THE EYE. 363 



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'") and broader (0-003—0' 004"') 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 consisting, as far as to the or a serrata, of a 

 single layer of well formed, almost regularly hexahedral, con- 

 tiguous cells, 0-006 — 0008"' in diameter, 0-004'" thick, dis- 

 posed 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 the retina, however, a narrow clear border is fre- 

 quently left free of colour, showing that the cells must, at one 

 time, have possessed contents or have 

 had a thickened membrane. From the « 



ora serrata onwards, the 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 pigment is composed 

 of minute, flattened, oval corpuscles, at most 0-0007"' 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 

 choroid 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 colourless. 



The iris differs from the choroid in containing true con- 

 nective tissue, the delicate, lax fasciculi of which, partly radiating, 

 in part circular, especially at the ciliary border, and much in- 



Fig. 300. Cells of the pigmentum nigrum of Man s a, viewed on the surface ; b, 

 on the side ; c, pigment-granules. 



