CHAPTER XVIII 



PATHOLOGICAL PIGMENTATION' 



MELANIN^ 



Melanin occurs normally as the coloring-matter of hair, of the 

 choroid of the eye, of the skin, in the pigment matter of many lower 

 animals, and most strikingly as a defensive substance in the "ink'" 

 ejected by squids to render themselves invisible in the water. Path- 

 ologically melanin occurs chiefly as the result of an excessive pro- 

 duction of this pigment by cells normally forming it, as in freckles, 

 melanotic tumors, and Addison's disease (probably). Cells that do 

 not normally form melanin probably do not acquire this power in 

 pathological conditions. Pathological failure to form melanin is also 

 observed, as in skin formed in the healing of wounds and after syphili- 

 tic lesions; or in albinis77i, in w'hich the failure to form melanin may be 

 attributed to hereditary influences.^ Occasionally in domestic 

 animals, especially in calves, a congenital melanosis is observed in- 

 volving many parts of the body."* A melanin or some similar pig- 

 ment may be found in nerve cells (e. g., substantia nigra), and DoUey* 

 beheves it to be a result of nuclear metabolism under conditions of 

 depression. The function of melanin is evidently that of protection 

 from light rays, and Young*' has found that isolated melanin from hu- 

 man skin absorbs violet and ultra-violet rays. Probably this protec- 

 tion is responsible, at least in part, for the relative infrequency of skin 

 cancers in the colored races.'' 



Melanin seems always to be produced through metabolic acti\'ity 

 of specialized cells. The idea, which was formerly advanced, that 

 it is derived from hemoglobin as a product of disintegration, seems 

 to have failed entirely of substantiation. In malaria we frequently 

 find a diffuse pigmentation of the skin of such a nature as to suggest 



' Literature by Oberndorfer, Ergebnisse Pathol., 1908 (12), 460, and Hueck, 

 Ziegler's Beitr., 1912 (54), 68. 



2 Literature and resume given by v. Ftirth, Cent. f. Pathol., 1904 (15), 617; 

 Handb. d. Biochem., 1, 742. 



' Gortner holds that dominant whites are due to the presence of antioxidase, 

 while regressive whites have neither the power to form pigments nor to inhibit 

 their formation (Amer. Naturalist, 1910 (44), 497). 



* See Caspar, Ergebnisse allg. Path., 1896 (III2), 772. 



' Science, 1919 (50), 190. 



« Biochem. Jour., 1914 (8), 460. 



' However, Hanawa found white areas in skin less affected by chemical irri- 

 tants and infections than dark areas. (Dermatol. Zeit., 1913 (20), 761.) This is 

 not in agreement with most observers who have found pigmented skin more re- 

 sistant. (See Hanzlik and Tarr, Jour. Pharm.. 1919 (14), 221.) 



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