136 SIMPLE HISTOLOGICAL ELEMENTS. 



seem to be intended, at the same time, for ornament also giving 

 the peculiar color to the eye. The variously colored cells on the 

 surface of the lower animals apparently subserve the latter object 

 alone. We cannot, however, remark the same in regard to the 

 colored spots on the peritoneum of some of them. 



The presence of pigment-cells in the epithelium of the skin of 

 man bears some relation to the degree of solar light and heat to 

 which it is exposed ; but which is not well understood. 



Pigment-cells abound, as has been stated, in the human brain and 

 spinal cord. That the function of these organs, or of even the cells, 

 does not depend on the color of the contained granules, may be 

 inferred from the fact that in the spinal cord of the frog the cells 

 contain colorless, instead of colored, granules. 



The fact that the pigment-granules in the epidermic cells of the 

 areolae around the mammilla of the human female are increased in 

 pregnancy, is well known, and its darker color is regarded as one 

 of the signs of that condition. We can only associate this fact 

 with the development of the whole mammary gland at the same 

 time, in consequence of the sympathy existing between it and the 

 uterus. It may also be added that an increased tendency to develop 

 pigment-granules is often manifested in other parts of the body 

 during the last weeks of pregnancy, in the form of freckles, espe- 

 cially of the face, neck, and upper part of the chest. 



Regeneration of Pigment-cells. 



In the case of a young man who had a congenital black spot on 

 the skin just below the angle of the mouth, a blister was applied to 

 remove the epidermis, and then the nitrate of silver till the corium 

 was completely abraded. Still, the pigment-cells were reproduced 

 as abundantly as before. In case of ephelis hepatica affecting the 

 forehead, cheek, or back of the neck, the same experiment has been 

 followed by the same result. After such failure, however, the pig- 

 ment has sometimes spontaneously disappeared. In negroes, if a 

 portion of the skin be lost, that which replaces it, for a long time, is 

 deficient in pigment-cells. In some cases, however, the new skin, 

 after many years, becomes as black as the original integument. 



Pathological Formation of Pigment-cells. 



The abnormal development of pigment-cells or granules in the 

 tissues constitutes melanosis. The latter is not, therefore, itself ma- 



