154 THE HUMAN BODY. 



of Ruysch, corresponds to the retina. These two layers, 

 attached to each other by their internal surfaces, are covered 

 externally with a layer of pigment, which is thicker next to 

 the retina than on the side toward the sclerotic. The choroid 

 is pierced behind by an opening, which gives passage to the 

 optic nerve; in front, near the circumference of the cornea, 

 it separates in order to form the ciliary circle and ciliary 

 processes. The ciliary circle, ring, or muscle, is a little band, 

 vascular, like the choroid, slightly adherent by its external 

 surface to the sclerotic, and united by its lesser circumference 

 to the cornea, at the point where the latter attaches itself to 

 the sclerotic. Behind the ciliary circle a series of membran- 

 ous rays are seen joined together, and forming a crown; 

 these are the ciliary processes (from processus, prolongation, 

 ray), the whole of which constitute the ciliary body or disk. 

 These rays, which are attached to the choroid, like the ciliary 

 circle, are of two kinds; one into which the crystalline is 

 set, and which gives attachment to its capsule, termed the 

 ciliary processes of the vitreous body; the others extend to 

 the iris, behind which they form a sort of circular curtain, by 

 folding back on themselves, and adhering to the larger cir- 

 cumference of this membrane. Thus fixed by one border, 

 the ciliary disk floats by the other, like a fringe behind the 

 iris, yielding to the slightest impulse which may be com- 

 municated to it. The ciliary processes are covered with a 

 thick layer of pigment. 



Iris. In the space between the ciliary circle and the 

 ciliary process the larger circumference of the iris is fixed. 

 This is a muscular membrane, according to some writers, and 

 vascular according to others, forming a vertical partition 

 behind the cornea. The iris is pierced in the middle by a 

 circular opening called the//////. It represents exactly what 

 is called a diaphragm in optical instruments. Its anterior 

 surface is coloured in different shades according to the indi- 

 vidual, but always remarkable for their delicacy or their 

 intensity; the variety of which has given to the membrane 

 the name of iris, or the rainbow. Its posterior face is 

 covered with a layer of pigment, which is called the uvea. 



It is well known that the pupil dilates in the dark, and 



