PHYSIOLOGICAL ANATOMY OF THE EYEBALL. 775 



and is covered by cells of tessellated epithelium. Just beneath this membrane, are a 

 number of irregularly-shaped pigmentary cells. 



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

 contains numerous small cells exceeding rich in pigmentary granules. Some anatomists 

 recognize this membrane only as the uvea. 1 



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, numerous 

 blood-vessels, and, probably, nerve-terminations. From a physiological point of view, 

 the arrangement of the muscular fibres is the most interesting. Directly surrounding 

 the pupil, forming a band about 4 of an inch in width, is a layer of non-striated muscu- 

 lar 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 anato- 

 mists 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 question of observa- 

 tion ; 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, 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 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. 



Pupillary Membrane. At a certain period of foetal life, the pupil is closed by a mem- 

 brane 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 it 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 way at the centre, gradually atrophies, and generally scarcely a 

 trace of it can be seen at birth. 



Retina. The retina is described by anatomists as the third tunic of the eye. It is 

 closely connected with the optic nerve, and the most important structures entering into 

 its composition are probably continuous with prolongations from the nerve-cells. This 

 is the membrane endowed with the special sense of sight, the other structures in the eye 

 being accessory. 



If the sclerotic and choroid be removed from the eye under water, the retina is seen, 

 in perfectly fresh specimens, in the form of an exceedingly delicate and transparent mem- 

 brane covering the posterior portion of the vitreous humor. A short time after death, it 

 becomes slightly opaline. It extends over the posterior portion of the eyeball to a dis- 



1 The name uvea was applied, at one time, to the choroid with the iris, again to the iris alone, and again to the 

 posterior, or pigmentary layer of the iris. To avoid confusion, this term will not be again used. 



