100 Applied Biophysics 



the results.^'*'' So long as the circulation is intact, recovery from 

 the direct effects of exposure is hastened ; when the blood supply 

 is compromised, the injurious results are additive. 



The indirect effect of radiation upon embryonic tissue has 

 been strikingly demonstrated by means of tissue-culture experi- 

 ments ^^^ in which it was shown that the cells of a six-day 

 embryo, irradiated i}i ova and explanted shortly afterwards, could 

 be cultivated in vitro in an apparently healthy condition for days. 

 If the embrvos were incubated iii ova for 21 to 25 hours after 

 irradiation, however, they showed no trace of growth when 

 explanted in vitro. The cause of cell death was shown to be due 

 to the absence of gaseous exchange in the tissues of the chick 

 when incubated in the shell, resulting from the arrest of the 

 blood circulation shortly after irradiation. 



The level to which the dose must be raised to affect the 

 circulation is considerably above that which causes a direct 

 effect upon tissue cells. For the chick the doses differ by a 

 factor of about 10. 



Of the various body systems, the blood vessels and l^lood- 

 forming tissue were among the first in whicli the direct effects 

 of radiation were observed. -•^' ^^' ^^^ These studies have recently 

 been greatly extended by the use of radioactive substances, intro- 

 duced into the body and selectively absorbed in the ])lood-forming 

 tissues, in place of external radiation by gamma or X-rays. 

 The range of sensitivity of these tissues is remarkable ; less 

 than 10 r of X-radiation is required to affect the leucocytes of 

 the blood, while a dose of 100,000 r has no demonstrable effect 

 on the isolated (frog's) heart. ^-^' ^-*^ 



Alteration in the blood count in man is an early and convenient 

 warning of injurious exposure to radiation, but there is no 

 agreed opinion as to where the danger line can be precisely 

 marked. ^^^ The lymphocytes show the more marked change in 

 patients who have been irradiated, while the polymorphonuclear 

 cells may be the first to show any change in blood counts of the 

 therapeutic staff. .Small doses of gamma rays spread over a long 

 time may lead to a specific aplastic anemia which is not seen after 

 X-radiation. 



