsistent with the "streamlining" desirable 

 in swiftly moving fishes. The mammalian eyeball is usually nearly 

 spherical and its cornea strongly convex. But in wholly aquatic mam- 

 mals, such as whales, the corneal surface is flattened. The lens in 

 fishes is approximately spherical, but in land mammals considerable 

 refraction occurs at the convex corneal surface and the curvature of 

 the lens is accordingly reduced so that the lens becomes a more or 

 less flattened biconvex body (Fig. 509). 



The sclerotic layer of the eyeball (Fig. 509) consists of dense 

 connective tissue but, except in monotremes whose eyes are in several 

 particulars more reptilian than mammalian, it does not produce 

 cartilage or bony plates such as occur in eyes of reptiles and birds 

 (Figs. 369, 415). The method of accommodation in birds and most 

 reptiles demands rigidity in the wall of the eyeball. This need is met by 

 a ring of small sclerotic bones surrounding the lens. The mechanism of 

 accommodation in mammals puts less strain upon the sclerotic layer. 



The pecten (see p. 533), highly elaborated in birds and represented 

 by the simpler conus papillaris of some reptiles, does not occur in 

 mammals, except that possible embryonic vestiges of it have been 

 observed. 



In many mammals a reflecting layer, the tapetum lucidum, 

 occurs at the surface of the choroid layer adjacent to the retina. It is 

 especially well developed in most ungulates and in carnivores, and 

 particularly in those of nocturnal habits. By reflecting into the retina 

 light which has already penetrated it, the stimulation of the retinal 

 sensory elements is intensified. It is this tapetum which causes eyes 



CARTILAGE 



Muscte 



Fig. 509. Diagram of median section of the eye of a vertebrate (riyht) and a 

 cuttlefish (left). While the two types of eye resemble one another in many funda- 

 mental characters, the retina of the vertebrate eye is inverted, while that of tin' 

 cuttlefish is not. The similarities are presumably an instance of convergent evolu- 

 tion. (Courtesy, Neal and Rand: "Comparative Anatomy," Philadelphia, The 

 Blakiston Company.) 



