Irradiation of the whole bodies of mice with neutron rays in sufficient quantities, leads to a clinical, bacteriological and anatomical picture similar to that following Roentgen irradiation. The mucosa of the small intestine and the lymphoid and hemapoietic tissues are the most radiosensitive. The mechanism of death after both forms of radiation seems to be a combination of tissue destruction and enterogenous infection, the former predominating in the acute deaths after large doses. Aside from any possible delayed effects from exposure to small doses over a long period of time, concerning which we have no information, these changes after relatively large doses make it imperative that workers in laboratories generating neutrons protect themselves from exposure by screening. For the same amount of ionization measured by a small bakelite-walled thimble chamber, neutrons are more biologically destructive than x-rays. The average daily dose to those working with neutrons should not exceed one-fourth of the tolerance dose accepted for x-rays. Whether daily doses of this magnitude, over a long period of time, will result in damage is not known. Also, if neutrons are tried therapeutically normal tissue must be protected from undue irradiation.

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