.) 



10 RADIATION HKH-OCY 



( )l" |);irliiul:ir iiilcicst in this regard is Xcfiro skin. Here melanin is not 

 only nioic plcntilul hut is normally present in large amounts in the eor- 

 ntuin rathci than being concentrated in the neighl)orhood of the basal cell 

 layer as in the unexposed skin of the white races. The Negro character- 

 istically has a high erythemal thi'eshold (Miescher, 1<)32), and it might 

 seem obvious to attribute this to the greater amount of melanin. How- 

 ever, it is generally agreed that the Negro corneum is considerably thicker 

 than that of the white races — although no extensive histological account 

 seems to have been pul)lished — and so the high erythemal threshold may 

 be due in this case to the protection afforded by a thick corneum rather 

 than to high melanin content. 



Because, as a rule, suntan and thickening of the corneum develop at 

 about the same time, it is natural to associate the former with the pro- 

 tection afforded by the latter. Actually, however, the thickness of the 

 coi-neum and the erythemal threshold both return to normal in about two 

 months if the skin is guarded from exposure to ultraviolet radiation. On 

 the other hand, suntan may persist for months or years, fading away 

 gradually, but at the same time subject to transient phases of bleaching 

 and darkening. While as a general rule a tanned skin is less susceptible to 

 sunburn than is an untanned one, this may be most misleading. There 

 are persons who develop very little tan, yet can expose themselves to sun- 

 light with relative impunity, and some who are fairly dark in complexion 

 are comparatively sensitive to sunburn. In studying the erythemal 

 thresholds of a considerable number of subjects, the author has been 

 unable to find any close correlation between complexion and sensitivity 

 to ultraviolet radiation. 



The possibility that the epidermal cells develop a local immunity, pre- 

 sumably by the formation of antibodies, was suggested by Perthes (1924) ; 

 who carried out experiments which he interpreted as demonstrating such 

 immunity, but these may have been confused by the inhibition of ery- 

 thema (Blum and Terus, 1946a). The findings subsequent to Perthes' 

 experiments indicate that it would be very difficult to demonstrate such 

 immunity if it occurred. Hausmann and Spiegel- Adolph (1927) sug- 

 gested that changes in the proteins of the corneum might lead to a 

 decrease in transmission. The thickening of the corneum seems so 

 obviously the major factor in limiting the amount of the erythemal ultra- 

 violet reaching the malphigian layer that other contributing factors would 

 seem to be relatively unimportant. 



Artificial Protection. During World War II the problem of evaluating 

 various agents designed for protection of the skin against sunburn assumed 

 certain minor importance. While the problem seemed at first relatively 

 simple, it proved to be quite difficult, and no completely satisfactory solu- 

 tion was found. Only as the study progressed did its complexity become 

 manifest, and in the end the most important thing gained was a somewhat 



