THE EYE. 735 



anastomose more closely, and run in a circular direction. The ciliary 

 muscle terminates at the most adherent portion of the ciliary processes, 

 though not in those processes themselves, and as regards the elements 

 of which it is composed, they are rather shorter (0-02 of aline) and 

 broader (0'003-0'004 of a line) than the common fibre-cells; being at 

 the same time finely granular, soft, and so perishable, as, in Man, not 

 readily to admit of being isolated. 



The pigmentum nigrum (Fig. 296 m) is a continuous, purely cellular 

 layer, completely investing the inner surface of the choroid and consist- 

 ing, as far as to the ora serrata, of a single layer of well-formed, almost 

 regularly hexahedral, contiguous cells, O'OOG-O'OOS of a line in diame- 

 ter, 0'004 of a line thick, disposed in an elegant mosaic manner, in 

 which the large quantity of brownish-black pigment usually prevents the 

 nucleus being apparent as more than a clear spot in the interior. On 

 the side towards u the retina, however, a narrow clear border is frequently 

 left free of color, showing that the cells must, at one time, have pos- 

 sessed contents or have had a thickened mem- 

 brane. From the ora serrata onwards, the Fig. 300. 



/L 



pigment-cells are disposed in several, at least 

 two, layers, become, rounded, smaller, and 

 entirely filled with pigment, so that the nuclei 

 even are scarcely visible. All the pigment- 

 cells have extremely delicate walls, and are 

 very easily ruptured under pressure ; the pig- 

 ment is composed of minute, flattened, oval corpuscles, at most 0-0007 

 of a line long, and presenting, sometimes even while contained in the 

 cell, but still better when liberated, the phenomenon of molecular motion 

 in the most marked manner. The pigment of the clioroid is wanting in 

 the eyes of albinoes, as well as, at any rate partially, in the region of 

 the tapetum in animals ; but the cells, which would elsewhere contain it, 

 exist in both these instances, only perfectly colorless. 



The iris differs from the choroid in containing true connective tissue, 

 the delicate, lax fasciculi of which, partly radiating, in part circular, 

 especially at the ciliary border, and much interlaced, constitute the prin- 

 cipal bulk of the stroma of this tunic, and, towards its surface, form a 

 more homogeneous layer. It contains a large number of elongated 

 nuclei, which, at any rate in part, are situated in fusiform cells, similar 

 to those of the choroid, only smaller; and also a few rigid, pale fibres, 

 which, as prolongations of the ligamentum pectinatum of the iris, or of 

 the "membrane of Demours," are continued over a part of the anterior 

 surface; lastly, the smootH muscular fibres of the iris, presenting ex- 

 actly the same characters as those of the choroid [ciliary muscle]. In 

 Man, these fibres constitute a very distinct occlusor muscle of the pupil 



FIG. 300. Cells of the pigmentum nigrum of Man : o, viewed on the surface ; 6, on the 

 side ; c, pigment-granules. 



