ORGAN AND BODY SYSTEMS 483 



the twelfth day and has its maximum varying from the fortieth to the 

 seventieth day. Twenty per cent variations in the dose caused no 

 appreciable change in the degree of reaction. 



The capillaries of the skin do not completely return to normal after 

 one roentgen ray erythema dose (273). The microscopic and macro- 

 scopic changes appear in cycles, without, however, a sharp interval 

 between the consecutive periods. One "medium erythema dose" of 

 ultra-violet rays produces capillary changes similar to those induced by 

 unfiltered roentgen rays, but they last for a shorter time and are followed 

 by complete recovery. The increased redness characteristic of the 

 erythema is due to the increased quantity of blood in the peripheral 

 vessels (135). According to David and Gabriel (60) not only the size 

 but also the number of capillary loops may be increased. 



Grenz rays, excited at 4 to 12 kv., produce in general the same reac- 

 tions as do higher voltage roentgen rays (303). In the rabbit ear, follow- 

 ing exposure, there is a dilation of the vessels, epilation, ulceration, and 

 crust formation. The reaction is dependent on the intensity of the 

 radiation and on the length of exposure (112, 113). In the human skin, 

 reddening appears early and continues at its height for five days (306). 

 It then gradually subsides during the next three months during which 

 time pigmentation develops. The histological changes are not unlike 

 those -produced by roentgen rays. Cell destruction, swelling of the 

 endothelium, infiltration of lymphocytes and leucocytes occur. 



The erythema produced by ultra-violet irradiation may vary in 

 intensity as much as 100 per cent in the same individual, depending on 

 the region which is exposed, and between different individuals (272, 273, 

 341, 398). Schall and Alius (320, 321, 322) using an "erythema meter" 

 found that the reaction develops in a wave which has its maximum 6 to 

 10 hr. after exposure. The latent period before the onset of the erythema 

 is usually from 1 to 2.5 hr. but may be more. These authors present 

 curves illustrating the several types of reaction, detailed description of 

 which is given. 



Motojima (239) has compared the effects of ultra-violet rays, roent- 

 gen, and gamma rays on the capillaries of the frog's tongue. In each 

 case dilation of the vessels occurred, a slowing of the blood stream with 

 stasis and diapedesis, and edema. These responses were most severe 

 following ultra-violet radiation, and recovery was most rapid. Beta 

 rays produced a rapid reaction which remained longer than that follow- 

 ing exposure to gamma rays. Roentgen rays produced the least response. 

 The impossibihty of measuring equal doses of these three types of radi- 

 ation prevents strict comparisons between the degree of effect produced. 

 The erythema following exposure to radium radiation (294) has a stronger 

 color and is darker than that produced by roentgen rays (296). Further- 

 more, the height of the redness is later, pigmentation is greater, and the 



