Ixiv 



PIGMENT. 



Fig. XXXII. 

 A 



Fig. XXXII. PIGMENT-CELLS PROM 

 THE CHOROID ; MAGNIFIED 370 DIA- 

 METERS (Henle). 



A, cells still cohering, seen on their 

 surface ; ft, nucleus indistinctly seen. 

 In the other cells the nucleus is con- 

 cealed by the pigment granules. 



B, two cells seen in profile ; a, the 

 outer or posterior part containing 

 scarcely any pigment. 



cuticle of the European. In different forms also they exist on certain parts 

 of the investing membrane (pia mater) of the spinal cord, in the membranous 

 labyrinth of the ear, and (with brownish yellow pigment) on the olfactory 

 region of the nose. 



The pigment cells of the choroid membrane (fig. xxxn.) are for the most 

 part polyhedral in figure, most generally six-sided, and connected together 

 like the pieces of a mosaic pavement ; others are spheroidal, and most of 



those on the back of the iris are of 

 that shape. The cells contain the pig- 

 ment, strictly so called, which consists 

 of black or brown granules or mole- 

 cules of a round or oblong shape, and 

 almost too small for exact measure- 

 ment. These molecules are densely 

 packed together in some cells ; in 

 others they are more scattered, and 

 then it may be seen that there is a 

 certain amount of colourless matter in- 

 cluded along with them. When they 

 escape from the ruptured cells, they 

 exhibit very strikingly the molecular 

 movement ; and in consequence of this 

 movement the apparent figure of the 

 particles is subject to change. It is 

 worthy of remark, that when viewed 

 singly with a very high magnifying 

 power they look transparent and almost 



colourless, and it is only when they are heaped together that their black- 

 ness distinctly appears. The cells have a colourless nucleus, which is very 

 generally hidden from view by the black particles. It contains a central 

 nucleolus. 



Examined chemically, the black matter is found to be insoluble in cold 

 and hot water, alcohol, ether, fixed and volatile oils, acetic and diluted 

 mineral acids. Its colour is discharged by chlorine. The pigment of the 

 bullock's eye, when purified by boiling in alcohol and ether, was found by 

 Scherer to consist of 58 '672 carbon, 5 '962 hydrogen, 13*768 nitrogen, and 

 21-598 oxygen; its proportion of carbon is thus very large. Preceding 

 chemists had obtained from its ashes oxide of iron, chloride of sodium, lime, 

 and phosphate of lime. 



The dark colour of the Negro is known to have its seat in the cuticle, and 

 chit-fly in the deeper and softer part named the rete mucosnm. It is 

 caused by cells containing dark-brown colouring matter, either diffused 

 through their substance or in form of granules, usually more densely 

 aggregated round the nucleus. These cells are found along with ordinary, 

 colourless cells, which in other respects they entirely resemble : and the 

 depth of tint depends on the proportion of each. It i* affirmed, on good 

 authority, that the nuclei of these epidermic pigment-cells are coloured, but 

 of this I have not been able to satisfy myself iu examinations of the Negro 

 skin. The dark parts of the European skin owe their colour and its dif- 

 ferent shades to intermixture in the cuticle of similar cells in different pro- 

 portions. Lastly, it cannot be doubted, that in both the coloured and white 

 races, the colouring matter of the skin is the same in its essential nature 

 as that of the choroid. In Albino individuals, both Negro and European, 



