104 



ZOOLOGY 



SECT. 



e.c 



e.cj 



tained 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 (E.) covered on its 

 outer or choroidal surface 

 with a layer of black pig- 

 ment (P. 2). It extends as 

 far as the outer ends of the 

 ciliary processes where it 

 appears to end in a wavy 

 line, the ora serrata (0. S.) : 

 actually, however, it is con= 

 tinued as a very delicate 

 membrane (p.c.E) over the 

 ciliary processes and the 

 posterior face of the iris. 

 The optic nerve (ON.)" 

 pierces the sclerotic and 

 choroid and becomes con- 

 tinuous with the retina, its 



fibres spreading over the inner surface of the latter. Microscopic 

 examination shows that these fibres, which form the innermost 

 layer of the retina (Fig. 740, o. n.\ turn outwards and become 

 connected with a layer of nerve-cells (n. c.). External to these 

 come other layers of nerve-cells and granules, supported by a 

 framework of delicate fibres, and finally, forming the outer surface 

 of the retina proper, a layer of bodies called, from their shape, 

 the rods and cones (?.). These are placed perpendicularly to the 

 surface of the retina, and their outer ends are imbedded in a 

 single layer of hexagonal pigment cells, loaded with granules of 

 the black pigment already referred to. 



Immediately behind and in close contact with the iris is the 

 transparent biconvex lens (Fig. 739, L.), formed of concentric layers 

 of fibres each derived from a single cell. The lens is enclosed in 

 a delicate capsule, attached by a suspensory ligament (sp. /.) to the 

 ciliary processes. The suspensory ligament exerts a pull upon the 

 elastic lens so as to render it less convex than when left to itself ; 

 when the ciliary muscles contract they draw the suspensory 

 ligament towards the iris and allow the lens to assume its normal 



FIG. 739. Diagrammatic horizontal section of the 

 eye of "Man. c. cornea; clt. choroid (dotted); 

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

 c. cj. conjunctiva; /.. o. yellow spot; /.iris; L, 

 lens ; ON. 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. L suspensory ligament ; Scl. sclerotic ; V. H, 

 vitreous. (From Foster and Shore's Physiology.) 



