534 



VISION 



of the anterior part of the zonule which would antagonize the meridional 

 fibers. 



Schon seeks to explain the change in the form of the lens in quite another 

 way. In his view the circular fibers, and to a less extent the inner meridional 

 fibers, of the ciliary muscle are the important parts. As a consequence of their 

 contraction the ciliary processes are moved inward and somewhat backward in 

 the direction of the arrow (Fig. 229) ; the lens, therefore, is pressed upon from 

 all sides and as the schema in Pig. 229 shows, must bulge forward. 



Space will not permit us to discuss these different hypotheses more fully. 

 We would merely mention the fact that Hess has brought forward a very weighty 

 argument in favor of the hypothesis of Helmholtz. When the mechanism of 

 accommodation is stimulated by dropping eserin in the eye, the ciliary proc- 

 esses push forward toward the cornea and, at the same time, inward toward 



the edge of the lens, so that the 

 ciliary processes are then found in 

 front of the equator of the lens. 

 Again, in accommodation for near 

 vision the lens is loose while in the 

 unaccommodated eye it is held firm- 

 ly in place. 



All authorities agree, that by 

 muscular action the eye can only 

 be adjusted for near vision and not 

 for distant vision. 



We have the following state- 

 ments from Hensen and Volckers 

 with regard to the innervation of 

 the ciliary muscle. By stimula- 

 tion of single ciliary nerves, the 

 choroid is drawn forward, its dis- 

 placement at the equator of the 

 eye being as much as 0.5 mm. ; the 

 anterior surface of the lens bulges 

 forward both in the uninjured eye 

 and after removal of the cornea 

 and iris ; and the posterior surface 

 of the lens pushes backward a little. 

 The fibers innervating the cili- 

 ary muscle arise from the oculo 

 motor. From clinical evidence 



which has recently been gathered by Stuelp it appears that the nuclear center 

 for the ciliary muscle lies close to that of the sphincter pupillas, and that the 

 accommodation center lies in the anterior medial nucleus of the oculo motor, 

 in front of the center for constriction of the pupil. [Fig. 230 represents the 



modern view as to the location of this center (cf. pages 615, 616). ED.] 



A convergence of the optical axes i. e., a contraction of the internal recti 

 muscles takes place in accommodation even when one eye is covered. But this 

 association of convergence and accommodation is not an inseparable one, for a 

 person can learn to converge the axes without accommodating, and vice versa. 



FIG. 229. Schema of the mechanism of accom- 

 modation, after Schon. The form of the lens 

 when the eye is adjusted for near vision is 

 shown by the dotted line. 



