ACCOMMODATION OF THE EYE. 



833 



near objects, it draws the margin of the choroid forward, and the zon- 

 ule, which is in intimate relation with it, is relaxed. As a result, the 

 lens assumes a more curved form, by virtue of its elasticity, so that it 

 becomes more convex as soon as the flattening tension of the zonule 

 relaxes. As the posterior surface of the lens rests in the saucer-shaped, 

 unyielding depression of the vitreous, the anterior surface, in becoming 

 more convex, must protrude further forward. 



Hensen and Volckers discovered the origin of the nerve of accommodation 

 in the most anterior portion of the oculomotor nucleus. Irritation of the posterior 

 part of the floor of the third ventricle produces accommodation; if the irritation 

 is applied a little further back, the pupil contracts. The fibers for the sphincter 

 of the iris and for the ciliary muscle are derived from the upper oculomotor 

 nucleus; close by this is the center for the elevator of the eyelid. If the boundary 

 between the third ventricle and the aqueduct of Sylvius is irritated, the internal 

 rectus muscle contracts; irritation of the posterior part of the aqueduct causes 

 contraction of the superior rectus, the inferior rectus and the inferior oblique. 



The movement during accommodation may be recognized by means of the fol- 

 lowing phenomena: 



as.' 



FIG. 282. Diagrammatic Representation of Accommodation for Near and Far Objects. On the right the con- 

 dition during accommodation is shown; on the left the condition during rest. On both sides one-half of 

 the contour of the lens is drawn as a continuous line, the other half as a dotted line. The letters appearing; 

 twice both on the right and left sides have the same significance; those on the right side are primed. A left, 

 B right half of the lens; C, cornea; S, sclera; C.S.. canal of Schlemm; V.K., anterior chamber; /, iris; P, 

 pupillary margin; V anterior, H posterior surface of the lens; R, equator of the lens; F, edge 9f the ciliary 

 processes; a and b, interval between them. The line Z X shows the thickness of the lens in the act of 

 accommodation for a near object; Z Y, the thickness of the lens when the eye is at rest. 



i. The images of Purkinje-Sanson. If the light of a candle is allowed to 

 fall on the human eye a little from the side, or, better still, if the light comes 

 through two small triangular openings in a piece of cardboard, placed one 

 above the other, the observer sees three pairs of reflected images in the eye. The 

 brightest and most distinct pair (virtual) are formed by the anterior surface 

 of the cornea (Fig. 283, a). The second pair (likewise virtual) are the largest, 

 but at the same time the faintest; they are reflected from the anterior surface 

 of the lens (b) and lie 8 mm. behind the plane of the pupil. (The images 

 produced by convex mirrors are the larger the longer the radius of curvature.) 

 The third pair are the smallest and stand midway in intensity; they are inverted, 

 and lie about in the plane of the pupil (c). These images are also virtual, because 

 they do not lie in the second medium, which is represented here by the air. The 

 posterior capsule of the lens, which reflects these last images acts as a concave 

 mirror. (If a luminous object is placed at a distance from a concave mirror, an 

 inverted, real image, reduced in size, is formed near the focal point of the mirror, 

 on the same side as the object.) While the observer watches these images, with 

 the eye of the person at rest, the latter is requested to accommodate suddenly 

 for a near object. Changes in the images are at once recognized. The middle pair 

 (from the anterior surface of the lens) become smaller, and brighter, and approach 

 each other (b), because the anterior surface of the lens becomes more convex. 

 At the same time they approach the corneal images, because the anterior surface 

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