The differential action of X-rays on tissue, growth and vitality.—Part IV.—The biological reaction to X-radiation in relation to time

In the investigation of a selective action of homogeneous X-radiation on the allantoic membrane of the chick (Moppett, 1929), a constant time of exposure was adopted since one cannot assume that the "time factor" is expressed by a linear function. It was observed at an early stage that a considerable fluctuation in current supply did not materially after the biological reaction if the normal time of exposure were given. An experimental investigation was carried out using a wave-length of 0·5 Å. which produces a hypertrophic reaction with an exposure of ½ hour and an atrophic reaction in 1¼ hours, when other conditions are similar. The reaction shows up abruptly when the above values are exceeded, suggesting the breakdown of a process of repair and the phenomenon is aptly described by the term threshold dose. Typical experiments are given in the following table (I) and the results are represented by the letter A for atrophy, H for hypertrophy, and HA for an intermediate stage. Experiment 147 may be taken as a standard in which a certain quantity of energy is given over a period of 1 hour, the reaction being HA. In experiment 135, approximately the same does was given over a period of 2 hours and the reaction was increased to A+. The reliability factor 5/5 indicated that five consistent pairs of experiments showed an increased reaction in association with a longer time of exposure. Experiment 142 illustrates the fact that an increased reaction is obtained even if the exposure is given intermittently over a long period of time.