110 



ZOOLOGY 



SECT. 



e.cj 



dense fibrous tissue, the 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 iris. 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 (C. 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, (H.), 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 (j;. c. R) over 



FIG. 784. Diagrammatic horizontal section ^ 

 eye of XKCan. c. cornea; Ch. choroid (d<,. 

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

 e. cj. conjunctiva ; /. o. yellow Fpot ; /. iris ; L, 

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

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

 P. E. pigmented epithelium (black) ; R. retina ; 

 sp. 1. suspensory ligament ; Scl. sclerotic ; V. H, 

 vitreous chamber. (From Foster and Shore's 

 Physiology. ) 



