PIGMENT-CELLS. 



135 



Section of skin of the nose, showing 

 pigment. (Queckett.) 



The sebaceous follicles of the skin being Fig. 74. 



in close relation with the hair-bulbs, are 

 sometimes found distended with pigment. 

 This is especially the case in acne, a dis- 

 ease essentially consisting of an enlarge- 

 ment and suppuration of the sebaceous 

 follicles, and in which masses of black 

 matter may be pressed from them. Fig. 

 74 shows a section of the skin of the 

 nose having a stratum of black pigment 

 in the deepest portion of the cuticle, 

 which also dips down into the sebaceous 

 follicleSj seen on each side of the two hairs growing from the corium. 



Development of Pigment-cells. 



The stimulus of solar light doubtless exerts an influence on the 

 development of the pigment-granules. In many persons a strong 

 sunlight produces freckles ; and exposure for several years to a 

 tropical climate renders the fairest complexion sallow. The Dutch 

 who have for several generations resided in Africa, have at length 

 become so black as to be distinguished from the natives by their 

 features only, and not by their color. The natives of the torrid 

 zone almost invariably have black hair and eyes. The infant 

 negro is scarcely darker than the white during the first few days 

 after birth. 1 The genital organs become colored on the third day, 

 and the whole body on the fifth and sixth. 



The stimulus of solar light is essential to the development of the 

 green pigment (chlorophyl) of plants, which also consists, in great 

 part, of carbon ; and with this fact the preceding may be naturally 

 associated. 



Functions of Pigment-cells. 



The pigment-cells in the pigmentum nigrum of the eye are im- 

 portant principally in the way of interrupting or absorbing light, 

 and thus contributing to the perfection of vision. Hence albinoes 

 cannot see well in the full light of a sunny day. In the substance 

 and on the posterior surface of the iris, however, the pigment-cells 



1 Some writers maintain that the different varieties of the human race are due 

 to climatic influences, exerted through a long succession of generations. If color 

 alone were to be taken into the account, this idea would be more plausible. 



