440 SPECIAL PHYSIOLOGY. 



in cases of paralysis of the recti muscles, the power of accommodation 

 of the eye to distance is not impaired, and it has even been proved 

 that these muscles are unable to produce any appreciable change in 

 the form of the globe. Nor can the adjustment of the eye as regards 

 distance be owing to the action of the oblique muscles; for under the 

 influence of belladonna, not only does the pupil dilate, but the normal 

 adaptive power of the eye is lessened, although the oblique muscles 

 remain unaffected, for the eyes can still perform all the movements 

 dependent on them. 



The necessary adjustment has also been attributed to an elongation 

 of the distance between the retina and the optic centre, by a forward 

 movement of the crystalline lens as a whole, but such a movement is 

 not known to occur. 



This accommodation of the eye has also been ascribed to changes in 

 the degree of convexity of the cornea, effected by the ocular muscles ; 

 and, as the refractive power of this transparent coat, and of the aque- 

 ous humor behind it, is so much greater than that of air, a very slight 

 increase in the convexity of the cornea would be sufficient to account 

 for the whole adjusting power of the eye for near objects; but the 

 convexity of the cornea is said to undergo no change, being the same 

 in near as in distant vision. This and the preceding explanation are 

 further opposed by the facts of a case, in which, although there was 

 paralysis of the third nerve, and consequently of all the ocular muscles, 

 excepting the superior oblique and external rectus, the power of ac- 

 commodation was unimpaired (Von Grafe). 



That some change in the position or form of the crystalline lens is 

 intimately connected with the power of accommodation, is shown by 

 the fact, that when this body is removed, an operation performed when 

 it becomes opaque, constituting the disease known as cataract, the 

 power of accommodation is almost wholly lost. It is now, indeed, 

 generally admitted, that the adaptation of the eye to vision at different 

 distances, or the correction in it of distantial aberration, is due to 

 changes in the shape of the lens, and that, in near vision, the convexity 

 of its anterior surface becomes much increased, so that a forward move- 

 ment of this surface ensues, the convexity of its posterior surface 

 remaining unchanged (Helmholz). If, in a dark room, a lighted can- 

 dle, or any other luminous body, be held on one side of the eye, at a 

 distance of about 18 inches, the observer, standing on the opposite 

 side to the light, will see three images of the candle; an anterior erect 

 one, reflected from the surface of the cornea, a middle, also erect image, 

 reflected from the anterior surface of the crystalline lens; and a pos- 

 terior inverted one, reflected from the posterior surface of the same. 

 The light and the observer should form an angle of about 20 with the 

 eye examined. The first two images are erect, because the cornea and 

 anterior surface of the lens, are convex reflecting surfaces ; the third 

 image is inverted, because the posterior surface of the lens acts as a 

 concave reflector forwards. The anterior erect image is the brightest 

 and the clearest of the three; the middle erect image is the largest, the 

 least defined and the least luminous; the posterior inverted image is 

 the smallest, and intermediate in clearness between the other two. 



