ACTION OF X-RADIATION ON LIVING TISSUES 33 



ducible, but biological units of dosage contain factors still largely un- 

 known. 



Biological units of dosage of x-radiation have been expressed in terms 

 of an erythema of the skin. An erythema may be defined as a reddening 

 of the skin within a week or ten days, followed by tanning within about 

 a month's time. 



In human beings the amount of radiation necessary to produce just 

 a mild erythema of the skin seems to depend upon the amount of pig- 

 mentation present and the thickness of the skin. An intensity which 

 may produce a mild erythema on the inside of the thigh or forearm of a 

 thin-skinned blond may cause no visible change in the skin of a dark 

 brunette. The palms, soles, and the back of the neck can tolerate much 

 more radiation than the dorsal surface of the forearm. 



It is difficult to distinguish the effects due to the changes in the epi- 

 thelial cells of the skin from the secondary changes brought about by 

 injury to the subcutaneous capillary bed. The basal cells of the skin, 

 or Malpighian* layer, seem to be the most sensitive, and they show 

 vacuoles, pycnosis, and lack of staining power after large doses of radia- 

 tion. 



It has been noted (Packard [1926]) that a sufficient dose of x-radiation 

 may bring the process of growth and repair of the matrix cells of the 

 hair to a standstill with the result that the hair is loosened and falls out. 



The single greatest secondary factor in skin changes following radia- 

 tion exposure is the modification of cell nutrition by the capillaries. It 

 is the engorgement of the capillaries of the area radiated that is identified 

 with the erythema noted at various periods after exposure. ^ 



With properly filtered hard rays, the radiation through very small 

 portals (holes in lead plates) does not produce an erythema, probably 

 because there is no participation of adjacent tissues in the general reac- 

 tion of the capillaries of the skin, so that an increase in blood circulation 

 could not be identified visibly. 



Bone and cartilage are resistant to x-radiation. It is common thera- 

 peutic knowledge that cartilage may be damaged by very intensive 

 irradiation, especially in the region of the neck if larynx and trachea are 

 subjected to intensive crossfire radiation. Experiments on young rats 

 have shown that, when the jaws are subjected to large doses (Leist 

 [1927]), the odontoblasts, which are especially sensitive, are destroyed; 

 smaller doses produce only pulp injury. 



The precursors of the circulating blood cells in bone marrow are very 

 sensitive to irradiation. 



* The deeper portion of the epidermis, consisting of cells whose protoplasm has not 

 yet changed into horny material. 



