Mechanisms of accommodation and refractive error.

The classical or Helmholtz theory of accommodation has formed the basis of optometric practice for many years. This theory maintains that the ciliary muscle receives only parasympathetic innervation and that the position of rest in the emmetropic eye lies at optical infinity. The suggestion of a dual sympathetic/parasympathetic innervation to the ciliary muscle also dates from the last century. Recent studies indicate that the "dark focus" or resting point of accommodation lies at some intermediate distance and that the refractive state in the absence of an adequate stimulus represents the tonus position of the ciliary muscle for minimal autonomic innervation. This paper considers the possible role of a sympathetic innervation to the ciliary muscle in the development and correction of refractive errors.