PIGMENT, 
203 
pigmentum nigrum , within these are found innumerable 
minute granules, which, if examined soon after death, 
exhibit a molecular movement. The size of these 
granules does not generally exceed the ^th of an inch 
in diameter, the depth of the colour depending upon the 
quantity aggregated within a certain space. The granules 
are said to consist of a peculiar animal principle, the 
chief constituent of which is carbon, and on this account 
neither the strongest acids nor chlorine is capable of 
destroying the colour. The most common form of the 
cells is hexagonal, as in the pigmentum nigrum before 
described ; but between the choroid and sclerotic they 
are somewhat fusiform, and occasionally have bifid 
extremities. In some of the lower animals, as the 
reptiles and fishes, they have a stellate appearance, and 
in some cases bear a strong resemblance to bone cells. 
The principal seat of pigment in animals is the cuticular 
layer of the skin, in which it also occurs in hexagonal 
cells. In the negro the black colour was formerly 
supposed to occur only in a distinct layer of the 
skin, termed the rete mucosum , but such is now 
found not to be the case, the rete mucosum being 
merely the layer of cuticular cells last formed, and 
which contain pigmental granules. Similar cells are 
present in Europeans, those of the last deposited 
layer being always darker than those nearer the free 
surface ; the difference in quantity of the pigment in the 
cells gives rise to all the varieties of colour which the 
skin of the human race presents. In Albinoes the cells 
