206 PHYSIOLOGY FOR BEGINNERS CHAP. 



of granules of black pigment ; a similar layer of cells loaded 

 with granules of black pigment always covers the back of the 

 iris. Just before the choroid becomes continuous with the 

 iris it is thrown into a number of folds or plaits arranged in 

 a radiating manner all round. These folds are called the 

 ciliary processes. They, like the rest of the choroid, are 

 covered by a layer of black pigment cells. 



Situated at the back of the pupil, immediately behind the 

 iris, is a transparent double convex body called the crystalline 



scl 



FIG. 96. Dissection of bullock's eye. 



scl, sclerotic ; c, cornea turned aside ; c', edge of cornea left ; /', iris ; f, part of iris 

 turned aside to show ciliary processes, cp ; I, dotted line to indicate edge of the 

 lens. 



lens. The crystalline lens is kept in place by a sheet of trans- 

 parent tissue passing off from its edge, and attaching it all 

 round to the choroid processes. This sheet, which forms a 

 ring all round the lens, is called its suspensory ligament. 



Lying on the choroid coat at the back of the eye is a third 

 membrane. This is the important membrane of the eye, and 

 is called the retina. The sclerotic and choroid coats are 

 chiefly to protect the retina and to supply it with blood. The 

 retina, which is loosely attached to the choroid, lines the hinder 

 two-thirds of the eyeball and ends just behind the ciliary pro- 

 cesses. The fibres of the optic nerve, entering the eyeball 

 behind, pierce the sclerotic and choroid coats, and so pass into 

 the retina in which they radiate out on all sides. Hence the 



