680 SPECIAL SENSES. 



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

 striated variety, many blood-vessels, and probably nerve-terminations. Di- 

 rectly surrounding the pupil, forming a band about -^ of an inch (0-5 mm.) 

 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 differ- 

 ent, however, for the radiating muscular fibres. Most anatomists describe, 

 in addition to the sphincter, non-striated fibres, which can be traced from 

 near the great circumference of the iris almost to its pupillary border, ly- 

 ing both in front of and behind the circular fibres. 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 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 ves- 

 sels 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 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 transparent, 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 atro- 

 phies, and 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 pro- 

 longations 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 a delicate, trans- 

 parent membrane covering the posterior portion of the vitreous humor. A 

 short time after death it becomes slightly opaline. It extends over the pos- 



