424 SPECIAL PHYSIOLOGY. 



glass set in its frame, is not quite circular, being somewhat broader 

 transversely than from above downwards. Its convexity varies in 

 different individuals ; it is more convex in early life, and in short- 

 sighted persons, and becomes flattened in old age. The radius of its 

 curve is said to range from rather more than Jth to nearly Jd of an 

 inch. Like the sclerotic, the cornea is composed of fibres, but these 

 are arranged more regularly, and are separable into closely -connected 

 layers; its anterior and posterior surfaces are formed by specially- 

 condensed, structureless, and highly elastic laminae. The inner sur- 

 face of the cornea forms the anterior boundary of a space within the 

 eyeball, called the anterior chamber -, a, and is lined by a single layer 

 of flat, polygonal, epithelial cells. Its anterior surface is covered by 

 a very fine extension of the conjunctiva, which reaches it from the 

 fore part of the sclerotic. Where the sclerotic joins the cornea, the 

 character and arrangement of the constituent fibres are altered, so 

 that they form a transparent, not an opaque coat. Both these struc- 

 tures are supplied with nerves; but the cornea is non-vascular, and 

 must receive its nutrient supply, indirectly, from the sclerotic and 

 conjunctival vessels. The sclerotic is about ^th of an inch, and the 

 cornea about j^th of an inch in thickness. 



The choroid coat, e, is a comparatively thin, tender, vascular, black 

 or brown, pigmentary membrane, which is perforated behind by the 

 optic nerve, and reaches forwards as far as a circular fibro- elastic 

 band, corresponding with, the line of junction of the cornea with the 

 sclerotic, named the ciliary ligament; with this, the anterior edge of 

 the choroid coat is firmly united. The outer surface of the choroid is 

 loosely connected with the sclerotic, by bloodvessels, nerves, and a fine 

 cellular web, the lamina fusca; within this, are curious whorled veins, 

 vence vorticosce, and numerous branching arteries, mixed with star- 

 shaped pigment cells; within these, is a network of exceedingly fine 

 and close capillaries; and lastly, the pigmentary layer, made up of 

 regularly-hexagonal nucleated cells, filled with pigment granules, Fig. 

 43, d. The choroid averages about aJ^th of an inch in thickness. 



The ciliary ligament, just mentioned, placed opposite the junction of 

 the sclerotic with the cornea, serves to connect those coats with the 

 choroid, and also supports numerous black or brown radiated folds or 

 rays, called the ciliary processes, Fig. 83, 6, which are prolongations of 

 the fore-part of the choroid. These processes, from 60 to 80 in num- 

 ber, are situated in a radiated manner around the margin of the crys- 

 talline lens, to be presently described; some of them, the larger ones, 

 are about T J th of an inch in length, and ^th of an inch thick, be- 

 tween which smaller ones are found. In these processes, the capillary 

 network is larger than in the choroid. 



On the surface of the fore-part of the choroid, is a yellowish-pink 

 band, about Jth of an inch broad, the ciliary muscle; this consists of 

 involuntary muscular fibres, some having a longitudinal, others a circu- 

 lar direction; the former arise from the line of junction of the cornea 

 and sclerotic, opposite the ciliary ligament, and posteriorly, are inserted 

 into the iris, the sclerotic ciliary processes, and the anterior part of the 

 choroid. This muscle has also been named the tensor of the choroid. 



