TUNICS OF THE EYEBALL. 535 



The Ciliary processes, from sixty to eighty in number, and 

 arranged in a circle, are formed by the plaiting or folding inwards 

 of the middle and inner layers of the choroid ; their circumfer- 

 ence is attached to the ciliary muscle, and their central border is 

 received between corresponding foldings of the suspensory liga- 

 ment of the crystalline lens, or zonula of Zinn. The anterior 

 face of these processes is turned towards the back of the iris, and 

 the posterior face, closely connected with the zonula of Zinn, 

 through which connection a communication is established between 

 the inner and choroid tunics of the eyeball. Externally, they are 

 covered with a thick layer of pigmentary cells. 



The Iris is a thin diaphragm or curtain, suspended imme- 

 diately in front of the crystalline lens, its periphery being 

 connected with the choroid coat and ciliary muscle, and its 

 centre pierced by an elliptical open- 

 ing, the pupil. It is variously 

 coloured, but in the horse is brown, 

 with more or less of a yellow tinge ; 

 sometimes, howevei', it is almost 

 white or grey, when the animal is 

 said to be "wall-eyed." Its anterior 

 surface is slightly convex, and its 

 posterior surface covered with a 

 deep purple pigment, called the 

 uvea, from its resemblance in 

 colour to a ripe grape. 



Two sets of fibres enter into the formation of the iris, one of 

 which, converging from the circumference towards the centre, has 

 the power of dilating the pupil ; the other, surrounding the mar- 

 gin of the pupil on its posterior surface, and blending with the 

 radiating fibres, has the power of contracting it. 



In the foetus, the pupil is closed by a delicate transparent 

 vascular membrane, the memhrana pupillaris, which begins to 

 be absorbed a short time before birth, the absorption com- 

 mencing at the circumference. At the period of birth it is 

 almost entirely removed. 



The small bodies situated on the upper pupillary margin of 

 the iris, three or four in number, are the corpora nigra, their 

 use appearing to be that of concentrating the rays of light, and 

 preventing their too direct passage through the pupil. 



Third Tunic. — The last tunic of the eye is the Ketina, which 



