CHAPTER VIII. 



THE 'CIRCULATION. 



In order that the blood may be of service in the body it 

 is necessary that it be kept in circulation. This for two 

 reasons: It must bring new nourishment and new oxygen 

 to the part in question, otherwise the tissue would soon be 

 exhausted, and it must carry away those products of de- 

 composition resulting from the activity of the tissues, other- 

 wise these products would accumulate to such an extent 

 as to become poisonous and prevent further work. To ac- 

 complish these purposes the entire amount of blood is kept 

 in one constant whirl, not stopping even for an instant 

 throughout the entire life of the individual. The stoppage 

 of blood an instant or two would at once entail death. 

 And when it is told that a single particle of blood is whirled 

 along at such a speed that it makes one complete circulation 

 in little more than half a minute, passing in that short in- 

 terval twice through the heart, through the capillaries of 

 the lung and over the system, one may realize with what 

 energy and rapidity the circulation is going on. The main 

 reason for this speed is found in the necessity for carrying 

 the oxygen to the tissues rather than the nutritive portions 

 of the blood. These nutritive substances soak rather slowly 

 through the tissues in the lymph, while the blood itself, con- 

 fined in definite vessels, is hurried along at pell-mell speed 

 back to the lungs to carry to the tissues new quantities of 

 oxygen. 



In many of the lower animals the necessity for a circu- 

 lation is not so great. Thus some of the very tiny forms are 

 able to have their nutritive substances soak through all parts 

 of their bodies by the simple process of osmosis, while the 

 air, too, may be taken in sufficient quantities through the 

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