THE EYE. 365 



by several authors as a special membrane (membrana pigmenti, 

 Krause, m. limitans, Pacini [?], Briicke, M. Jacobi, Arnold), 

 and, in fact, in eyes that have been kept some time, and on 

 the addition of alkalies, may be raised in places from the pig- 

 ment. But since, in such instances, the pigmentary layer is 

 always without any defined outline, and its granules are ex- 

 posed and dispersed, it appears to me that this membrane is 

 nothing more than the conjoined outer cell-walls of the 

 pigment-cells, which, as is known to be the case elsewhere 

 (intestinal villi for instance), are raised in their totality, and 

 apparently as a special membrane. The cellular layer of the 

 anterior surface of the iris is a simple epithelium of rounded 

 and much flattened cells, which, when viewed in a fold of the 

 iris, are seen to constitute, not a continuous, clear border of 

 uniform breadth throughout, but on the contrary, only distinct, 

 slight elevations. This layer is better seen after the removal 

 of the posterior pigment, in a horizontal view, and also by 

 scraping or shaving off* the anterior surface of the iris} The 

 colour of the iris, in blue eyes, depends simply upon the pos- 

 terior pigment seen through its substance ; whilst in brownish- 

 yellow, brown, and black eyes, it is owing to a special iris- 

 pigment, which is very unequally distributed, and thus produces 

 the peculiar markings of the anterior surface. This pigment 

 is seated, in the first place, in the stroma itself, and in fact, 

 chiefly in its fusiform cells, but also, as it appears to me, occurs 

 free among the fibres and vessels, and in the fibre-cells of the 

 sphincter pupillce; lastly, in the anterior epithelial layer, it 

 consists of larger and smaller cells, gold-yellow or brownish 

 irregular-sized granules, aggregations of granules and streaks, 

 never of the regular pigment-granules of the true ocular pigment. 

 The vessels of the tunica vasculosa are extremely numerous, 

 and are variously disposed in its different parts. The choroid 

 receives its blood from the short posterior ciliary arteries, 

 about twenty small vessels, penetrating the sclerotic, in the 

 posterior part of the eye-ball, at a greater or less distance 

 from the optic nerve, and which, dividing in a dichotomous 

 manner in the middle or vascular layer of the choroid, run 

 anteriorly, and subdivide into three sets of branches : 1. external, 

 which, having attained a certain fineness by continued division, 

 1 [Vid. note, p. 357. — Eds.] 



