ACTION OF THE IRIS. 423 



see any circular fibres, Dr. Monro describes them minutely, 

 and has published a plate of them. He considers them as 

 forming a circle immediately round the pupil, which circle 

 makes about one-fifth part of the breadth of the iris.* 



In the human subject, the iris does not appear to be divisible 

 into lamina.f It is abundantly supplied with arteries, which 

 form two circles upon it, and it has also a large supply of nerves. 



The operations of the iris, in contracting the pupil, upon 

 exposure to light, and dilating it, when light is diminished or 

 withdrawn, have been explained very differently by persons 

 who have different sentiments respecting its structure. Some 

 of those persons, who do not believe in the existence of mus- 

 cular fibres in the iris, suppose its motions to depend upon the 

 sudden turgescence or depletion of its blood-vessels : while 

 others impute it to a peculiar quality, exclusively enjoyed by 

 this membrane. 



Dr. Monro, who refers the contraction of the pupil to the 

 action of the circular fibres, which is excited by the stimulus 

 of light applied to the retina, considers this operation as analo- 

 gous to that of the abdominal muscles, in those cases of cough- 

 ing, which are produced by irritation of the glottis. The 

 action of the muscles, in these instances, being excited by sub- 

 stances applied to the glottis, which would have produced no 

 irritation if they had been applied to the muscles themselves. 



The dilatation of the pupil appears, in the human species, if 

 not in other animals, to depend upon a different cause. It 

 seems to be a passive state of the eye, which takes place when 

 the stimulus is withdrawn, and the consequent action ceases. 

 There is some elasticity of the iris ; and a great many, perhaps 

 all, of its natural or healthy motions in the human species, 

 may be explained by the contraction of the circular fibres, 



* Si-e his publication, entitled Three Treatises: On the Brain, the Eye, and 

 the Ear. 



f According to H. Cloquet, the iris consists of two lamina, intimately united 

 near the pupil, but separable towards the greater circumference ; the anterior 

 layer appears to be composed mainly of the radiating muscular fibres, and the 

 posterior especially near the pupillary border of the circular or sphincter 

 muscle. It is, besides, lined, in front and behind with the transparent serous 

 inembraae of the aqueous chambers. — p. 



