V CHAPTER IV. 







THE INTEKNAL MEDIUM. 







The External Medium. During the whole of life inter- 

 changes of material go on between every living being and the 

 external world; by these exchanges material particles that 

 one time constitute parts of inanimate objects come at an- 

 other to form part of a living being; and later on these 

 same atoms, after having been a part of a living thing, are 

 passed out from it in the form of lifeless compounds. As 

 the foods and wastes of various organisms differ more or 

 less, so are more or less different environments, suited for 

 their existence; and there is accordingly a relationship be- 

 tween the plants and animals living in any one place and the 

 conditions of air, earth and water prevailing there. Even 

 such simple unicellular animals as the amoebae live only in 

 water or mud containing in solution certain gases, and in sus- 

 pension solid food-particles; and they soon die if the water 

 be changed either by essentially altering its gases or by taking 

 out of it the solid food. So in yeast we find a unicellular 

 plant which thrives and multiplies only in liquids of certain 

 composition, and which in the absence of organic compounds 

 of carbon in solution will not grow at all. Each of these 

 simple living things, which corresponds to one only of the in- 

 numerable cells composing the full-grown Human Body, thus 

 requires for the manifestation of its vital properties the pres- 

 ence of a surrounding medium suited to itself: the yeast 

 would die, or at the best lie dormant, in a liquid containing 

 only the solid, organic particles on which the amoeba lives ; 

 and the amoeba would die in such solutions as those in which 

 yeast thrives best. 



The Internal Medium. A similar close relationship be- 

 tween the living being and its environment, and an inter- 

 change between the two like that which we find in the amoeba 

 and the yeast-cell, we find also in even the most complex 

 living beings. When, however, an animal comes to be com- 



