VISION 341 



behind the front of the cornea, and about any one of three axes, the visual, 

 transverse, and vertical. 



Besides the larger movements produced by the extrinsic muscles and 

 apparent both from within and, to others, without, there are numerous 

 minute unconscious and entirely involuntary movements of extreme 

 quickness and jerky in nature, whose full purpose is yet by no means 

 quite clear. They are discernible only by delicate means, and most 

 readily when the eye is made to fixate rigidly a point or to trace carefully 

 along a given line. It is likely that these movements are intended to 

 avoid the tiring of the sensitive rod or rods and cones of the retina by 

 distributing the stimulus over a larger number than else would be inner- 

 vated. Another and perhaps a more important function of these minute 

 movements, of which the seer is always entirely unconsciousness, may 

 be to help the synthesis of disparate visual impressions into a "field of 

 vision," a homogeneous general impression. The synthesis of actual 

 vision is not wholly the work of the visual centers or of "the mind/' 

 but in part at least of the afferent end-organ (retina) as well. Experts 

 in these matters are at present discussing the rather primary question 

 as to whether the individual sees during these quick, short, intermittent 

 eye-movements or during the very short periods of rest between them. 

 Woodworth's contention for the former seems well-grounded both in 

 theory and in facts. 



ACCOMMODATION is the reflex adjustment of the refracting mechanism 

 of the eyes to the ever-changing conditions in the intensity and distance 

 of the object. The mechanism concerned includes the ciliary muscle, 

 the lens, the suspensory ligament, and the iris. 



The chief need of accommodation lies in the optical fact that rays 

 coming from an object near the eyes, and therefore directed in more 

 divergent directions, require a stronger (convexer) lens to converge them 

 upon the fixed retina than do more nearly parallel rays coming from a 

 distance. In a camera or in a compound microscope this focusing is 

 brought about by moving the lenses away from their screen or object, 

 but in the living eye it is more convenient to change the refracting power 

 of the lens itself. This change is easily brought about because of the 

 softness of the lens-protoplasm, especially in its interior. The anterior 

 capsule of the lens is thick and some of its lamellae are directly continuous 

 with the fibers of the suspensory ligament. 



The theory of accommodation originating with the great Helmholtz 

 is still held (in a condition somewhat modified by Hess, Smith, and 

 Einthoven) by the great majority of physiologists. The accompanying 

 illustration (Fig. 195), devised by Schoen to show how the lens changes 

 shape (on a theory opposed to that of Helmholtz), gives a good notion of 

 what appears to take place in accommodation to near-vision. The 

 anterior surface of the lens bulges into the shape shown in the figure 

 (although in a degree much less marked of course than that shown). 

 This is the tendency of the lens itself, because of its structure, whenever 

 the restraint of the suspensory ligament and the ciliary muscle exerted 



