562 STUDIES IN ADVANCED PHYSIOLOGY. 



of the change of form of lens in the accommodation of the 

 eye cannot be desired. 



This change also accords with what we know of the 

 property of lenses. We know, for instance, that when an 

 object is moved closer to a lens the image which the lens 

 forms is pushed further back, and if this pushing back is 

 not desired the rays of light must be bent more; that is, 

 brought to a focus sooner, a result easily produced by in- 

 creasing the convexity of the lens. The problem, there- 

 fore, narrows itself down to the explanation of the manner 

 in which this change of lens is brought about. The first 

 element of this explanation is found in the elasticity of the 

 lens. It is a quite elastic structure made up of almost trans- 

 parent cells, and is in its usual position continually unnatu- 

 rally flattened. This is proved by seeing that when the lens 

 is removed from the eye it at once becomes more nearly 

 round than before. It is flattened in spite of its elasticity 

 by the pull which the capsule of the hyaloid membrane 

 exerts upon it, and this pull is brought about by the pres- 

 sure of the vitreous humor which it exerts upon the hyaloid 

 membrane around it. The lens is flattened out much like 

 a foot-ball would be flattened out if it should be put under 

 a person's coat and the individual should then distend his 

 lungs with air. To make the analogy more complete we 

 might imagine the foot-ball placed between the outer cloth 

 of the coat and the lining, and the coat then buttoned up. 

 If now the chest were expanded, as the vitreous humor ex- 

 pands the hyaloid membrane, the foot-ball would be mate- 

 rially flattened. Thus, to repeat, in the natural resting 

 condition of the eye the lens is flattened, and if the eye be 

 a normal one, therefore adjusted to far objects. An in- 

 crease in the convexity of the lens is brought about by 

 producing a slack in the capsule and suspensory ligament 

 surrounding the lens. This slack is produced by the action 

 of the ciliary muscles already described. These, it will be 

 remembered, are muscles imbedded in the choroid coat, one 

 end fixed to the point where the choroid coat is firmly 



