CHAPTER I 

 THE CIRCULATING LIQUIDS OF THE BODY 



IN the living cells of the animal body chemical changes are 

 constantly going on ; energy, on the whole, is running down ; 

 complex substances are being broken up into simpler combina- 

 tions. So long as life lasts, food must be brought to the tissues, 

 and waste products carried away from them. In lowly forms 

 like the amceba these functions are performed by interchange 

 at the surface of the animal without any special mechanism ; 

 but in all complex organisms 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 

 distinguished : blood, lymph, and chyle. But it is to be re- 

 marked 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 everything 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. 



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

 and probably performing a nutritive function, since they seem to con- 

 tain blood-plasma, have been described by S chafer and others in the 

 liver cells. 



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