CHAPTER XVII 



THE STRUCTURE AND FUNCTIONS OF BLOOD AND 



LYMPH 



Introductory. We turn at this point from study of the mech- 

 anism by which the Body adapts itself to its surroundings to a 

 consideration of the structures and processes engaged in body 

 maintenance. These have the task of providing the living tissues 

 of the Body with supplies of energy-yielding material and of 

 keeping them in good working order. Their dependence upon 

 the environment is not so obvious perhaps as is that of the adaptive 

 mechanism proper, although as a matter of fact, changes in the 

 environment do influence the maintenance mechanisms, and often 

 very promptly and strikingly. For example, variations in the sur- 

 rounding temperature bring about adaptive responses in the 

 mechanism for keeping the Body at the proper degree of warmth. 

 We shall have constant occasion, therefore, to recall the facts 

 brought out in preceding chapters. 



The External Medium. During the whole of life interchanges 

 of material go on between every living being and the external 

 world; by these exchanges material particles that one time con- 

 stitute 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 thing, are passed out from it in the form 

 of lifeless compounds. As the foods and wastes of various or- 

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

 ments suited for their existence; and there is accordingly a re- 

 lationship between 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 suspension 

 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 



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