THE IMAGE-FORMING MECHANISM OF THE EYE 



66 I 



(79, p. 158; 86, p. 201) proved that the lens;th of the 

 eye does not change with accommodation by placing 

 the front and back of one of his eyes between the jaws 

 of a clamp and noting that there was no change in 

 the pressure phosphene at the back of the eye as the 

 eye changed accommodation. The X-ray method of 

 measuring the length of the eye described abo\e is 

 now available for demonstrating this fact. Young also 

 showed that the cornea does not change its curvature 

 during accommodation by noting the reflections from 

 the front surface. He also immersed his eye in water 

 which has about the same index as the aqueous and 

 showed that the power of accommodation was not im- 

 paired. The final proof offered by Young that the lens 

 alone pro\ides the mechanism of accommodation is 

 the fact that the eye assumes a fixed focus when the 

 lens is absent. 



Bv measuring the curvature of the front and back 

 surface of the lens and the distances of the two surfaces 

 from the cornea, Helmholtz 79, p. 143) showed that 

 the lens increases in thickness and moves forward 

 slightly, that the curvature of the back surface also 

 increases slightly but that the most important change 

 is in the curvature of the front surface of the lens. 



Helmholtz believed these changes to be brought 

 aljout by a decrease in tension of the zonule which 

 attaches the lens to the ciliary body surrounding the 

 lens. The lens was regarded as a pliable body enclosed 

 in an elastic capsule. Such a body tends to assume an 

 ellipsoidal form when a centripetal tension is applied 

 all along its equator but tends to assume a more 

 spherical form when released from this tension. 



Helmholtz believed that the release in tension on 

 the zonule required for accommodation is brought 

 about by a contraction of the ciliary muscle which acts 

 partly as a sphincter in reducing the diaineter of the 

 ciliary margin and partly as a system of radial fibers 

 pulling forward the choroid to which it is attached. 

 This increases the pressure of the vitreous on the back 

 side of the lens and neutralizes the tendency of the 

 lens to bulge on its back side. The mechanical pressure 

 of the iris on the peripheral part of the front surface 

 of the lens would help to increase the curvature of the 

 front surface. 



Fincham (26, p. 42) has demonstrated in a patient 

 with aniridia that the mechanical pressure of the iris 

 is not an essential part of the mechanism of accommo- 

 dation. In this case he could observe the decrease in 

 diameter of the lens and of the margin of the ciliary 

 body during accommodation. Finchain (26, p. 50) 

 has described a person with an eye in which the lens 

 substance had been dissolved out of the capsule and 

 showed that when the other eve accommodated the 



tension on the capsule decreased and left it free to 

 dangle. Hensen & \'oelkers (86, p. 199) demonstrated 

 the forward movement of the choroid during accom- 

 modation by inserting a needle through the sclera and 

 choroid near the ora serrata and then making the 

 ciliary muscle contract. The protruding part of the 

 needle moved backward as the choroid ino\ed 

 forward. 



Young (86, p. 208) demonstrated that the spherical 

 aberration of the eye becomes partially corrected 

 when the eye accommodates and Tscherning (86, 

 p. 211) used the specular reflections from the front 

 surface of the lens to demonstrate that this was due 

 to a flattening of the peripheral part of the front sur- 

 face of the lens. The Helmholtz theory had not taken 

 this fact into account. 



It remained for Fincham (26, p. 59) to demonstrate 

 that this is due to a variation in the thickness of the 

 capsule. The details of the theory of how an elastic 

 ellipsoidal inembrane having zones of varying thick- 

 ness from the pole to the equator affects the form of 

 the lens as a result of a change in tension of the zonule 

 have not yet been worked out. 



Fincham (26, p. i 7) demonstrated the elastic force 

 of the capsule by puncturing the lens and noting that 

 the lens substance protruded through the capsule. 

 Also by direct experiment Fincham (26, p. 24) demon- 

 strated that the zonule behaves like an elastic mem- 

 brane. Furthermore, when the capsule of the lens is 

 removed, the lens assumes a form of its own in which 

 the decapsulated lens is more spherical (26, p. 65). 



Hess (26, p. 52; 79, p. 398) observed that when a 

 maximal efTort of accommodation is made, tension on 

 the zonule is relaxed so that the position of the lens 

 in the eye is aflfected by gravity. This can be demon- 

 strated by showing that the amplitude of accommo- 

 dation is greater when the head points downward than 

 when it points upward. The lateral displacement of 

 the lens when the head is laid on one side or the other 

 can be demonstrated entoptically when there exists a 

 small opacity near the front or back pole of the lens. 

 Hess concluded from this finding that it may not be 

 necessary for the eye to make a maximal contraction 

 of the ciliary muscle in order to relax the lens com- 

 pletely from the tension of the zonule. 



Lancaster & Williams (53) have shown that when 

 a maximal state of accommodation is maintained over 

 a period of time, the lens develops a set so that it can- 

 not immediately relax to the zero level of accommo- 

 dation. It takes several minutes to overcome this set. 

 Lancaster & Williams regarded this as evidence that 

 the lens is completely released from the tension of 

 the zonule at the maximum level of accommodation. 



