PHYSIOLOGICAL ANATOMY OF THE EYEBALL 655 



membrane covering the general anterior surface of the iris is extremely 

 thin and is covered with cells of tessellated epithelium. Just beneath 

 this membrane are a number of irregularly-shaped pigment-cells. 



The posterior layer of the iris is thin, easily detached from the 

 middle layer, and contains a number of small cells rich in pigmentary 

 granules. Some anatomists recognize this membrane only as the uvea. 



The middle layer constitutes by far the greatest part of the substance 

 of the iris. It is composed of connective tissue, muscular fibres of the 

 non-striated variety, many bloodvessels and probably nerve-terminations. 

 Directly surrounding the pupil, forming a band about -^ of an inch 

 (0.5 millimeter) in width, is a layer of non-striated muscular fibres, 

 called the sphincter of the iris. In addition to the sphincter, are radi- 

 ating fibres, which can be traced from near the circumference of the 

 iris almost to its pupillary border, lying both in front of and behind the 

 circular fibres. 



The bloodvessels of the iris are derived from the arteries of the 

 choroid, from the long posterior ciliary and from 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 

 of the iris empty into a circular sinus situated at the junction of the 

 cornea with the sclerotic. This is called the circular venous sinus, or the 

 canal of Schlemm. 



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

 sensory, and the short ciliary, from the ophthalmic ganglion. The cir- 

 cular fibres are animated by filaments from the third nerve, which pass 

 through the ophthalmic ganglion. The radiating fibres receive filaments 

 derived from the cervical sympathetic, probably through the carotid 

 plexus and the ganglion of Gasser. 



Pupillary Membrane. At a certain period of foetal life the pupil is 

 closed with a membrane connected with the lesser circumference of the 

 iris, called the pupillary membrane. This is not distinct during the first 

 months ; but between the third and the fourth months, it is readily seen. 

 It is most distinct at the sixth month. The membrane is thin and trans- 

 parent and completely separates the anterior from the posterior chamber 

 of the eye. It is provided with vessels derived from the arteries of the 

 iris, anastomosing with each other and turning back in the form of 

 loops near the centre. At about the seventh month it begins to give 



