TROPICAL CLIMATOLOGY 



ftrst all parts were slightly influenced by the heat, and then those 

 parts which were unprotected or covered with rock-crystal became 

 red and inflamed, and later desquamated and became pigmented, 

 while the parts covered by glass and Indian ink, after the slight 

 initial inflammation due to the heat, did not further react. This 

 experiment is interesting as showing that the effects due to heat 

 appear at once and pass off quickly, while those due to the chemical 

 rays do not begin until after a lapse of three hours, which agrees 

 with the well-known fact that a sunburn takes some time to develop. 



Freund, as the result of his experiments, concluded that the 

 chemical rays penetrated into the skin. Bernard and Morgan found 

 that the ultra-violet rays were the active agent in producing sun- 

 burn. 



The histology of solar erythema is not well known. Leredde and 

 Pantrie made a biopsy on the skin of the shoulder of one of their 

 friends who was suffering from sunburn of three days' duration. 

 The skin was in a condition of acute erythema, without oedema or 

 effusion. Under a low power of the microscope the epidermis 

 appeared normal in thickness and disposition, but the horny layer 

 was exfoliated in places. The dermis was richer than normal in 

 cellular elements, and the connective-tissue bundles were swollen. 

 Under a higher power the intercellular spaces appeared larger than 

 normal. The vessels of the dermis were dilated, and there was 

 a slight leucocytic infiltration, while the connective-tissue cells were 

 swollen. These appearances are exactly like those produced by the 

 rays of an electric light. 



If the sun's action stops at this, the only change will be the 

 deposit of the yellowish-brown pigment in the skin so well known 

 in the tropics. If, however, the action is more intense, an exuda- 

 tion appears, which may be sero-fibrinous, cellular, or bloody, while 

 the depth to which these changes may extend depends upon the 

 intensity of the light. The epithelium becomes swollen, and bullse 

 may form, and the connective tissue of the dermis be swollen. 



The pigmentation of the epidermis is important, and will be 

 referred to at greater length in the next section. 



Chronic Skin Irritation.— The chronic effects produced on the 

 skin by the chemical energy of light are :— 



(1) Pigmentation. 



(2) Vascular modification. 



(3) Disease. 



(i) Pigmentation. — It is well known that pigmentation follows 

 sunburn, but until recently it was not evident that it was of a 

 protective nature. 



In 1888 Wedding, confirmed by Charcot, first made this point 

 clear, though Unna in 1885 was the first to say that it was to be 

 regarded as useful, inasmuch as it prevented the rays penetrating 

 too deeply, thus preventing inflammation. 



In 1896 Finsen painted a black ring 2 inches wide round his arm 



