DIOPTRIC MECHANISMS OF THE EYEBALL 549 



ring, so that there is an actual diminution of the distance between the lens and the 

 retina. 



In every class of animals, except in the cephalopod and in birds, species are found 

 which possess no power of accommodation at all, or only to a very slight extent. This 



A 



FIG. 276. Diagrams from BEER to show mode of accommodation (for distance) 



in a fish. 



A, vertical section of the eyeball ; B, view of eye from front ; L, lens ; Ls, 

 suspensory ligament ; J, iris ; Rl, retractor lentis or ' campanula ' ; C, changes 

 in position of lens when eye is accommodated for an object at varying distances. 



FIG. 277. Accommodation in eye of sepia. (BEER.) 

 R, at rest ; A, during accommodation (for distance). 



is the case in frogs, alligators, vipers, and in many rodents. Many of these animals 

 are distinguished by nocturnal habits, and in daylight their pupils may be constricted 

 to such an extent as to render accommodation unnecessary. In many of them, too, 

 the exact form of an object is not so important as the power to follow its movements. 

 In such cases the movement of the extrinsic ocular muscles or of the head are more 

 important than the exact focusing of the object on the retina. 



