394 



Comparative Animal Physiology 



in accordance with principles of geometric optics. The aperture of the lens 

 is varied by the iris, and the eyelids are used as a lens cover to exclude light 

 and lessen the possibilitv of mechanical injury to the corneal surface. The 

 photosensitive screen is backed by a layer of black pigment which reduces 

 internal reflection and thereby enhances the clarity of the image. Part of 

 the focusing mechanism (i.e., the lens), is also variable (by contraction of 

 the ciliary muscle), so that objects at very different distances may be seen 

 with equal sharpness if they are not viewed simultaneously. 



posterior chamber 

 limbal zone 



coryuncfiVo 

 anal of Sc hie mm 

 ciliary muscle 



rectus 

 tencfon 



sclera 

 chorioid' 



lamina cribrosa 



macula 



Fig. 109. Horizontal section of the right human eye, X 4. From Walls.^"* 

 Modified from Salzmann. 



Focusing of light on the retina is brought about partly by the curvature 

 of the cornea, which has a refractive index (1.376) much higher than that 

 of air (1.00) and partly by the lens, which has a refractive index (1.42) 

 slightly higher than that of the aqueous and vitreous humors (both 1.33) 

 Since the greatest difference in refractive index, and also the greatest curva- 

 ture, is found at the air-corneal interface, this is really the most important 

 refracting surface in the formation of the image. The lens serves in a capacity 

 comparable to that of a fine adjustment on a microscope, giving a delicate and 

 accurate control over a short range. The range over which the lens can 



