338 IMS STRUCTME. 



effected by the radiating fibres, drawing it apart, wliile at 

 the same time the circular ones are relaxed. 



The posterior surface is deeply stained, as stated, with 

 the black pigment, which is said to be protected by the 

 very delicate membrane of the aqueous humor. 



The iris is abundantly supplied with blood vessels and 

 nerves. The arteries come from the long ciliary, which, 

 after entering the sclerotic behind, pass forward between 

 this coat and the choroid, one on either side of the eye, 

 parallel to the equator owdi, to the ciliary ligament, here 

 each of them divides, and after forming a circle round the 

 circumference of the iris, sends off radiating branches which 

 converge towards the pupil, and there form anastomotic 

 arches. Some muscular branches of the ophthalmic also 

 supply the iris from the front. The veins are more numer- 

 ous than the arteries and return the blood either by the 

 ciliary veins, or into the vasa-vorticosa. The nerves of the 

 iris are large, and are supplied by the ciliary nerves, which 

 come from the ophthalmic ganglion. This ganglion is con- 

 nected with a sentient branch of the ophthalmic, one of the 

 divisions of the fifth, a motive nerve, one of the branches 

 of the third, and with branches of the sympathetic. From 

 this source the ciliary nerves, about 12 or 14 in number, 

 pass through the sclerotic behind, and then go forward 

 on the surface of the choroid to the ciliary ligament, which 

 they enter in great numbers, where they are distributed to 

 the iris. From this bountiful supply of nerves, both of 

 sensation and motion, we can readily understand the great 

 sensibility, and the delicate and rapid movements of 

 the iris. 



The iris is three or four times thicker than the choroid, 

 and becomes thinner towards the pupillary margin. The 

 pupil is closed the greater part of uterine life, by a delicate 

 membrane, called membrana pupillaris. It is represented 

 as most distinct about the fifth month, and as disappearing 

 about the seventh. This membrane separates the anterior 

 and posterior chambers of the eye, and prevents any com- 

 munication between them till, as just stated, after the 



