Walsh & Hoyt: Accomodation and the Autonomic Nervous System

As noted above, when normal persons shift fixation from far objects to near objects, the near response occurs. During this response, both eyes converge, pupillary constriction occurs, and a change also occurs in the optical system of the eye. This optical change, termed accommodation, is controlled by an elaborate sensorimotor mechanism that monitors the clarity of the image focused upon the retina. The peripheral motor effector for this mechanism is the muscle of the ciliary body. This smooth muscle is activated primarily by the mesencephalic, parasympathetic outflow but is also influenced by cervical sympathetic outflow as well. The ciliary body containing the ciliary muscle encircles the anterior, inner surface of the eye as a girdle. It conforms externally to the curvature of the overlying sclera and has a triangular shape in cross-section, with the base of the triangle facing the anterior chamber and the apex directed posteriorly toward the retina. Anteriorly, the ciliary body is continuous with the iris, whereas posteriorly, it is continuous with the rest of the uveal tract and with the overlying peripheral retina at the ora serrata. The external surface of the ciliary body lies adjacent to the sclera, being separated from it by the suprachoroidal space. The internal surface faces the vitreous. The ciliary body contains three layers: (a) unpigmented ciliary epithelium; (b) pigmented ciliary epithelium; and (c) the ciliary muscle, which controls accommodation. The ciliary muscle consists of meridional, radial, and circular muscle fiber orientations. Rohen suggested that the whole muscle is interconnected, with the muscle bundles forming a three-dimensional reticulum consisting of muscle cells that show considerable interweaving from layer to layer.