568 



THE NERVOUS SYSTEM. 



at the termination of b ; when focussed for near objects and the 

 anterior surface of the lens moves forward, the image is seen at 

 the termination of b' i. e., nearer the corneal image ; it is also 

 smaller and brighter. The change in the convexity of the anterior 



surface of the lens may also 

 be shown by looking at the 

 eye from the side, as in ac- 

 commodation the iris may be 

 seen to move forward, being 

 pushed in that direction by 

 the anterior surface of the 

 lens, with which it is in con- 

 tact. 



Phakoscope of Helmholtz 

 (Fig. 345). This apparatus 

 was devised by Helmholtz 

 to demonstrate the changes 

 which have just been de- 

 scribed, and which were ad- 

 vanced by him to explain 

 accommodation. 



The eye of the person 

 whose accommodation is to 

 be studied is placed at 0. 

 For near vision the needle 

 at D is to be looked at, and 

 for distant vision some ob- 

 ject in the same direction. 

 At B and B f there are two 

 prisms, in front of which a candle-flame is placed. The eye of 

 the observer at A sees two sets of the three reflected images, each 

 image being a square spot of light ; those reflected from the ante- 

 rior surface of the lens approach those reflected from the cornea, as 

 already explained, and also approach each other (Fig. 346). 



Tscherning's Theory of Accommodation. This authority ex- 

 plains positive accommodation by supposing that by the contrac- 

 tion of the anterior part of both the radiating and circular fibers 

 of the ciliary muscle the ciliary processes are drawn backward, and 

 that this pulls the zonule of Zinn (suspensory ligament) backward 

 and outward. This increases the tension of the ligament and the 

 pressure upon the lens, the external or softer portion of which is 

 caused to bulge out, this change being especially marked on its 

 anterior surface. The contraction of the posterior portion of the 

 ciliary muscle pulls forward the choroid, and thus makes tense, so 

 to speak, the vitreous, preventing it from yielding when the lens 

 is pressed against it by the anterior portion ; thus the pressure of 



FIG. 346. Purkinje-Sanson images: A, in 

 the absence of accommodation ; B. during 

 accommodation for a near object. The upper 

 pair of circles enclose the images, as seen when 

 the light falls on the eye through a double slit 

 or a pair of prisms ; the lower pair show the 

 images seen when the slit is single and tri- 

 angular in shape (Stewart). 



