Autonomic innervation of the ciliary body: a modified theory of accommodation.

One of the more interesting and contro­ versial aspects of ocular physiology in recent years has been the relative importance of parasympathetic and sympathetic portions of the autonomie nervous system on visual ac­ commodation. As early as 1801 Young pro­ posed that changes in the refractive power of the eye were brought about by changes in the radius of curvature of the crystalline lens, and not by changes in eyeball axial length, corneal curvature or position of the lens. The Young hypothesis was rediscov­ ered by Helmholtz who strongly cham­ pioned this mechanism of refractive change. The "Helmholtz theory of accommodation," which is almost universally accepted, today, states that when the eye accommodates for a near object, the ciliary muscle contracts, op­ posing the elastic force of the choroid layer, decreasing the tension exerted by the sus­ pensory ligament on the lens capsule thereby allowing the elastic forces of the capsule to mold the lens to a more spherical form. This action of the ciliary muscle results when it is stimulated by the oculomotor nerve, part of the parasympathetic system. The role and relative importance of the sympathetic system in controlling accommo­ dation has been less clearly delineated. Since Horner's* initial observation in 1869, it has been known that individuals with long standing cervical sympathetic lesions evi­ denced no apparent alteration of distance accommodation. This phenomenon is in con-