116 



ZOOLOGY. 



light 



increases, 

 of the 

 adapted 

 varying 

 by the 



the outer face of this as a thin, transparent epithelium, 

 called the conjunctiva, The choroid coat is continued in 

 front by a muscular curtain, the iris, which shows through 



the cornea as the 

 coloured portion of 

 the eye. This iris 

 has a central aper- 

 ture (the pupil), and 

 as the amount of 

 diminishes or 

 the size 

 pupil is 

 to the 

 conditions 

 greater or 

 less contraction of 

 the muscle-fibres of 

 the iris: this 

 adaptation is a 

 reflex action. The 

 iris serves precisely 

 the same function as the diaphragms or " stops " used in 

 the photographic camera or under the stage of the 

 microscope, viz. the cutting off of oblique rays of light 

 that tend to fog the image. 



Just behind the iris is the transparent biconvex lens, the 

 curvature of whose surfaces may be changed at will by the 

 contraction of the ciliary muscles which pass from the 

 sclerotic into the choroid, and pull the latter forward. By 

 this alteration of the curvature of the lens and therefore 

 of its focal length, the image of either a near or a distant 

 object can be clearly focussed on the retina. The lens 

 separates off from the main chamber of the eye containing 

 vitreous humour, a smaller anterior chamber containing 

 a much more liquid aqueous humour. 



9. The Retina. Fig. 63 shows in a very diagrammatic 

 way the structure of this essential layer of the eye. The 

 only structures which are actually sensitive to light are 

 certain modified cells, known as the rods and cones. These 



Fig. 62. EYE OF BABBIT. 

 Diagram of a median section. 



