PRINCIPAL METHODS OF ACCOMMODATION 257 



pupil. These animals are Encheliophis, the sea-snakes, Tarsius, Pedetes, 

 and the two-toed sloth (see Table VIII, pp. 272-3). 



4. The employment of two separate visual mechanisms which are per- 

 manently set for two particularly useful distances. This method is very 

 common among the arthropods, where it is expressed in the combination 

 of compound eyes and simple ocelli as seen in the average insect. Among 

 the vertebrates, it is used only in the tubular eyes of deep-sea fishes, 

 whose lenses are relatively so enormous that adjusting them very much 

 is quite out of the question. Here there is often a second (sometimes 

 even a third) 'accessory' retina far up the side of the eye, close to the 

 lens (Fig. 136, p. 400). Distant objects can be seen with this retina while 

 nearby ones are imaged farther from the lens, on the orthodox retina at 

 the bottom of the eye. The effect of this arrangement is essentially like 

 that of the bifocal spectacles to which we resign ourselves in presbyopia. 

 An even closer approach to a literal bifocal lens is seen in the kingfishers 

 and particularly in the famous 'four-eyed fish' Anableps, though with 

 different significances (see pp. 434-5 and 442). 



Vertebrate Methods of Accommodation — Few eye-minded verte- 

 brates have eyes small enough to get along without accommodating, or 

 have produced one of the four substitute devices described above. The 

 vast majority alter the optical system dynamically, either by pushing or 

 pulling the lens backward (Fig. 98) or forward (Fig. 99) — the group 

 of methods employed by all of the Ichthyopsida (fishes and amphib- 

 ians); or by changing the shape of the lens (Fig. 100). This may be 

 accomplished : 



A. By squeezing the lens at its equator positively and vigorously by 

 means of the ciliary body, and with the sphincter of the iris sometimes 

 called into play to help deform the anterior surface of the lens. This 

 method is used by all of the Sauropsida (reptiles and birds) except the 

 snakes, whose ancestors lost the mechanism during their early ocular 

 degeneracy. The snakes have evolved, as a substitute, a version of the 

 ichthyopsidan method which is all their own. 



B. By relaxing, through muscular effort, a tension which normally 

 exists (when the muscles are at rest) in the fibers of the suspensory liga- 

 ment of the lens — thus allowing the elasticity of the lens capsule to mold 

 the soft lens cortex into a new form with a sharper curvature. This is the 

 method of the Mammalia (and man) , and differs considerably from the 

 ancestral sauropsidan one because of the disappearance, in the early mam- 

 mals, of some structures essential to the complete sauropsidan mechanism. 



