860 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



case, also, a distinct image might be formed by substituting a lens of a 

 less degree of curvature. 



In the eye accommodation is accomplished by muscular action through 

 which the shape of the lens is changed. When the eye after viewing an 

 object at a distance is adjusted to form a sharp image of a nearer object 

 the crystalline lens becomes thicker through an increase of the curvature 

 of its anterior surface. This change may be represented in the following 

 diagram, after Helmholtz (Fig. 382). 



The left portion of the figure represents the eye adjusted for distant 

 objects, while the right half is accommodated for near objects. It is here 

 seen that the anterior surface is increased in convexity, moving nearer 

 the cornea, and has carried the iris with it. It is well known that the 

 refraction of light rays caused by a convex lens increases with its increase 

 of curvature, and that the focal length of one lens will be longer than 



Fig. 382.— Scheme of Accommodation for Near and Distant Objects, 

 after Helmholtz. (Landois.) 

 The right side of the figure represents the condition of the lens during accommodation for a near 

 object, and the left side when the eye is at rest. The letters indicate the same parts on both sides ; those 

 on the right side are marked with a stroke. .4, left, B. right half of lens; C, cornea; S, sclerotic; C.S., 

 canal of Schlemm ; V.K., anterior chamber ; J, iris ; P, margin of the pupil ; V, anterior surface, H, 

 posterior surface of the lens ; R, margin of the lens ; F, margin of the ciliary processes ; a ft, space be- 

 tween the two former ; the line Z X indicates the thickness of the lens during accommodation for a near 

 object ; Z Y, the thickness of the lens when the eye is passive. 



that of one of greater curvature — in other words, the latter will produce 

 a greater convergence of the light-rays. 



A similar state of affairs holds in the adjustment of the lens in the 

 eye. When the object of our vision is closely approached to the eye 

 the lens is more convex than when the distance is greater, and its refract- 

 ing power is, therefore, increased and the formation of an image on the 

 retina rendered possible. Of course, with every variation in distance 

 there must be a corresponding variation in the degree of curvature of 

 the lens. 



The mechanism by which the change in the curvature of the lens is 

 accomplished is the ciliary muscle. The capsule of the lens is attached 

 at its edge to the zone of Zinn, which radiates outward and keeps the 

 lens in a state of constant tension. At the point where the fibres of this 



