Mammalia: Nervous System 



683 



Fig. 510. Diagrams illustrating the accommodation of the eye by means of 

 the change in shape of the lens. The elastic lens is flattened by tension of the 

 suspensory ligaments. When the circular muscle of the iris contracts, tension is 

 relaxed and the lens assumes a more nearly spherical shape as a result of its elas- 

 ticity. (A) The lens adapted for distant vision. (B) The accommodation of the lens 

 for near vision. The diagrams exaggerate the increase in convexity. (After Kahn: 

 "Der Mensch," Zurich, Albert Miiller.) 



whales, especially toothed whales, and squirrels have considerable 

 accommodation. Probably many mammals sharpen the image on the 

 retina by narrowing the pupil rather than by manipulation of the lens. 

 In mammals, as in reptiles, the "resting" eye (i.e., in the absence 

 of any accommodating muscular tension) is focused for seeing rela- 

 tively distant objects. For sharp seeing of near objects, the focal length 

 of the optical system of the eye must be decreased. This is done by 

 increasing the convexity of the lens. The mechanism whereby this 

 increase is effected in mammals is unique. The lens is swung in place 

 between the iris and the vitreous humor (Figs. 509, 510) by a system of 

 delicate fibrils which radiate from the periphery of the lens to their 

 attachment at that region of the eyeball where the choroid layer 

 merges into the iris and the sclera into the cornea — i.e., the region of 

 the ciliary processes, which are vascular thickenings of the choroid 

 arranged in a series encircling the eyeball. These fibrils constitute the 

 zonula ciliaris (zonule of Zinn or suspensory ligament). Ex- 

 ternally adjacent to the ciliary'processes are two systems of nonstriated 

 muscle-fibers. Those of one system form a circular ciliary muscle, 

 encircling the eyeball. Those of the other extend in the direction of 

 meridians of the eyeball ("meridian" in relation to the optic axis as 



