520 



PHYSIOLOGY 



the suspensory ligaments are shallow pockets into which the ciliary processes 

 fit closely. In this way the lens is held firmly in position, while at the same 

 time by the movement of the ciliary processes under the action of the ciliary 

 muscle, traction can be applied to the suspensory ligaments, thus effecting the 

 change in curvature of the lens, which will be shown later to be necessary 

 for accommodation. 



Histologically the lens is composed of a number of radially arranged fibres 

 each of which is a modified epithelial cell. These fibres are arranged in 

 concentric layers, the more peripheral being soft, nucleated and of low 

 refractive index, while the central form a dense non-nucleated mass of high 

 refractive index, the fibrous layers between having an intermediate structure 

 and index. 



REFRACTION BY THE CRYSTALLINE LENS. Kefraction occurs 

 whenever light passes from a medium of one optical density into another. 

 It is due to the fact that the waves of which the beam of light is com- 

 posed travel more slowly in a dense medium, than they do in one of less 

 density. Rome of the effects which this produces are shown in the diagram 

 below. 



l-'n:. L'til. Diagram showim; ivfr;trtion of li 



(A) By an inclined surface, (B) by a lens, (C) by a plate of greater density at 

 its lower end than at its upper, (D) by a plate of greater density at its centre than 

 at its edges, (E) by a lens of greater density at its centre than at its edges. 



At A plane waves are seen entering a dense medium at an angle. At 

 B the medium is lens shaped. At C the medium has a plane surface but 

 has a greater density on the right than on the left. At D the medium 

 has a plane surface but a greater density at its centre than at its edges. 

 At E the medium is lens shaped as at B, and also varies in density as at D. 

 The very great refracting power of such a structure is \\cll shown. Since 



