XL BLOOD AND LYMPH. 



After digestion and absorption the food is ready for the 

 use of the cells. Absorption has placed this food in the 

 blood for delivery to the cells, and it is in its character as 

 a carrier of digested nutrients and other substances to 

 the cells that we are to investigate the functions and 

 structure of the blood. 



Blood vessels. — The blood in our bodies is contained 

 in a set of tubes called blood vessels, and these vessels are 

 so constructed and connected as to form a completely 

 closed system. In other words, while the blood absorbs 

 matter into itself and gives out matter from itself to the 

 cells of the body, this exchange takes place through the 

 walls of the blood vessels, and never, except in case of 

 accident (bleeding), does the blood (as blood) escape from 

 the system of closed tubes. In this respect it is like a 

 train on a circular track. We may load goods into this 

 train and .take goods from it, but the train itself keeps 

 on continually and, except in case of accident, never 

 leaves the track. In our body the blood is kept in con- 

 stant movement within the tubes by the beating of the 

 heart and for this reason the blood vessels and their con- 

 tents are spoken of collectively as the circulatory system. 

 If it were not for this constant motion it would be mani- 

 festly impossible for food taken in from the alimentary 

 tract to reach the other parts of the body. Before con- 

 sidering the way in which this liquid is kept in circulation 

 we shall first consider its composition. 



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