ANATOMY OF THE EYEBALL. 53 



the substance of the iris. It is composed of connective tissue, 

 muscular fibres of the non-striated variety, numerous blood- 

 vessels, and, probably, nerve-terminations. From a physio- 

 logical point of view, the arrangement of the muscular fibres 

 is the most interesting. Directly surrounding the pupil, 

 forming a band about -fa of an inch in width, is a layer of 

 non-striated muscular fibres, called the sphincter of the iris. 

 The existence of these fibres is admitted by all anatomists. 

 It is different, however, for the radiating muscular fibres. 

 Most anatomists describe, in addition to the sphincter, fibres 

 of the same variety, which can be traced from near the great 

 circumference of the iris almost to its pupillary border, lying 

 both in front of and behind the circular fibres, which are, as 

 it were, enclosed between them. A few observers deny that 

 these fibres are muscular ; but they recognize a thick muscular 

 layer surrounding the arteries of the iris. This is merely a ques- 

 tion of observation ; but the weight of anatomical authority 

 is greatly in favor of the existence of the radiating fibres ; and 

 their presence explains certain of the phenomena of dilatation 

 of the iris which would otherwise be difficult to understand. 



The blood-vessels of the iris are derived from the arteries 

 of the choroid, the. long posterior ciliary, and the anterior 

 ciliary arteries. The long ciliary arteries are two branches, 

 running along the sides of the eyeball between the sclerotic 

 and choroid, to form, finally, a circle surrounding the iris. 

 The anterior ciliary arteries are derived from the muscular 

 branches of the ophthalmic. They penetrate the sclerotic a 

 little behind the iris, and join the long ciliary arteries in the 

 vascular circle. From this circle, the vessels branch and pass 

 into the iris, to form a smaller arterial circle around the pupil. 

 The veins from the iris empty into a circular sinus situated 

 at the junction of the cornea with the sclerotic. This is 

 sometimes spoken of as the circular venous sinus, or the canal 

 of Schlemm. 



The nerves of the iris are the long ciliary, from the fifth 

 cranial, and the short ciliary, from the ophthalmic ganglion. 



