IN COMPARISON with the severe changes seen in many tissues, reports concerning the effects of ionizing radiation on the adult central nervous system have usually indicated a relative radioresistance on the part of the brain.* This observation is supported by experiments suggesting that sensitivity to irradiation depends in part on the rate of synthesis of nucleic acids, the lymphocyte and the maturing neuroblast appearing more radiosensitive than the adult neuron.5 Despite these general statements, reports from both clinical \ s=d\ and experimental sources \ s=dd\ reveal that irradiation can produce extensive neuronal pathology and overt neurological disorders. Of key interest here has been the question of the relative importance of direct and indirect neuronal injury: direct by primary alteration of nerve cells, and indirect from changes secondary to alterations in neuroglia or the blood-brain barrier. In this connection, primary impairment of neurons, of neuroglia, and of the vascular system has each been cited as the principal cause of the neurological abnormalities observed.As evidenced by edema and vascular fragility, nonspecific permeability changes may occur in the central nervous system after irradiation. § The concept of a selective barrier between the blood and the brain in the normal animal has generally been accepted, since it has been shown that certain negatively charged substances cir¬ culating in the blood are uniquely incapable of entering the nerve tissue. This phenomenon can be demonstrated grossly by intravenous or intraperitoneal injection of certain vitally staining acid aniline dyes, such as trypan blue, and has often been described since Ehrlich's experiments at the end of the last century. In the normal animal, trypan blue so injected stains the tissues of every organ except the central nervous system. The dura mater, choroid plexus, and walls of cerebral vessels stain vitally, as do the neural stalk and the area postrema; but the dye normally does not penetrate the central nervous system proper.

TWELVE FIGURESMost investigations of the effects of irradiation on the thymus (reviewed by Murray, '48) hare emphasized the damage done to the thymic cells in terms of injuries to the nuclei. The present study is concerned with the distribution of lipids in the cytoplasm of the cells of thymuses from irradiated mice. It was found that lipids accumulated as soon as 4 hours after exposure to x rays, and decreased in amounts after 5 days. MATEETAL AND METHODSThe mice used in this work were mostly of the C57 B1/6 strain obtained from the Roscoe B. Jackson Memorial Laboratory, Bar Harbor, Maine, and were divided into three age groups: 20-30 days, SO-100 days and mice as old as could be procured. The latter were animals of different strains approximately 360-540 days old. The mice were held in a plastic cage (Haber, '49) and received single, total body, sublethal doses of 410 r. The radiation factors were 250 ICV., no filter, 3 min., 50 em, TSD.Mice from each age group were killed at 4, 8, 12, 18, 24, 36, 72 hours and 5, 7, and 9 days after x-radiation. Thymuses were studied from 125 experimental animals and 35 non-This investigation was