THE CILIARY BODY AND LENS 



821 



this power altogether, while others possess it in only a very rudimentary degree. 

 This is true of the frog, alligator, vipers and certain rodents. Inasmuch as these 

 animals are chiefly dependent upon near vision and are nocturnal in their habits, 

 their accommodation is never subjected to wide variations. Furthermore, their 

 associations are so poorly developed that there is really no necessity for their being 

 able to discern the exact details of an object, as long as they can' perceive its 

 simplest movements. They are essentially shadow-animals. 



In the cephalopod molluscs, such as sepia, we observe that the thin globe ©f 

 the eye is strengthened by a transverse ring of cartilage, immediately adjoining an 

 exceptionally delicate ring of tissue (Fig. 427). The anterior wall to this eye con- 

 tains bands of meridional muscle fibers which are attached to the cartilaginous ring 

 (C) and are inserted in the ciliary body. On contraction this muscle pulls the entire 

 anterior half of the eye backward, in this way bringing the lens nearer the retina. 

 This movement necessitates, of course, a redistribution of the intraocular pressure 

 which is made possible by a bulging of the thinned wall of the eyeball directly 

 behind the cartilaginous ring. It need scarcely be mentioned that this approxi- 

 mation of the lens to the retina enables this animal to accommodate for far objects. 



Fig. 427. — Diagram Il- 

 lustrating THE Process of 

 Accommodation in the Eye 

 OF Sepia. 



The anterior half of the 

 eyeball is drawn toward the 

 cartilaginous ring C on far 

 vision. 



Fig. 428. — Diagram Illustrating 

 Process of Accommodation in the 

 Eye of the Fish. 



C, cornea; L, the lens is pulled 

 toward the retina on far vision by 

 R, the muse, retractor lentis. 



The eyes of the amphibians and many reptiles, such as the snakes, are normally 

 adjusted for far vision. In these animals accommodation is effected by increasing 

 the distance between the lens and the retina. This change is accomplished in this 

 way: As the ciliary muscle contracts, it pulls the sclerotic-corneal junction back- 

 ward, thereby increasing the pressure in the vitreous cavity. In consequence of 

 this increase in pressure, the lens is pushed forward into the aqueous cavity and 

 approaches the cornea. An equalization of the pressure in this chamber is made 

 possible by a displacement of the aqueous humor into its lateral angle which has 

 just been enlarged by the retraction of the ciliary body. It is evident that this 

 removal of the lens from the retina must increase the posterior focal distance 

 and must accommodate the eye for near objects. 



The eyes of the fishes are normally set for near objects (Fig. 428). They are 

 not in possession of ciliary processes nor of ciliary muscles, and their almost spher- 

 ical lens is suspended in the visual axis by means of flat bands of connective tissue, 

 forming the so-called suspensory ligament. The lower and inner pole of the lens 

 gives attachment to a number of horizontal strands of muscle fibers which on 

 contraction pull the lens backward, thereby diminishing the distance between it 

 and the retina. This muscle, known as the retractor lentis, lessens the posterior 

 focal distance of these eyes and accommodates them for far objects. Consequently, 



