ACCOMMODATION IN MAMMALS 285 



This method, seen at its best in man (see Chapter 2, section B) , makes 

 use of the elasticity of the lens capsule to furnish the actual force of 

 accommodation. The contraction of the ciliary muscle, by easing the 

 tension in the fibers of the zonule which normally hold the lens flat- 

 tened, merely releases this elastic force and lets it go to work. In some 

 amphibious mammals, as in the turtles, water-snakes, and diving birds, 

 the sphincter iridis comes into play also to aid in accommodation (Chap- 

 ter 11, section C). Even in these mammals, the ciliary muscle still 

 apparently does most of the work, for it is more massive than in strictly 

 terrestrial species. 



Their employment of capsule elasticity is probably wholly original 

 with the mammals. The elasticity is not a useful factor in sauropsidan 

 accommodation which can be regarded as having been simply exagger- 

 ated by the mammals. Reptilian lenses do take on something like their 

 accommodated shape, when they are cut free from their attachments. 

 But the zonule fibers are probably not under greater tension in the rest- 

 ing eye than in the accommodating one, as they are in mammals. More 

 likely their tension increases in accommodation, since they apparently 

 serve as check-ligaments rather than as the real supports of the lens. 



Among the land mammals, the ciliary muscle is well developed only 

 in ungulates, carnivores, and primates. It is seldom so compact as in man. 

 More often there is much connective tissue between the fibers, so that 

 although the muscle is bulky, it is not strong. In many small, large-lensed 

 mammals (e.g., mice) it consists of but a few fibers, or is even entirely 

 lacking. Even where it can be made out easily, as in domestic ungulates, 

 it may accomplish nothing because of the great size of the lens and the 

 relative weakness of the capsule. The horse, sheep, and pig have no 

 accommodation, and such instances serve to emphasize that though the 

 ciliary muscle may propose, it is the elasticity of the lens capsule which 

 disposes — just as in a presbyopic human being. 



Circular ciliary muscle fibers, forming a 'muscle of Miiller' with an 

 especially efficient orientation (see p. 33), are known to occur only in 

 seals, primates (best in man) and in some toothed whales and some 

 ungulates. This distribution is important to remember; for every so often 

 someone comes along with experiments based upon pharmacological 

 responses, which 'prove' that the radial and circular portions of the 

 ciliary muscle in mammals are antagonists, the circular fibers adjusting 

 the eye for near and the radial ones, just as actively, for distance. Such 



