PHYSIOLOGY OF RADIATION INJURY 977 



local irradiation is thought to be due to alternate regions of constriction 

 and dilatation in the same vessels (Borak, 1942a, b, c). Several mecha- 

 nisms have been proposed to account for the vascular reactions, including 

 a direct effect of radiation on blood vessel walls and the release of humoral 

 agents (H substances) in the irradiated area (Ellinger, 1951 ; Light, 1935). 

 Diffusible substances are postulated to account for the observation that 

 two radiation fields a distance apart show less injury than areas that are 

 closer together (Jolles, 1950). 



Scratch tests of erythematous skin lead to a persistent vasoconstriction, 

 and wheal formation is not induced by histamine (Larkin, 1942). Skin 

 temperature is unchanged during the latent period between irradiation 

 and erythema, but, with the appearance of the latter, the temperature 

 may rise several degrees centigrade, suggestive of marked vasodilatation 

 (Larkin, 1942). It has been pointed out that the cutaneous vessels show 

 an increased sensitivity to dilator substances and a decreased responsive- 

 ness to constrictor stimuli during the period of skin erythema (Lazarew 

 and Lazarewa, 1926). Blanching of the hyperemic regions can occur 

 after injection of epinephrine, however (Larkin, 1942). 



Direct in vivo observations of blood vessels in the frog's web or bat's 

 wing reveal that radiation is a nonspecific vascular damaging agent 

 (Painter et al., 1947; Smith, Svihla, and Patt, 1951). A similar conclu- 

 sion may be drawn from studies of the nail-fold area (Braasch and Nick- 

 son, 1948). Detailed studies of circulation have been made in the wing 

 of the bat, Myotis lucifugus, following total-body irradiation and following 

 local irradiation of a portion of the wing (Smith, Svihla, and Patt, 1951). 

 Of interest is the radioresistance of the hibernating bat. 1 Circulatory 

 changes do not occur in the wing unless the total-body dosage with 

 250-kv X rays exceeds 10,000 r. With dosages over the range of 10,000 

 to 60,000 r, adherence of leukocytes to blood vessel walls, clumping of red 

 cells, and stagnation of blood are prominent. There are, however, no 

 consistent changes in vessel diameter or in venomotor activity following 

 a total-body exposure and hemorrhage is not apparent. On the other 

 hand, there is some evidence of an increase in capillary permeability. 

 After local irradiation of the wing, vascular reactions are confined to the 

 irradiated area, and the threshold for circulatory disturbance is in the 

 neighborhood of 50.000 r for 50-kv X rays. With the exception of red 

 cell clumping, the intravascular changes noted after local exposure, e.g., 

 leukocyte sticking and clumping, platelet thrombi, and stagnation, appear 

 to be related to radiation dosage. Platelet thrombi and leukocyte clumps 

 have also been observed in tissues taken from patients and from animals 

 treated with radium and X rays, and it is presumed that these changes 

 lead to an impairment of blood flow (Pullinger, 1932). Hepatic blood 



1 Although the bats were collected while in hibernation, observations were carried 

 out at room temperature. 



