110 



ZOOLOGY 



SECT. 



e.c. fi 



dense fibrous tissue, tlie optic capsule or sclerotic (Fig. 784, scL). 

 On the outer or exposed portion of the eye the sclerotic is replaced 

 by a transparent membrane, the cornea (c), formed of a peculiar 

 variety of connective-tissue, and covered on both its outer and 

 inner faces by a layer of epithelium. The whole external. coat of 



the eye has thus the charac- 

 ter of an opaque spherical 

 case — the sclerotic, having 

 a circular hole cut in one 

 side of it and fitted with 

 a transparent window, the 

 cornea. The curvature of 

 the cornea is not the same 

 as that of the sclerotic; 

 the former is almost flat in 

 Fishes, but bulges outwards 

 in terrestrial Vertebrates. 



Lining the sclerotic is 

 the second coat of the eye 

 — the choroid (ch.) — formed 

 of connective-tissue abund- 

 antly supplied with blood- 

 vessels. At the junction of 

 sclerotic and cornea, it be- 

 comes continuous with a cir- 

 cular membrane (/), placed 

 behind but at some distance 

 from the cornea, and called 

 the ii'is. This latter is 

 strongly pigmented, the 

 colour of the pigment vary- 

 ing greatly in different species, and giving, as seen through the 

 transparent cornea, the characteristic colour of the eye. The iris 

 is perforated in the centre by a circular or slit-like aperture; the 

 pupil, which, in the entire eye, appears like a black spot in the 

 middle of the coloured portion. Except in Fishes, the pupil can be 

 enlarged by the action of a set of radiating unstriped muscle-fibres 

 contained in the iris, and contracted by a set of circular fibres ; 

 and the anterior or outer portion of the choroid, where it joins the 

 iris, is thrown into radiating folds, the ciliary processes {0. P.), 

 containing unstriped muscular fibres, the ciliary muscle. 



Lining the choroid and forming the innermost coat of the eye is 

 a delicate semi-transparent membrane, the retina {li.), covered on 

 its outer or choroidal surface with a layer of black pigment {P.E.). 

 It extends as far as the outer ends of the ciliary processes where 

 it appears to end in a wavy line, the ora serrata{o. s.) : actually, 

 however, it is continued as a very delicate membrane (fJ. c. B) over 



Fig. 784. — Diagi-aramatic horizoiital section of the 

 eye of man. c. cornea ; Ch. choroid (dotted) ; 

 C. P. ciliary processes ; e. c, epithelium of cornea ; 

 e, cj. conjunctiva ; / o. yellow spot ; /. iris ; i, 

 lens ; 0. iV. optic nerve ; os. ora serrata ; o — x, optic 

 axis ; p.c.ii. anterior non-visual portion of retina ; 

 P. B. pigmented epithelium (black) ; li. retina ; 

 s;?. I. suspensory ligament ; Bel. sclerotic ; V. if, 

 vitreous chamber. (From Foster and Shore's 

 Physiology. ) 



