932 



TEE SENSORY AIWA RAT USES. 



fibres which are arruugcd in orbicular fasciculi, or extend backwards fand are 

 lost in the choroid, behind the ciliary processes). These fibres are mixed in the 

 plexus of ciliary nerves, on the track of which small ganglia are formed. By its 

 contractions, the ciliary circle (or muscle) plays an important part in accommo- 

 dating the eye to the perception of objects at different distances. (In Birds, the 

 muscular fibres are striped.) 



The ciliary body ox processes are formed by intercrossed fasciculi of (fibrillated) 

 connective tissue, vessels, and some unstriped muscular fibres ; their inner 

 surface is covered ))y pigment, like that of the choroid zone. 



4. The Iris (Figs. 502,/; 505, 6). 



The iris forms in the interior of the eye — at the anterior opening of the 

 sclerotic, and in front of the crystalline lens — a veritable diaphragm pierced with 

 a central opening — the pupil — which contracts or dilates according to the 

 intensity of the light and the distance of the objects to which the vision is 



Fig. 505. 



THE EYE (human) WITH THE SCLEROTIC COAT REMOVED. 



1, Sclerotic coat ; 2, veins of the choroid ; 3, ciliary nerves ; 4, veins of the choroid, or vence 

 vorticosa ; 5, ciliary ligament ; 6, iris. 



directed. This diaphragm divides the space between the cornea and the anterior 

 face of the lens, and internal extremities of the ciliary processes, into two 

 compartments or chambers of unequal size ; the anterior space is the largest, the 

 posterior having only a virtual existence, as the iris is close to the crystalline lens. 



In shape, the iris is elliptical, like the cornea and the sclerotic aperture. 



Its anterior face is flat or very slightly convex, and has very marked circular 

 furrows and radiating striae, noticeable only at the outer circumference of the 

 membrane. It is diversely coloured, not only according to species, but also in 

 individuals. In ^^olipeds, it has nearly always a brownish yellow tint ; though 

 sometimes it is nearly white or bright grey, when the animal is said to be " wall- 

 eyed." 



The posterior face, in relation with the lens and ciliary processes, is covered 

 by a very thick layer of pigment named the uvea. Portions of this pigment, 

 supported by a small pedicle, frequently pass through the pupillary aperture and 

 appear in the anterior chamber of the eye, where they are known as "soot-balls," 

 or corpora nigra. (There "are frequently several of these black spongy masses, 



