Page:Handbook of Ophthalmology (3rd edition).djvu/29

PRINCIPLES OF ACCOMMODATION. 23 reason to believe that they fully explain all the changes of optical adjustment. It only remains to show more exactly how the changes in the lens are caused.

We start from the assumption that with perfect relaxation of accommodation the eye is adjusted for its far point, and that with each effort of accommodation it is adjusted for some less distant point. The simple suspension of accommodation causes a return to the optical adjustment for the far point. The truth of this sup- position is established by the fact that the eye may be adjusted for its far point artificially, by the use of atropine, or pathologically, by the paralysis of accommodation.

The phenomenon of accommodation is reduced accordingly to an antagonism between the elastic strength of the lens on the one. side and the zonula on the other, which is excited by the action of the ciliary muscle. The lens from its elasticity has a tendency to approach a spherical shape ; Helmholtz,* who first announced this fact, also found that with absolute relaxation of accommoda- tion for distant vision, the thickness of the lens is less than it is after death.

The zonula, which is stretched between the ciliary processes and the equator of the lens and is attached to both, tends by reason of its elasticity to flatten the lens. So soon as the zonula is re- laxed the lens will follow its own elastic tendency and will assume a greater convexity.

It may with the greatest probability be assumed that both the radiating and the circular fibres of the ciliary muscle act simul- taneously to relax the zonula. The elasticity of the lens then comes in play, and the above-described changes in its form occur; upon the cessation of muscular contraction the elastic tension of the zonula acquires the ascendency, and then occurs that flattening of the lens which corresponds to the condition of relaxed accom- modation.

It is advantageous in practice to possess a uniform standard of measure for the power of accommodation, in order to compare the


 * Physiol. Optik, pag. 110.