52 SPECIAL SENSES. 



border, and thickest at about the junction of its inner third 

 with the outer two-thirds. It slightly projects anteriorly and 

 divides the space between the lens and the cornea into two 

 chambers, anterior and posterior, the anterior chamber being 

 much the larger. Taking advantage of a property of the 

 crystalline lens, called fluorescence, which enables us, by con- 

 centrating upon it a blue light, to see the boundaries in the 

 living eye, Helmholtz has demonstrated that the posterior 

 surface of the iris and the anterior surface of the lens are 

 actually in contact, 1 except, perhaps, for a certain distance 

 near the periphery of the iris. This being the case, the pos- 

 terior chamber is very small, and only exists near the margins 

 of the lens and the iris. 



The color of the iris is very different in different indi- 

 viduals. Its anterior surface is generally very dark near the 

 pupil, and presents colored radiations toward its periphery. 

 Its posterior surface is of a dark-purple color, and is covered 

 with pigmentary cells. 



The entire iris presents three layers. The anterior layer 

 is continuous with the membrane of the aqueous humor. At 

 the great circumference, it presents little fibrous prolonga- 

 tions, forming a delicate dentated membrane, called the liga- 

 mentum iridis pectination. The membrane covering the 

 general anterior surface of the iris is extremely thin, and is 

 covered by cells of tessellated epithelium. Just beneath this 

 membrane, are a number of irregularly-shaped pigmentary 

 cells. 



The posterior layer of the iris is very thin, easily detached 

 from the middle layer, and contains numerous small cells ex- 

 ceeding rich in pigmentary granules. Some anatomists rec- 

 ognize this membrane only as the uvea. 3 



The middle layer constitutes by far the greatest part of 



1 HELMHOLTZ, Des progres recents dans la throne de la vision. Revue des 

 cours scientifiques, Paris, 1868-1869, tome vi., p. 217. 



2 The name uvea was applied, at one time, to the choroid with the iris, again 

 to the iris alone, and again to the posterior, or pigmentary layer of the iris. To 

 avoid confusion, this term will not be again used. 



