METHODS 81 



man or any mammal, we introduce our reagent into that 

 portion of the environment which is most directly in con- 

 tact with the living substances of the given animal that 

 which we call the interior medium of the individual. In 

 fact, we find in all these higher beings that the individual 

 may be considered a closed sack traversed by a digestive 

 tube something like a lady's muff. This closed sack (Fig. 

 3) contains a colloid, in which bathe all the living elements 

 of the cells ; it is this not-living colloid which is called the 

 interior medium. 



In the relations existing between the living being and 

 the environment, there are always two stages : 1. Ex- 

 change between the environ- 

 ing medium and the interior 

 medium ; 2. Exchange be- 

 tween the interior medium 



and the living elements. To / ^^f _/_ Digestive 



avoid the first stage, which is 

 itself a biological phenome- 

 non, and to be undisturbed ' ^ IG 3 

 by the superposition of two 



different phenomena, we employ the method of subcut- 

 aneous injections. By breaking through the enclosing 

 surface of the sack, we introduce our reagent directly 

 into the interior medium of the individual. Then we 

 wait for the effect of the reagent. It concerns us only 

 when the individual survives the operation ; it is the passage 

 from the living state A t to the equally living state A 2 that 

 particularly interests us. When the individual dies, biology 

 has no longer to study it. 



What we have said in the first part of this book with 

 regard to the scale of phenomena makes us think that it 

 will particularly interest our studies to use reagents of the 



6 



