THE SENSE OF SIGHT. 599 



of layers of connective tissue which are arranged transversely and 

 longitudinally. It is pierced posteriorly by the optic nerve about 

 3 or 4 millimeters internal to the optic axis. By virtue of its firmness 

 and density the sclera gives form to the eyeball, protects delicate 

 structures enclosed by it, and serves for the attachment of the muscles 

 by which the ball is moved. The cornea is the transparent mem- 

 brane forming the anterior one-sixth of the ball. It is nearly circular 

 in shape, measuring in its horizontal meridian 12 mm., in its vertical 

 meridian n mm. The curvature is therefore sharper in the latter 

 than in the former. The radius of curvature of the anterior surface 

 at that central portion ordinarily used in vision is 7.829mm.; that 

 of the posterior surface about 6 mm. 



The substance of the cornea is made up of thin layers of delicate 

 transparent fibrils of connective tissue continuous with those found 

 in the sclera. Lymph-spaces are present throughout the cornea, in 

 which are to be found lymph-corpuscles. The anterior surface of 

 the cornea is covered with several layers of nucleated epithelium 

 supported by a structureless membrane, the anterior elastic lamina. 

 The posterior surface also is covered by a layer of epithelium sup- 

 ported by a similar membrane, the membrane of Descemet, which 

 at its periphery becomes continuous with the iris. At the junction 

 of the cornea and sclera there is a circular groove, known as the 

 canal of Schlemm. 



The Chorioid, Iris, Ciliary Muscle, and Ciliary Processes. 

 The chorioid is the dark brown membrane which extends forward 

 nearly to the cornea, where it terminates in a series of folds, the 

 ciliary processes. Posteriorly, it is pierced by the optic nerve. It- 

 is composed largely of blood-vessels, arteries, capillaries, and veins, 

 supported by connective tissue. Externally it is loosely connected 

 to the sclera; internally it is lined by a layer of hexagonal cells con- 

 taining black pigment which, though usually described as a part of 

 the chorioid, is now known to belong, embryologically and physio- 

 logically, to the retina. Lying within the outer layer of arteries and 

 veins there is a thick layer of small arterioles and capillaries, known 

 as the chorio-capillaris. The chorioid with its contained blood-vessels 

 bears an important relation to the nutrition and function of the eye. 

 It provides a free supply of lymph and presents a uniform temperature 

 to the retina in contact with it (Fig. 277). 



The iris is the circular, variously colored membrane in the anterior 

 part of the eye just behind the cornea. It presents a little to the 

 nasal side of the center a circular opening, the pupil. The outer or 

 circumferential border is united by connective tissue to the cornea, 

 sclera, and ciliary muscle; the inner border forms the boundary of 

 the pupil. The iris consists of a framework of connective tissue sup- 

 porting blood-vessels, muscle-fibers, and pigmented connective-tissue 



