VISION 343 



about its edge is relaxed. According to this view, then, the chief activity 

 of the ciliary muscle is to relax the centrifugal support of the suspensory 

 ligament, thus allowing the lens to bulge somewhat in the manner shown. 



The notion of Tscherning and of Schoen is that the ciliary muscle in 

 contracting stretches the ligament instead of relaxing it, and that it is 

 this centripetal pressure on the edge and sides of the lens which causes its 

 soft substance to become more convex in accommodation to a near-point. 

 The discovery of Hess, that when the ciliary muscle is made to contract 

 strongly by eserin the lens wobbles back and forth within the suspensory 

 ligament, seems to dispose of this supposition. 



With the increase in the convexity of the lens in accommodating to 

 near objects (amounting at its maximum to an increase in strength of 

 14 diopters up to 34 diopters) occur two other adjustments of the eyes. 

 One of these is the convergence of the eyes necessary to place a near 

 object on both fovea. This is produced by contraction of the internal 

 recti muscles, the degree of convergence depending on the nearness of 

 the object seen. The internal recti are relatively strong muscles because 

 used so much, and when the eyes are quite at rest (as in sleep) tend to 

 converge the eyes mechanically so that the visual axes would meet if 

 prolonged 40 cm. from the eyes. One sees this atonal condition often, 

 also, when persons are in a "brown study," one of those lapses of con- 

 sciousness which occur at times, and which are really just the opposite 

 in nature from study. 



The other adjustment which is properly part of the process of accom- 

 modation is contraction of the iris (pupil). This shuts out the peripheral 

 rays from the eye, prevents spherical aberration and so makes the defini- 

 tion of the object's retinal image sharper. The iris has movements other 

 than those associated with accommodation. It serves as a diaphragm 

 for adjusting the eye to differences in the intensity of light. Its move- 

 ments are closely watched in conditions of anesthesia out of many a 

 serious condition involving the central nervous system. Its size is 

 affected furthermore by many drugs, contracting to opium, nicotine, 

 and muscarine, and dilating to atropine. (See the experiments in the 

 Appendix, p. 521.) 



Visual Theories. When one has summarized thus briefly some of the 

 most essential structures of the eye and some few of the principles on 

 which these structures work he has had suggested to him many interesting 

 phenomena, but he has still left, quite unconsidered, the essential part of 

 vision, namely, how and in what different ways the eyes see. In other 

 words, we would have explained to us, if we might, exactly what takes 

 place in the minute retinal rods and cones when the rays of light, thus 

 carefully admitted to them, stimulate them and cause them to send to the 

 visual centers of the brain the several different sorts of vision that most 

 of us experience. For example, if our eyes, etc., are normal we see light, 

 the forms and colors of objects, and the spatiality of objects. We can 

 see in the dazzle of noonday and form, at least, in the quasi-darkness of 

 night. We experience after-images and our eyes are prone to be deceived 



