422 STRUCTURE OF THE IRIS. 



covered with a black pigment like the choroides, and has been 

 commonly called Uvea* 



This black pigment, on the internal or posterior surface of 

 the iris, has been supposed to have a great effect in determining 

 its general color; and there is some reason for this opinion, 

 as the iris is partly transparent ; but there is a color inhe- 

 rent in it, which is most evident when the black pigment is very 

 carefully washed away.f 



Thus, the light hazel color remains unchanged after the black 

 pigment is removed ; and it is not probable that light blue is 

 much influenced by the black pigment. 



The iris is capable of dilatation and contraction to a very 

 considerable degree, by which the pupil, or central vacuity is 

 enlarged or diminished. By this means it regulates the quantity 

 of light admitted into the eye. Upon the first exposure to 

 strong light, the pupil of every healthy person is observed 

 to be diminished, and upon the diminution of light, to be 

 enlarged. 



It has been ascertained by experiment, that this motion of 

 the iris is not excited by the action of light immediately upon 

 it, but on the interior surface of the eye ; and this circumstance 

 has occasioned the greatest attention to the structure of this 

 extraordinary and important membrane. 



Anatomists of the greatest respectability hold opinions 

 extremely different, indeed, respecting the structure of the iris. 

 They agree that there are many radiated fibres on the posterior 

 surface, commencing at its circumference, which can be readily 

 seen when the black pigment is washed away ; but while many 

 respectable anatomists declare that they have not been able to 



* So called from its color, which is dark brown, resembling the skin of a 

 raisin. The inherent color of the iris is either light blue or orange, which, with 

 the pigment on its back part that varies in thickness and in intensity of die in 

 different subjects, form all the variety of colors of the human eye. p. 



f Sometimes the pigment is strewed here and there in patches, thicker than 

 in other parts, which gives a marbled appearance to the iris. The pigment, 

 which, like that of the choroid coat, is a secretion from the vessels of the part, 

 has reflected over it, the serous membrane of the posterior chamber of the 

 eye. p. 



