820 THE SENSE OF SIGHT 



backward to be inserted in the choroid at and behind the ciliary 

 process. The circular fibers are most numerous in the base of the 

 ciliary body, and pursue a course circularly around the aperture in 

 which the lens is suspended. They are most clearly in evidence in 

 hypermetropic persons.^ 



The aperture between the margins of the ciliary body is occupied 

 by the lens. The latter is invested by a transparent and elastic 

 capsule, measuring in front 6.5 to 25 /x in thickness. Its substance 

 is composed of a firm central and softer cortical portion, both of which 

 appear as hexagonal, prismatic lamellae of homogeneous elastic tissue. 

 A single layer of cuboidal cells, 2.5 to 10/x in height, forms their outer 

 investment. It is to be noted especially that the edge of the lens is 

 not in absolute contact with the margin of the ciliary body, but re- 

 mains at some distance from it, the intervening space being occupied 

 by ligamentous bands which extend straight across from its capsule to 

 the surface of the ciliary body. Furthermore, these ' ligamentous 

 fibers pursue a peculiar diagonal course, some of them arising upon 

 the posterior aspect of the ciliary body and terminating upon the 

 anterior surface of the lens, while others arise upon the anterior surface 

 of the ciliary body and end upon the posterior surface of the lens. We 

 shall have occasion to refer to these data again later on, while dis- 

 cussing the accommodation of the eye for near objects. 



The Process of Accommodation in Different Animals. — The pur- 

 pose of the lens is to bring rays of light to a precise focus upon the 

 retina. In this function it is aided in a slight measure by the other 

 refractive media of the eye. But, since objects are situated at different 

 distances from the eye, some means must be provided by which their 

 images may be retained upon the retina ; in other words, the eye must 

 be able to accommodate itself to these varying distances. To ap- 

 proach this subject in the most logical way, inquiry should first be 

 made regarding the manner in which the ordinarj^ photographic camera 

 may be adjusted for far and near objects. Two ways are open to us, 

 namely, to move the sensitive plate either nearer to or farther away 

 from the lens, or to move the latter either nearer to or farther away 

 from the plate. A third method would be to permit the screen as well 

 as the lens to remain stationary and to adjust the focal distance of 

 the camera by interposing other lenses of different refractive power. 

 This procedure, however, is not in common use, because it is less 

 convenient than the other two. 



In the animal kingdom, however, the third method is imitated by 

 varying the convexity of the lens, while its position, as well as that of 

 the retina, remains unchanged. 



Indeed, the mechanism of accommodation is so diversified among the different 

 animals that really all of the physical means just enumerated find their prac- 

 tical application. 2 To begin with, it should be noted that certain species lack 



1 Iwanoff, Archiv fur Ophthalmol., xv, 1869, 1. 



2 Beer, Wiener klin. Wochenschr., 1898, No. 12. 



