4IO 



CHORDATE ANATOMY 



lens, vitreous humor, and retina. The curved cornea serves to converge 

 rays of Ught, and the lens increases the convergence. 



The lens is a biconvex translucent and elastic body surrounded by an 

 elastic capsule. Fibers which extend from the periphery of the lens to the 

 ciliary body form a suspensory ligament which holds the lens in position 

 back of the iris. The ciliary body is a ring of vascular and muscular 

 tissue, wedge-shaped in cross section, which projects into the cavity of the 

 eye-ball, just back of the iris. The ciliary muscles, circular and meridional, 

 act on the suspensory ligament of the lens, and accommodate the eye for 



RADIAL MUSCLE FIBERS 



CILIARY BODr' 



IRCULAR MUSCLE 



— SUSPENSORY LIGAMENTS 



CIRC. MUSC. CONTRACTED 



i— CILIARY BODY 



RADIAL MUSCLE FIBERS 



. Fig. 360. — A diagram illustrating the mechanism of accommodation of the eye by 

 means of the change of shape of the lens. The lens is flattened through the 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 natural elas- 

 ticity. A shows the lens adapted for distant vision, B the accommodation of the lens 

 for near vision. (Redrawn after Fritz Kahn, " Der Mensch," Albert Miiller, Zurich.) 



near or far vision. When the meridional fibers contract, the suspensory 

 ligament tightens, and flattens the lens for distant vision. When the 

 circular muscle contracts, the ligament is loosened, and the lens by its 

 own elasticity becomes more nearly spherical. When, in later life, this 

 elasticity is lost, we put on convex glasses. 



The iris is a muscular diaphragm, usually pigmented, which projects 

 from the ciliary body into the cavity of the eye, between lens and cornea, 

 and is perforated at its center by the pupil. It has two sets of muscles, 

 a radial and a circular, which are controlled by the sympathetic system. 

 In dim light, the radial muscles contract and the pupil enlarges. In 

 strong light, the circular muscles contract and reduce the opening. 



