THE IRIS. 311 



ments is always well defined, since that of the ciliary processes 

 is provided, as far as to its point of contact with the uvea, with 

 a layer of the ciliary portion of the retina. 



Histologically the uvea is composed of cells, the protoplasm 

 of which is infiltrated with pigment granules, which completely 

 conceal the nucleus. In specimens teazed out with needles, 

 clumps with rough surfaces, and of the most various size, are 

 commonly met with under the microscope. It is impossible 

 from these fragments to determine the form of the cells. The 

 nuclei, when completely freed from pigment, are spheroidal 

 and slightly granular. 



The free surface of the uvea possesses a series of radiating 

 slightly projecting folds, which extend from the pupillary to 

 the ciliary border in the form of regularly arranged straight 

 lines ; their number is from 70 to 80. 



In Man there is no investing membrane for this pigment 

 layer. That which was at one time described under the names 

 of the membrana limitans Pacini, Jacobi, pigmenti, is, accord- 

 ing to Kolliker, " the coalesced external cell walls of the pigment 

 cells ; " according to Henle, it is the border of the cementing 

 substance which holds the pigment granules together, a view 

 which appears so much the more probable, as no walls can be 

 perceived in the cells of these layers. 



The tissue of the iris, like that of the choroid, is composed of 

 vessels, muscles, nerves, and stroma. 



The vessels of the iris are characterized, speaking generally, 

 by the extraordinary thickness of their walls (Arnold), and 

 especially of their adventitia (Henle), which is considerably 

 thicker than all the rest of their coats put together. The mus- 

 culature of the vascular walls is also remarkably developed 

 (Arnold and Hiittenbrenner). 



The movements of the iris are effected by means of two 

 muscles, the sphincter, by which the pupil is contracted, and 

 the dilatator, by which it is enlarged. 



The sphincter of the pupil (fig. 365, a) occupies the pupillary 

 zone of the iris, and extends outwards to a distance of from 

 0*9 1'3 millimeters. At the pupillary border it is thin (having 

 a thickness of 0' 10 of a millimeter) ; but further outwards it 

 becomes thicker, and not far from its external margin it attains 



