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HUMAN PHYSIOLOGY 



if the eye is not accommodated, they illuminate two different points 

 of the retina because its two rays of light unite back of the retina. 



Mechanism of accommodation (see Fig. 30). The lens 

 L lies inclosed in the two leaves of the capsule of the lens. 

 The capsule of the lens at its border passes over into the 



FIG. 30. CHANV.KS IN THE LENS DVRINC, ACCOMMODATION. 

 (After Helmholtz.) 



/"', far vision; N, near vision; w, musculus ciliaris; :,r, CjC, , xonule of /inn; 

 r, ciliary process; L, lens. 



zonule of Zinn, zz and z^z^ a folded membrane whose 

 periphery is united with the choroid where this passes over 

 into the corpus ciliare (Y). 



The intra-ocular pressure produced by the transudation 

 of tissue fluid from the blood vessels into the inner part of 

 the eye causes the. coats of the eye to be taut, and thereby 

 stretches the part at which the zonule is inserted into the 

 choroid. Hence the zonule and the capsule of the lens are 

 stretched and the lens is pressed together and flattened in 

 its anterior-posterior direction. This is the condition of the 

 lens in the resting eye (Fig. 30, F). 



The fibres of the muscle of accommodation ;// (tensor of 

 the choroid or ciliary muscles) in the ciliary body pass from 

 the insertion point of the zonule to the place where the 

 choroid has grown together with the boundary between the 

 cornea and the sclerotic. By the contraction of these 

 muscles the part at which the zonule is inserted is pulled 

 slightly forward and inward (toward the axis of the eye) and 



