The function of the lens capsule in the accommodation of the eye

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Abstract
Although abundant evidence is now available in support of the general principle of Helmholtz's theory of accommodation, no definite explanation has been given of the formation of the "lenticonus" discovered by Tscherning, which led him to challenge that theory. In a paper which appeared in these Transactions in 1925 the present author suggested that the varying thickness of the capsule covering the anterior surface of the lens would account for the type of surface which Tscherning had described. The present paper describes the determination of the form of the anterior surface of the lenses of freshly dead animals, by making photographic records of the image reflected from the surface. The lenses of a number of different species were examined and records made when the lens was held under tension by its normal suspensions and also when the suspensions were removed. The thickness of the capsule in each case was then measured microscopically. The results show that in primates the anterior surface of the lens assumes a somewhat conical form with an area of increased curvature in the centre, when the suspensions are severed. The capsule of the primate lens has a zone of increased thickness surrounding a central thin area. Photographs of the profile view of a monkey's lens, which was removed from the eye and suspended in oil, showed it to be in the typical accommodated form. A similar view of the lens without its capsule showed a close approach to the unaccommodated form. The anterior lens capsule of animals of an order lower than the primates was found to be approximately uniform in thickness. The lenticonus is not found in these cases, and the removal of the capsule does not cause an appreciable change in the form of the lens. From these findings the theory is formulated that accommodation consists of a relaxation of tension upon the lens by the contraction of the ciliary muscle as stated by Helmholtz. This relaxation allows the capsule to press upon the lens substance and mould it into the accommodated form. In the unaccommodated state the tension under which the lens is held by its supporting fibres does not permit this compression. The unaccommodated lens substance is therefore in its unrestricted or natural form and not under compression as supposed in the Helmholtz theory.

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