CHAPTER II 

 THE CIRCULATING LIQUIDS OF THE BODY 



IN the living cells of the animal body chemical changes are con- 

 stantly going on; energy, on the whole, is running down; complex 

 substances are being broken up into simpler combinations. So long 

 as life lasts, food must be brought to the tissues, and waste products 

 carried away from them. In lowly forms like the amoeba these 

 functions are performed by interchange at the surface of the 

 animal without any special mechanism; but in all complex organ- 

 isms they are the business of special liquids, which circulate in 

 finely branching channels, and are brought into close relation at 

 various parts of their course with absorbing organs, with eliminating 

 organs, and with the tissue elements in general. 



In the higher animals three circulating liquids have been dis- 

 tinguished: blood, lymph, and chyle. But it is to be remarked 

 that chyle is only lymph derived from the walls of the alimentary 

 canal, and therefore, during digestion, containing certain freshly- 

 absorbed constituents of the food; while both ordinary lymph and 

 chyle ultimately find their way into the blood, and are in their turn 

 recruited from it. The blood contains at one time or another 

 everything which is about to become part of the tissues, and every- 

 thing which has ceased to belong to them. It is at once the 

 scavenger and the food-provider of the cell. But no bloodvessel 

 enters any cell;* and if we could unravel the complex mass of 

 tissue elements which essentially constitute what we call an organ, 

 we should see a sheet of cells, with capillaries in very close relation 

 to them, but everywhere separated from them by a thin layer of 

 lymph. And to describe in a word the circulation of the food 

 substances we may say that the blood feeds the lymph, and the lymph 

 feeds the cell. 



SECTION I. MORPHOLOGY OF THE BLOOD. 



The blood consists essentially of a liquid part, the plasma, in 

 which are suspended cellular elements, the corpuscles. When the 

 circulation in a frog's web or lung or in the tail of a tadpole is 



* Fine intracellular canaliculi, communicating with the blood-capillaries, 

 and probably performing a nutritive function, since they seem to contain 

 blood -plasma, have been described by Schafer and others in the liver cells. 



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