226 



ELEMENTS OF BIOLOGY 



The Eye of Vertebrates. The essential features of the verte- 

 brate eye are: A spherical cup at the base of which are the nerve 



ciuAnr MuscLS 



AQUEOUS HUMO/f 



supcmon rectus muscle 



VITREOUS HUMOR 

 SCLERA 



IRIS 



PUPIL 



LENS 



CORNEA 



SUSPENSORr LIGAMENT 



OPTIC NERVE 



HYALOID CANAL 



INFERIOR RECTUS MUSCLE 



SUPERIOR OBLIQUE 



SUPERIOR RECTUS 



INTERIOR PECTUS 



. Fig. 154. — Diagram of a longitudinal section through the human eye. 



endings and the receptors with the light-sensitive substances that, 



when affected by light, activate 

 the nerves; a lens suspended in 

 front of the nerve endings in such 

 a way as to converge the rays of 

 light and to form an image on 

 the sensitive area of retina. As 

 the source of light is moved far- 

 ther away or nearer to the eye, the 

 lens is accommodated to the 

 changing distance. In the eye of 

 the fish this is accomplished by 

 means of a mechanism that moves 

 the lens forward or backward, as 

 one focuses the lens of a camera. 



INFERIOR RECTUS 



EXTERIOR RECTUS 



INFERIOR OBLIQUE 



Fig. 155. — The arrangement of muscles 

 which operate the human eye. 



