32 A TYPICAL VERTEBRATE EYE: THE HUMAN 



curvature to accomplish this (Fig. 14), and the structures most involved 

 are the lens capsule, the zonule fibers, and the muscle cells in the ciliary 

 body. The latter must contract to focus the eye for nearby objects, relax 

 partially for more distant objects up to twenty feet away, and relax com- 

 pletely for objects beyond twenty feet. This is why it is restful to the 

 eyes to gaze out of a window at distant objects for a few moments 

 occasionally, when doing close work of any kind. 



The ciliary muscle fibers are formed into two muscles which blend 

 with each other and are really only one, since one mass of fibers is de- 

 rived from the other in the embryo and the two masses have a common 

 nerve supply and act together, having the same effect upon accommo- 

 dation in spite of their great difference in orientation within the ciliary 

 body. 



The 'radial' or 'meridional' fibers, as seen in a sagittal section of the 

 eye, are arranged fanwise, the small end of the mass being fastened 

 at the scleral roll and the other end being frayed out and distributed 

 along the whole ciliary body, most of the fibers ending along its inner 

 surface (Fig. 3; Fig. 5, mb). When this radial muscle (of Briicke) con- 

 tracts, the effect is a stretching of the flat orbiculus region of the ciliary 

 body so that its anterior border moves forward — the ora terminalis being 

 fixed. The corona ciliaris, that portion of the ciliary body bearing the 

 ciliary processes, is telescoped, its posterior border moving forward but its 

 anterior attachment at the iris angle remaining fixed. The result of this 

 forward movement of the region of junction between corona and orb- 

 iculus is a relaxation of the taut guy-wires of the lens, the zonule fibers. 

 These are normally in a state of considerable tension when the ciliary 

 muscle is not contracted; for, as the eyeball grows, before and after 

 birth, its diameter increases proportionately faster than that of the lens. 

 Hence the suspensory-ligament fibers, once they have grown out from 

 the ciliary epithelium and attained connection with the young lens cap- 

 sule, are placed under constantly increasing lengthwise stress which is 

 not entirely removed by any compensatory increase in length on their 

 part. This brings about a slow broadening and flattening of the growing 

 lens and a permanent state of tension in the suspensory ligament, which 

 can be relieved only by the contraction of the ciliary muscle. 



A portion of the ciliary muscle fibers, the number being often greater 

 in far-sighted eyes and less in near-sighted ones (where they may even 

 be entirely lacking) are organized into a ring-like muscle (of Mixller), 

 analogous to the sphincter pupillae. Although the fibers in Miiller's muscle 



