QUANTITY AND PERCENTAGE AFFECTED 395 



increases in exposure within this range do not necessarily produce death 

 more rapidly. The animals survive 3 days, and during this time they 

 refuse to eat, lose weight, and become progressively weaker. Patho- 

 logic damage is produced in practically all vital organs. It is as if all 

 further growth and nutrition have been stopped but the animal is able 

 to survive for a limited time on material already provided. 



At extremely high exposures, still another time curve is demonstrated. 

 This "instantaneous death" may occur during the exposure itself or a 

 few minutes afterward. The changes in the animal leading to death 

 vary in different mammals and range from convulsion and shock to gen- 

 eral necrosis. 



Quastler (51) has studied survival time and dosage in mice and has 

 given some mathematical formulae for the time-dose relations. The 

 study has been extended to mice of different weights and ages, and he 

 finds these to be factors in determining the radiosensitivity as measured 

 by reduction in survival time (52). For further details concerning ani- 

 mal deaths at different times after single exposures see Brues (3), El- 

 linger (10), Evans (13), Henshaw (28, 29, 30), Krebs (38), and Rajew- 

 sky (54). 



Influence of Quantity on Percentage of Population 



Affected 



For several years lower organisms have been employed to yield quan- 

 titative data regarding the relative effectiveness of different dosages of 

 different kinds of ionizing radiation. Such material has the advantage 

 of permitting the use of large numbers of individuals per exposure, and, 

 because of small size, of being irradiated more homogeneously through- 

 out the organism. More recently, the need for quantitative data re- 

 garding the effect of different kinds of radiation exposure on the mammal 

 has encouraged the use of such animals as the mouse, the rat, and the 

 guinea pig. On a smaller scale, larger animals such as the rabbit and 

 dog have been employed to investigate radiosensitivity under various 

 conditions of irradiation. When it has been possible to use sufficient 

 numbers of individuals of the same strain, sex, and weight, rather sym- 

 metrical and satisfactory dose-effect curves have been obtained. Such 

 curves are sigmoid in character (see Fig. 2), and the slope appears to 

 vary according to the general sensitivity, the species, and certain ex- 

 posure factors. A frequency-distribution diagram is included in Fig. 2 

 to illustrate an explanation of the sigmoid character of the dose-survival 

 curve. This interpretation would indicate that even in a carefully se- 

 lected population a few individuals are especially sensitive, a few are un- 



