354 ADVANCED PHYSIOLOGY 



Astigmatism. We have assumed that the surface of the cor- 

 nea in front and of the lens behind it were parts of true spheres, 

 curving equally in all directions. The majority of eyes, how- 

 ever, are astigmatic, i. e. there is somewhere, either in the 

 cornea or lens, an irregularity so that the combined shape of 

 these bodies is more like that of a football than like a true 

 sphere, curving in a circle in one direction, but in an oval in 

 the other. As a rule there is one plane in which persons 

 with astigmatic eyes can see well, but in all other planes the 

 image will be blurred; Fig. 179. To remedy this very serious 

 defect it is plain that neither a perfectly convex nor a per- 



FIG. 179. DIAGRAM OF DEFECTS IN AN ASTIGMATIC EYE 



Showing that rays from a point are not properly focused. A, source of light; 

 B, C, retinal surface; D, lens. Rays in one plane come to a focus at a point; in 

 the other plane they are distributed over B C in a line. 



fectly concave lens will suffice. The need, then, is for a lens 

 ground at two different curvatures, the opposites of those in 

 the eye lens. 



THE RETINA AND ITS FUNCTIONS 



We have seen that the images of objects in the external 

 world are focused upon the retina, a fact which suggests that 

 this is the sensitive, i. e. the real seeing part of the eye. 



The actual thickness of the retina at its thickest part, at 

 the back, is only about one seventy-fifth of an inch and it is 

 much thinner than that on the sides of the eye. Its border 



