to the cornea, where it terminates in a series of folds, the ciliary processes. In its structure the choroid is highly vascular, consisting of both arteries and veins. Externally it is connected with the sclerotic by connective tissue; internally it is lined by a layer of hexagonal pigment cells which, though usually classed as belonging to the choroid, is now known to belong, embryologically and physiologically, to the retina. From without inward may be distinguished the following layers : 1. The lamina supra choroidea. : 2. The elastic layer of Sattler, consisting of two endothelial layers. 3. The chorio-capillaris, choroid proper, or membrane of Ruysch, a thick, elastic network of arterioles and capillaries lying within the outer layer of veins and arteries called the vena vorticosæ. 4. The lamina vitrea or internal limiting membrane. The choroid with its contained blood-vessels bears an important relation to the nutrition of the eye; it provides for the blood supply, for drainage from the body of the eye, and presents an uniform and high temperature to the retina. The iris is the circular variously-colored membrane placed in the anterior portion of the eye just behind the cornea. It is perforated a little to the nasal side of the center by a circular opening, the pupil. The outer or circumferential border is connected with the cornea, ciliary muscle, and ciliary processes; the free inner edge forms the boundary of the pupil, the size of which is constantly changing. The framework of the iris is composed of connective tissue blood-vessels, muscular fibers, and pigmented connective-tissue corpuscles. The anterior surface is covered with a layer of epithelial cells continuous with those covering the posterior surface of the cornea; the posterior surface is lined by a limiting membrane bearing pigment epithelial cells continuous with those of the choroid. The various colors which the iris assumes in different individuals depend upon the quantity and disposition of the pigmentary granules. The muscular fibers of the iris, which are of the non-striated variety, are arranged in two sets, the sphincter and dilator. The sphincter pupillæ is a circular flat band of muscular fibers surrounding the pupil close to its posterior surface; by its contraction and relaxation, the pupil is diminished or increased in size. The dilator pupillæ consists of a thin layer of fibers arranged in a radiate manner; at the margin of the pupil they blend with those of the sphincter muscle, while at the outer border they arch to form a circular muscular layer. The ciliary muscle is a gray circular band consisting of unstriped muscular fibers about one-tenth of an inch long running from before backward. It is attached anteriorly to the inner surface of the sclerotic and cornea, and posteriorly to the choroid coat opposite the ciliary processes. At the anterior border of the radiating fibers and internally are found bundles of circular muscular fibers, constituting the annular muscle of Müller. The ciliary muscle thus consists of two sets of fibers, a radiating and circular, both of which are concerned in effecting a change in the convexity of the lens in the accommodation of the eye to near vision. The Retina forms the internal coat of the eye. In the fresh state it is a delicate, transparent membrane of a pink color, but after death soon becomes SCLEROTIC COAT REMOVED TO SHOW THE CHOROID, CILIARY MUSCLE, AND NERVES. a. Sclerotic coat. b. Veins of the choroid. c. Ciliary nerves. d. Veins of the choroid. e. Ciliary muscle. f. Iris.-From Holden's Anatomy. opaque; it extends forward almost to the ciliary processes, where it terminates in an indented border, the ora serrata. In the posterior part of the retina at a point corresponding to the axis of vision is a yellow spot, the macula lutea, which is somewhat oval in shape and tinged with yellow pigment. It presents in its center a depression, the fovea centralis, corresponding to a decrease in thickness of the retina; about one tenth of an inch to the inner side of the macula is the point of entrance of the optic The arteria centralis retina pierces the optic nerve near the sclerotic, runs forward in its substance, and is distributed in the retina as far forward as the ciliary processes. nerve. |