CHAPTER IV. 



THE INTERNAL 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 another to form part of a living being; and later on 

 these same atoms, after having been a part of a living cell, 

 are passed out from the Body in the form of lifeless com- 

 pounds. As the foods and wastes of various living things 

 differ more or less, so are more or less different environ- 

 ments suited for their existence; and there is accordingly 

 a relationship between the plants and animals living in 

 anyone place "and the conditions of air, earth, and water 

 prevailing there. Even such simple unicellular animals as 

 the amcebae live only in water or mud containing in solu- 

 tion certain gases and, in suspension, solid food particles; 

 and they soon die if the water be changed either by essen- 

 tially 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 innumerable cells 

 composing the full-grown Human Body, thus requires for 

 the manifestation of its vital properties the presence 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. 



