684 Comparative Morphology of Chordates 



determined by the optic nerve and the center of the pupil). The 

 circular muscle is weakly developed or even lacking in many mammals. 



The lens is elastic throughout, and its outermost layer, the lens 

 capsule, is a thin but tough and highly elastic membrane which is in 

 a state of perpetual tension, thus exerting pressure on the deeper 

 substance of the lens. The fibrils of the zonula ciliaris are likewise 

 under more or less tension which, exerted centrifugally at the periphery 

 of the elastic lens, tends to flatten it. 



Contraction of the ciliary muscles, both circular and meridional, 

 serves to pull or stretch the adjacent region of the choroid layer toward 

 the iris, thereby slightly decreasing the diameter of the circular ring of 

 ciliary processes and, accordingly, decreasing the diameter of that 

 circular zone of the choroid layer at which the fibers of the zonula 

 ciliaris are attached (Fig. 510). The result is to diminish the tension in 

 the fibers of the zonula and therefore to lessen their centrifugal pull 

 on the lens capsule, which thereupon contracts and increases the 

 convexity of the lens. Because of inequalities in the thickness of the 

 lens capsule, the increase affects mainly the external surface of the lens. 

 The human lens, in later life, suffers some loss of elasticity. The 

 resulting inability to accommodate for near vision is corrected by use 

 of eyeglasses having convex lenses. 



This mammalian method of accommodation is in sharp contrast to 

 that which obtains in most reptiles and in birds. In these sauropsids 

 (except snakes and crocodilians) contraction of a strong circular muscle 

 situated in that part of the wall of the eyeball adjacent to the lens 

 (apparently corresponding to the mammalian circular ciliary muscle) 

 impinges directly upon the periphery of the lens, thereby exerting upon 

 it a centripetal force which causes it to bulge. Thus, in the sauropsids, 

 direct external muscular pressure compels the lens to bulge. In mam- 

 mals muscles act indirectly to decrease tension in the zonula, thus 

 permitting the lens to bulge by virtue of its inherent elasticity. 



The extrinsic muscles of the mammalian eyeball are essen- 

 tially like those of other vertebrates, consisting of four rectus mus- 

 cles converging from the eyeball to a posterior point of attachment on 

 the wall of the orbit, and two oblique muscles, a dorsal and a ventral 

 (Fig. 511). These extend from their points of insertion on the eyeball 

 toward more anterior points on the wall of the orbit where, by analogy 

 with the condition in other vertebrates, both might be expected to 

 attach. The ventral muscle does attach there, but the long tendon of 

 the dorsal one bends through a loop of tendon-like tissue which is 

 fixed to the wall of the orbit at that point and then passes into the 

 belly of the muscle, which continues backward to an attachment 



