PRACTICAL CONSIDERATIONS : THE IRIS. 1461 



T^e nerves of the iris are branches of the ciliary nerves. They follow the course 

 of the blood-vessels and, branching, form a plexus of communicating nonmeduUated 

 fibres, which supply sensory, motor and vasomotor impulses. The human iris prob- 

 ably contains no ganglion cells. 



Practical Considerations. — The iris may be partially or completely absent, 

 when by bringing down the eyebrows and partially closing the lids, the patient 

 will make an effort to shut of! the excess of light, as in albinism, and the eye will 

 frequently be nystagmic. 



A congenital coloboma or deficiency in the iris is usually in the lower part, and 

 may be associated with a corresponding defect in the ciliary body and choroid. The 

 pupil may be eccentric in position (corectopia), unusually small (microcoria), 

 irregular in shape (discoria), or it may be represented by several pupils (polycoria). 

 The pupillary membrane of the foetus, covering the pupil, not infrequently persists 

 for a short time after birth. A portion of it persisting permanently is one of the 

 commonest congenital anomalies of the eye. 



The color of the ins varies according to the amount and location of the pigment 

 in it. When the coloring matter is absent from the stroma, and present only in the 

 •posterior layer of epfithelium, the eye is blue. If such an iris is thicker than usual 

 the opacity will be greater and the eye will tend to be grayish. When there is pig- 

 ment only in slight amount in the stroma, the eye is greenish, and when in marked 

 quantity in the stroma, the eye is brown or even black, as in negroes. The deepest 

 tints of brown are usually called black. 



In albinism there is an absence of pigment in the iris, and in the other parts of 

 the body where pigment i« usually found. The eyes are pinkish in color, because 

 the light enters through the tunics and is not absorbed by the choroid and retina, 

 owing to the absence of pigment in it. The retina is therefore intolerant of light, 

 so that the patient tries to shut it out by screwing up the eyebrows and lids, and by 

 contraction of the iris. He will frequently show nystagmus or oscillation of the 

 eyeball, and amblyopia, or subacuteness of vision. 



The two eyes are not always of the same color, and even in the same eye, one 

 part of the iris may be blue and another brown (piebald iris). One eye may have 

 its color permanently changed as the result of inflammation, so that the difference in 

 color may be an important diagnostic sign of previous disease. 



The iris acts as a colored curtain to shut off excess of light, as more or less 

 light is necessary for the definition of images. Too much light impairs the defini- 

 tion and injures the retina. The pupils are usually of equal size in health, and any 

 marked inequality has a pathological significance. The iris does not hang in a verti- 

 cal plane, but is pushed slightly forward and supported at its pupillary margin by the 

 lens. If the lens is absent or dislocated, the pupillary margin of the iris may be 

 seen to quiver with the movement of the eyes. The iris in spite of its great vascu- 

 larity may not bleed much when wounded, probably because of the contraction of 

 its abundant muscular fibres. The iris is continuous with the ciliary body, and 

 through the latter with the choroid, the three taken together making up the uveal 

 tract, or middle tunic of the eye. Any inflammation of the one may easily spread 

 to the others. This usually occurs, but as the inflammation is predominant in 

 one, we speak of an iritis, a cyclitis, or a choroiditis, and not of the whole pro- 

 cess as a uveitis. In an iritis the exudation which affects the stroma as well as 

 the anterior and posterior aqueous chambers can be studied by inspection. It 

 thickens and discolors the iris, renders the aqueous fluid turbid, and leaves a 

 deposit on the contiguous surfaces of the cornea and. lens. Since the pupillary 

 margin of the iris is in contact with the lens on the posterior surface the exudate 

 causes adhesions of this margin to the lens (posterior synechiae). Since the pupil 

 is contracted in inflammation, when these adhesions form, dilatation of the pupil 

 normally or under the influence of atropine, gives rise to a very irregular pupil, 

 the unattached portion dilating, the attached portions not. Sight need not be 

 affected if the pupil is large enough. If the whole margin of the pupil is attached 

 to the lens, or the pupil is occluded by exudate, the normal flow of fluid from 

 the posterior to the anterior chamber cannot take place, and glaucoma {vide supra), 



