STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 255 



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 con- 

 taining 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 between 

 the living being and its environment, and an interchange 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 composed of many cells, some 

 of which are placed far away from the surface of its body and 

 from immediate contact with the environment, there arises a new 

 need a necessity for an internal medium or plasma which shall 

 play the same part toward the individual cells as the surrounding 

 air, water and food to the whole animal. This internal medium 

 kept in movement and receiving at some regions of the bodily 

 surfaces materials from the exterior, while losing substances to 

 the exterior at the same or other surfaces, forms a sort of middle- 

 man between the individual tissues and the surrounding world, 

 and stands in the same relationship to each of the cells of the Body 

 as the water in which an amoeba lives docs to that animal, or 

 beer-wort does to a yeast-cell. We find accordingly the Human 

 Body pervaded by a liquid plasma, containing gases and food- 

 material in solution, the presence of which is necessary for the 

 maintenance of the life of the tissues. Any great change in this 

 medium will affect injuriously few or many of the groups of cells 

 in the Body, or may even cause their death; just as altering the 

 media in which they live will kill an amoeba or a yeast-cell. 



In a body so large and complex as that of man, moreover, the 

 internal medium must do more than merely bring food to the 

 individual cells and carry waste materials away from them. All 

 the cells have to be kept at just the right degree of warmth, but 

 some produce more heat than others; so part of its work is to 

 maintain an even distribution of heat over the whole Body or 

 when excess is generated to provide for its escape. Many bodily 

 processes, particularly the slower ones, such as growth, are not 



