THE SKIN. 



385 



tive-tissue cells are found immediately beneath the epidermis, sending 

 out processes which may be traced outward between the cells of 

 the stratum Malpighii (Aeby). This fact has led some authors to 

 believe that the connective tissue is in reality the source of the pig- 

 ment, and that by some unknown process the latter is taken up and 

 conveyed to the cells of the epidermis. This theory would preclude 

 a direct production of pigment granules in the epidermal cells. But 

 although it can not be denied that the pigment may be derived from 

 the connective tissue, it is hardly logical to assume a priori that 

 epithelial cells are not capable of pigment production, since, in other 

 regions of the body, pigment formation may be observed in cells of 

 undoubted epithelial origin, as, for instance, in ganglion cells and in 



Fig. 308. A reconstruction showing the arrangement of the blood-vessels in the 

 skin of the sole of the foot (Spalteholz): a, Stratum Malpighii and corium ; b, boundary 

 between cutis and subcutis, in the region of the coiled portions of the sweat-glands ; 

 f, subcutis; of, subpapillary arterial network ; e, cutaneous arterial network; _/, g, //, and 

 i, first, second, third, and fourth venous plexuses. 



the pigment epithelium of the retina. An interesting proof that the 

 processes of pigmented connective-tissue cells actually penetrate the 

 epidermis is afforded by the case reported by Karg, of transplanta- 

 tion of a piece of skin from a white man to a negro. After some 

 time the piece of white skin became pigmented. Reinkehas demon- 

 strated that the pigment in certain cells is in combination with 

 certain definite bodies. The latter have been given the botanical 

 name of troplioplasts. If the pigment be removed, colorless tropho- 

 plasts are left. They may be tinged with certain stains. In the 

 epidermis of the white race troplioplasts are also constantly present, 

 although they are only slightly or not at all pigmented (Barlow). 

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