MECHANISM OF ACCOMMODATION 107 



forward. A fundamental objection to Helmholtz's theory, as pointed 

 out by Stilling [1912], is that the choroid cannot move forward when 

 the longitudinal fibers contract. 



In Fig. Ill— 5, the ciliary muscle, suspensory ligaments, and lens are 

 shown diagrammatically and a solution is given, on the assumption 

 that the tissue at b is elastic. The longitudinal muscle L is shown slop- 

 ing at an angle of 45 degrees; the circular fibers (c./.) lie directly below 

 them. Let F c represent the force of contraction developed by the 

 longitudinal muscles in a coordinate system whose Y axis represents 

 the direction of a radius of the lens passing through the circular fibers 

 above it and perpendicular to the optical axis of the lens. This force 

 may be resolved into two components at right angles to each other, F a 

 acting along the optical axis of the lens, and F r acting along a radius of 

 the lens in a direction pointing from the periphery at right angles to the 

 optical axis. 



If the radial ciliary muscles contract, lying as they do in the base of 

 the ciliary processes, this contraction causes the apices of the processes 

 to come together and form a smaller circle. In such a circular con- 

 striction the forces must again act radially to decrease the circumference 

 of the circle; this latter force is F r > in the diagram. It will be noted 

 that both F r and F T , act in the same direction, tending to slacken the 

 tension on the suspensory ligaments. The horizontal component of 

 the force F a which is directed forward tends to slacken the tension in 

 those suspensory ligaments which originate at b and which are attached 

 to the anterior face of the lens. This force does not change the tension 

 on those suspensory ligaments running from the anterior surface of the 

 ciliary process to the posterior side of the lens. These keep the posterior 

 face of the lens under constant tension and do not allow the hydrostatic 

 pressure in the lens to change the radius of curvature of that face. The 

 decreased tension over the anterior surface of the lens allows the lens 

 to bulge in the anterior direction, but not with a uniform change in 

 curvature. The distribution of the suspensory ligaments and the 

 changed thickness of the capsule tend to prevent a uniform change in 

 curvature. 



Helmholtz's theory assumes that in a condition of rest the suspensory 

 ligaments which run from the ciliary processes to the capsule of the lens 

 exert a tension upon the capsule which keeps the lens flattened, particu- 

 larly along its anterior surface, since the ligaments are attached more 

 numerously and more tangentially to this side. The above analysis 

 attributes this greater flattening of the anterior surface of the lens to 

 the increased tension applied to the capsule, to produce the far accom- 

 modation, or vision for parallel light, in the normal eye. 



