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 

 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 per- 

 formed 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 

 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 every- 

 thing 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 



