212 ATOMIC MEDICINE In normally nondividing cell systems as in the liver, chromosome damage can be “unmasked” by inducing cell division as a result of partial organ extirpation. It is stil] an assumption that the chromosome damage known to oceur is responsible for decreased proliferative capacity—-such damage could be present concomitantly with other lesions responsible for the defect. , 9.5.5 Circulating Toxins. The production and ‘or absorption and cireuJation of toxic substances from irradiated tissues or absorption from the bowel whose selective permeability has been changed, have been considered by some as contributing to the development of radiation illness. Others doubt its importance. Bacterial pyrogenic substances, digestive enzymes, and “‘enterotoxins” from the bowel and histamine-like substances from tissue in general have been considered by various investigators as playing some role in the pathogenesis of radiation illness. To date, the available evidence in favor of indirect effects upon the blood-furming organs by circulating toxie substances has not been conclusive (Lawrence; Valentine and Dowdy; Campo, Bond and Cronkite). The histamine theoryof radiation illness has been advocated by Ellinger, who considers that many of the effects of radiation are due to the production of histamine-like substances. Other investigators have failed to demonstrate significantly increased amounts of histamine in the blood andin tissues. Weber and Steggerda have obtained convincing information showing that there is a correlation between the increase in histamine levels of rat plasma and depression in the blood pressure following x-irradiation. Techniques for detecting histamine are difficult and failure of some investigators may have been due to technical difficulties. In addition, the presence of in- creased levels of histamine does not necessarily mean that histamineis the cause of the various phenomena that are seen after irradiation but may simply be a result of the fundamental defect produced by the effects of ionizing radiation on tissue. Conard has shown in a study of the motility, tonus, and contracticn of the bowel immediately after irradiation locally that the behavior of the bowel is not identical to that produced by histamine baths or injections. Other evidence which suggests that histamine may play an important part in the pathogenesis of radiation illness is the fact that conditions that increase the histamine content of tissues mayalso increase the lethality rate from x-rays. For example, induced hyperthyroid- ism increases the histamine content of tissues (Parrot) and increases the lethality of total body x-rays in mice (Blount and Smith). Adrenalectomy also increases the histamine content of tissues of the rat (Rose and Brown) and definitely increaxes the lethality of x-rays (Cronkite and Chapman; Kaplan; Edelman and Campo). 9.5.6 The Role of the Adrenals tn the Acute Radiation Syndrome. Vhe