34 A TYPICAL VERTEBRATE EYE: THE HUMAN 



just as if the latter had been severed and the lens entirely isolated. We 

 may express these antagonisms and cooperations as a series of equations : 



Lens - capsule = lens in situ + relaxed ciliary muscles (no accom.) ; 



Lens + capsule - zonule = lens in situ + contracted ciliary muscles; 



Lens + capsule - accommodation = lens - capsule; 



Lens + capsule + zonule + accommodation = lens + capsule — zonule; 

 and so on. 



The thinnest portion of the lens capsule is a large central area of its 

 posterior part. This is surrounded by a greatly thickened band which 

 lies fairly close to the equator. The equatorial region itself is again thin. 

 On the anterior surface is another thickened zone which lies a little 

 farther from the equator than the posterior thickening and leaves a 

 smaller thin central area than occurs on the posterior capsule. This 

 central thin area of the anterior capsule is also slightly thicker than the 

 posterior central thin area (Fig. 14). 



Ordinarily all of the light used for vision passes only through the 

 anterior and posterior central thinnings of the capsule — the pupil does 

 not dilate widely enough to expose the periphery of the lens to incoming 

 light. The posterior surface of the lens fits the vitreous body so closely, 

 with incompressible fluid in the retrolental space between the two, that 

 it cannot change its curvature materially during accommodation. The 

 anterior leaf of the zonule is probably relaxed more completely than the 

 weaker posterior leaf at a given stage of accommodation, and the net 

 result is that only the anterior lens surface is free to deform when the 

 zonule is relaxed by the contraction of the ciUary muscles. The anterior 

 zone of thickening in the capsule then proceeds to reduce its diameter 

 and is stiff enough to force the thin central area of the capsule to form 

 a bulge, into which the body of the lens is molded. This sharpening of 

 the curvature of the useful portion of the anterior lens surface increases 

 the refracting power of the eye and holds the image forward on the 

 retina in spite of the approach of the object within the 'commencement 

 point' of accommodation — that is, within the critical twenty-foot distance. 



The amount of accommodation which is being exerted at any one 

 time, and the total amount of which the individual is capable, can be 

 conveniently expressed in the same units used for designating the focus- 

 ing power of a lens. The unit in question — the diopter — is not really a 

 unit at all, for it has a sliding value. The strength of a lens in diopters 

 is the reciprocal of its focal length in meters. That is, a one-diopter lens 

 focuses parallel rays at a point one meter away, and a two-diopter lens 



