A single large dose of x-rays over the abdomen will cause a definite injury of the mucosa of the small intestine and the severity of the clinical intoxication seems to parallel this recognizable epithelial injury. This clinical intoxication lasts 4 to 6 days if the x-ray dose is sublethal. Subsequent doses of radiation given within this period of clinical intoxication give recognizable evidence of summation or a cumulative effect.

Small but repeated doses of radiation given within a 5 or 6 day period will cause practically the same cell injury and clinical intoxication as will a single dose representing the sum of the small doses expressed in milliampere minutes. Doses of radiation given at 6 day or longer intervals show no evidence of summation.

The reaction of this relatively sensitive intestinal epithelium to radiation may be similar to the reaction of certain deep lying tumor tissues to x-ray therapy and our experiments may give information of value to physicians concerned with x-ray or radium therapy.

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