280 THE ACTION OF THE LIVING CELL 



gration, cytost may be liberated from the exposed cells 

 in sufficient quantity to evoke a response in neighbor- 

 ing cells. 



Much the same argument obtains in the case of sun- 

 light, whose therapeutic value has been definitely 

 recognized since the time of Hippocrates. Today it 

 is common knowledge that the ultra-violet portion of 

 the spectrum of sunlight is of importance in connec- 

 tion with the synthesis of the antirachitic vitamin, but 

 the beneficial efifects of exposure to sunlight cannot 

 be attributed to this fact alone. 



Experiments have shown that the penetration of 

 sunlight into the skin is limited; hence the systemic 

 effects which follow prolonged exposure to such radi- 

 ation must be due to the absorption of cellular prod- 

 ucts liberated from the radiated area. That such prod- 

 ucts, including cytost, are liberated is evident, since 

 prolonged exposure to sunlight frequently leads to a 

 severe sunburn and the eventual onset of symptoms 

 typical of cytost intoxication. It seems highly prob- 

 able that even brief exposures to sunlight must like- 

 wise result in the release of cytost, for the character- 

 istic erythema resulting from such appears to be 

 identical with that which follows the release of cytost 

 by any other mild localized injury, such as has been 

 discussed in the previous text. 



Upon extending this line of thought, it is readily 

 conceived that the magnitude of the cytost released 

 under the influence of such environmental agencies 

 must vary in some quantitative manner with the in- 

 tensity of such injury. Thus when one unaccustomed 

 to intense sunlight is foolish enough to expose a con- 

 siderable area to the sun's rays for a protracted period. 



