562 



MANUAL OF PHYSIOLOGY. 



capsule, near its margin. The lens and its capsule, together with 

 the vitreous humor, may be said to be enclosed in the hyaloid 

 membrane, which is thickened and fixed to the capsule, and to 

 the ciliary part of the choroid. Thus any tension exercised by 

 the suspensory ligament tends to tighten the anterior part of the 

 capsule, and flatten the anterior surface of the lens. 



FIG. 219. 



./* 



A further stage of the development of the lens. (Cadiat.) a, Elongated epithelial cells 

 forming lens; 6, Capsule; c, Cutaneous tissue becoming conjunctiva; d, P, Two layers of 

 optic cup forming retina ; /, cell of mucous tissue of the vitreous humor ; g, Intercellular 

 substance ; h, Developing optic nerve. 



The shape of the lens varies at different times of life, being 

 nearly spherical in the infant and tending to become less convex 

 in old age (Fig. 217). The lens is developed from the outer 

 layer of the embryo by the gradual thickening and growing 

 inward of the epithelium, which meets the optic cup, and after a 



