422 - Multicellular Animals, Especially Man 



MUSCLE 

 CILIARY) 



CONJUNCTIVA 



NERVE 

 OPTIC) 



Fig. 23-5. Section of the human eye. 



makes the eyeball more opaque and prevents 

 a scattering of light not absorbed by the 

 retina. And lastly, a tree rotation of the eye- 

 ball within its bony socket is effected by six 

 small muscles, which permit the vision to be 

 directed according to the situation (see Fig. 

 23-5). 



The lens of the eye accommodates for near 

 and distant vision by changes of curvature; 

 and these curvature changes are controlled 

 partly by the elasticity of the lens itself, and 

 partly by the action of the ciliary muscle 

 (Fig. 23-5). This circular band of muscle 

 surrounds the margins of the lens, exerting 

 a tension on the lens by means of the sus- 

 pensory ligament (Fig. 23-5). Accommoda- 

 tion for near vision results when the ciliary 

 muscle contracts, reducing the tension on 

 the suspensory ligament and lens. When it 

 is released of tension, the elastic lens assumes 

 its unstrained form, which is quite thick 

 and nearly spherical. For distant vision, 

 on the other hand, the ringlike ciliary muscle 

 relaxes, increasing its circumference. This 

 restores the strain upon t lie suspensory liga- 



ment and forces the lens to become flatter and 

 broader. 



The refractive power of the lens, which 

 effects a precise focusing of the image on the 

 retina, is augmented by the curvature of the 

 cornea. The space in front of the lens con- 

 tains a relatively nonviscous fluid, the aque- 

 ous humor; whereas the region behind the 

 lens is filled with a viscous liquid, the vitreous 

 humor. 



The retina, or true photosensory surface of 

 the eye, is composed mainly of a prodigious 

 number of specialized receptor cells, called 

 the rods and cones (Fig. 23-6). In each human 

 retina there are more than 100 million rods 

 and about 6 million cones. The cones are 

 more numerous in the central area of the 

 retina; they are concerned with ordinary 

 "bright-light vision," in which colors and 

 sharp outlines are clearly appreciated. Vision 

 is most acute in a small depressed area, the 

 fovea centralis, where rods are not present 

 and the concentration of cones is at a maxi- 

 mum (Fig. 23-6). 



The rods are most numerous in the periph- 



