20 A TYPICAL VERTEBRATE EYE: THE HUMAN 



accumulates in the posterior chamber and bulges the iris forward, this 

 being one of the many possible causes of glaucoma. 



The body of the lens, released from its capsule in the fresh condition, 

 is a glutinous, almost inelastic mass which gives no hint of its true 

 histological structure. This is best disclosed by crushing a lens which has 

 been hardened in formalin or alcohol, when it is seen that the lens is 

 composed of innumerable layers, like the coats of an onion, each layer 

 in turn being made up of fine fibers. An individual lens fiber in a given 

 layer runs from an anterior point, near the axis of the lens, circumfer- 

 entially around to a point in the posterior half of the lens — again near 

 the axis. No fibers could each have both ends exactly on the lens axis, 

 or the lens would be greatly elongated, pointed anteriorly and poster- 

 iorly, and would then be quite unsuited to its optical function. Fibers 

 running in one radius or meridian of the lens meet fibers in the diamet- 

 rically opposite meridian, end-to-end, along radial planes called 'lens 

 sutures' (see Chapter 5, section A; Figs. 40, 41, pp. 110-1). These suture 

 planes necessarily branch more and more elaborately as the lens body 

 grows by the addition of new layers of fibers at its surface, in order to 

 accommodate the increase in number of fibers in each layer over the 

 smaller number in the next innermost, slightly older layer, A given lens 

 fiber tends to lie along the convex curvature of a fiber in the next inner- 

 most layer of fibers, and along the concave curvature of one in the next 

 outermost layer. Radial lamellae of fibers are thus built up so that the 

 lens, besides having an 'onion' aspect, can also be thought of as being 

 built like an orange with many hundreds of segments. Since the diameter 

 of a single fiber is quite constant, the number of fibers per layer in- 

 creases as the lens grows, and the number of radial lamellae perforce 

 increases from time to time so that a maximum can be counted at the sur- 

 face, fewer and fewer farther and farther in toward the center of the lens. 



The lens is contained in an unbroken, homogeneous, elastic envelope, 

 the lens capsule. The capsule is not uniform in thickness everywhere but 

 has definite thickened zones at particular locations, whose importance 

 will be explained in connection with accommodation. Covering the an- 

 terior half of the lens, to and slightly beyond the equator, is a single 

 layer of cuboidal cells, the lens epithelium (Fig. 5, le). This layer lies 

 just beneath the capsule. It is of no importance optically, but is all-im- 

 portant for the growth of the lens. It is believed to secrete the capsule 

 or at least to be more efficient in this than the lens fibers which have 

 their sides against the posterior half of the capsule, for the anterior por- 



