THE SENSE OF SIGHT. 



643 



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

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

 posterior surface about 6 mm. 



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

 parent 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 supported by a similar membrane, the posterior elastic lamina or 

 the membrane of Descemet, 

 which at its periphery be- 

 comes continuous with the 

 iris. At the junction of the 

 cornea and sclera there is a 

 circular groove, known as 

 the canal of Schlemm. 



The posterior elastic lam- 

 ina, near the margin of the 

 cornea, breaks up into fibers 

 to form a network structure, 

 the intervals between the 

 fibers of which are known 

 as the spaces of Fontana. 

 These spaces are in com- 

 munication with the canal of 

 Schlemm. 



The Chorioid, Iris, 



Ciliary Muscle, and Ciliary FIG. 302. CHORIOID COAT OF THE EYE. i. Optic 



nerve. 2, 2, 2, 2, 3, 3, 3, 4. Sclerotic coat divided and 

 turned back to show the chorioid. 5, 5, 5, 5. The 

 cornea, divided into four portions and turned back. 

 6, 6. Canal of Schlemm. 7. External surface of the 

 chorioid, traversed by the ciliary nerves and one of the 

 long ciliary arteries. 8. Central vessel into which 

 open the vasa vorticosa. 9, 9, 10, 10. Chorioid zone, 

 ii, n. Ciliary nerves. 12. Long ciliary artery. 13, 

 I3> T 3> I 3- Anterior ciliary arteries. 14. Iris. 15, 15. 

 Vascular circle of the iris. 16. Pupil. (Sappey.} 



Processes. The chorioid is 

 the dark brown membrane 

 which extends forward nearly 

 to the cornea, where it ter- 

 minates in a series of folds, the 

 ciliary processes. Posteri- 

 orly, 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 containing black pigment which, though usually 

 described as a part of the chorioid, are now known to belong, embryolog- 

 ically and physiologically, 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. 



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 



