SECT. 225.] THE IRIS. 549 



cornea ; it rather appearing to me to commence in the substance 

 of the iris at the ciliary border. It consists of numerous narrow 

 fasciculi, which, far from forming a conuected membrane, run 

 inwards independently between the vessels, on the posterior sur- 

 face of the iris more than on the anterior, and become inserted 

 at the border of the sphincter. 



The iris also differs from the choroid in possessing a layer of 

 cells on its anterior and posterior surfaces. The latter, the so- 

 called uvea of authors, or the black pigment of the iris, is a layer, 

 o - oo8'" to O'oi'" thick, of small cells densely filled with pigment, 

 similar to those of the corpus ciliare, with which they are also 

 uninterruptedly connected; this layer covers the whole posterior 

 surface of the iris, and extends up to the border of the pupil. On 

 folding the iris, the pigment layer appears limited on its free 

 surface by a fine but well marked line, which has been described 

 by several anatomists as a special membrane ('menibranapigmenti/ 

 Krause ; 'membrana limitans/ Pacini, Briicke, and H. Midler ; 

 M. Jacobi, Arnold)', and in truth it may be partially raised from 

 the pigment in old eyes, or after the addition of alkalies. Since, 

 however, in such cases the pigment-lamina is always destitute of a 

 sharp contour, and its granules are exposed and scattered, this mem- 

 brane appears to me to be nothing but the outer walls of the pig- 

 ment-cells united together, so that they may be separated apparently 

 as a special membrane. — The layer of cells of the anterior surface 

 of the iris, is a simple epithelium, of roundish and considerably 

 flattened cells ; and when the iris is folded, this layer does not 

 appear as a continuous clear edge of uniform width, but is dis- 

 tinguished rather by the separate slight elevations on it. This 

 layer may be still better recognised when the iris is viewed on its 

 surface, after the removal of the posterior pigment, and also in 

 scrapings of the anterior surface of the iris. — The colour of the iris, 

 in blue eyes, is simply owing to the posterior pigment seen through 

 it; while in yellowish-brown, brown and black eyes, the colour 

 is due to a special pigment, which is very irregularly distributed 

 through the iris, and then produces the peculiar appearances seen on 

 the anterior surface. The pigment is situated, firstly, in the stroma 

 itself, especially in its plasmatic cells ; secondly, it appears to me 

 to exist free between the fibres and vessels, and in the fibre-cells 

 of the sphincter pupillse; lastly, it is found in the anterior epi- 

 thelial layer, where it consists of larger and smaller granules, 

 heaps and stripes irregularly disposed, and having a yellow, 





