MULTICELLULAR ANIMALS 



227 



The mammalian eye (Fig. 154) accommodates to near and far vision 

 bv changes in the curvature of the lens. It is thought that the lens 

 substance is quite elastic and tends to assume the shape of a sphere. 

 It is suspended by a ligament and is enclosed in a capsule of trans- 

 parent tissue. The lens ligament is anchored to the crest of the 

 ciliary muscle, the fibres of which are chiefly at right angles to the 

 pull exerted by the ligament (Fig. 156). When an object is brought 

 near the eye, the ciliary muscles contract, thus shortening the base 

 attachment of the lens ligament. This allows the ligament to loosen 



CILIARY MUSCLE 



LENS 



SUSPENSORY LIGAMENT OF LENS 



Fig. 156. — Diagram to illustrate the theory of accommodation of the human eye 

 to changes in distance. In the first figure the eye is at rest and is said to be focused 

 at infinity. When vision is accommodated to observe a near object the ciliary 

 muscles contract. This lessens the tension on the suspensory ligament of the lens. 

 Then the lens, being elastic, tends to become spherical. It will be noted that in 

 this state the ciliary muscles are expending energy; hence prolonged close vision 

 causes fatigue. 



since the crest of the ciliary body is thereby raised. The relief of 

 tension on the ligament and lens capsule allows the elastic lens to 

 assume a more spherical shape by taking up the slack in the liga- 

 ment. Thus the curvature of the lens is increased. At rest the ciliary 

 muscles are relaxed but by reason of their elasticity and that of 

 associated tissues there is a draw exerted on the lens capsule which 

 flattens the lens. Thus the resting eye is set for far vision, that is, 

 focused at infinity. The mechanism of accommodation is very sen- 

 sitive and constantly in action during waking hours. 



Defects in Vision. The most common defects in human vision 

 are near-sightedness, myopia, far-sightedness, presbyopia, and astig- 



