934 RADIATION BIOLOGY 



differing in weight because of dietary restrictions, vary in their sensitivity 

 to neutron irradiation, the smaller rats being more sensitive (Ely and 

 Ross, 1947). Rabbits of either sex weighing more than 2 kg are reported 

 to be somewhat less sensitive than animals weighing less than 2 kg (Hagen 

 and Sacher, 1946). The heavier rabbits survive longer in any given 

 dose group. It has also been observed that survival time is related to 

 body weight in mice (Quastler, 1945b). An explanation of the protective 

 influence of weight or age is not obvious ; it is probably not attributable to 

 a simple relation between the rate of growth and sensitivity. Differences 

 in the amount and distribution of body fat can account for small differ- 

 ences in sensitivity on the basis of the low effective atomic number of fat 

 (Spiers, 1946). 



A number of constitutional factors must influence the breakdown 

 following irradiation. It is thought that preirradiation tests of func- 

 tional capacity would be useful in sorting out many of these influences, 

 thereby eliminating much of the so-called "inherent variability." Unfor- 

 tunately, there is little information relating to this aspect of the problem, 

 which is of both practical and theoretical importance. 



INDUCED RADIO-SENSITIVITY OR RESISTANCE 



Many procedures have been employed in an attempt to modify the 

 response to radiation. Some of these act in a general way to change 

 the response of the organism to stress. Others are concerned more 

 specifically with the decisive events responsible for injury to critical 

 physiological systems or for recovery from such injury. Some of the 

 procedures within the more specific category may be considered to act 

 primarily upon the biological system, while others probably influence the 

 pathways of energy dissipation within it. 



Needless to say, the criterion will influence any evaluation of radio- 

 sensitivity factors. The death of an irradiated animal may be a con- 

 siderable distance downstream from death of any particular cell or group 

 of cells in the same animal. Since the death of an animal is more com- 

 plex than the death of a cell, it is not surprising that more factors are 

 found to influence sensitivity of the animal than of the cell. Likewise, it 

 is necessary to distinguish the modification of immediate and delayed, 

 or acute and chronic, effects. Protection against acute lethal action 

 does not necessarily imply protection against the more chronic sequelae 

 in so far as different mechanisms may be involved in their development. 



In general, any agent that changes the body economy sufficiently modi- 

 fies the lethal action of radiation. Adrenal insufficiency (Cronkite and 

 Chapman, 1950), protein depletion (Jennings, 1949; Elson and Lamerton, 

 1949), vitamin deficiency (Johnson et al., 1946), infection (Taliaferro and 

 Taliaferro, 1951; Shechmeister and Bond, 1951), trauma (Brooks and 



