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CHAPTER XIX. 



THE CIRCULATION OF THE BLOOD. 



1. THAT the blood is constantly circulating throughout 

 the human body, was unknown till \Harveyjmade the disco- 

 very, two hundred years ago| Before this time, air was 

 supposed to circulate through the arteries, or air tubes ; 

 hence their name. The reason why this great truth was 

 not found out sooner, was, that/ on examining dead bodies, 

 the arteries were always found" empty of blood ; owing to 

 the contractile force with which they are endowed^ 



2. The circulation is called one of the vital functions, 



t>ecause it is essential to lifel Its suspension for a short 

 ime throughout the body is certainly fatal. Hence we find 

 that diseases of the heart and great vessels are apt to termi- 

 nate in sudden death, while in other diseases the approach 

 of death is gradual. 



3. Life, in all the organs, is maintained (by the presence 

 of arterial blood) Without it they could not be nourished, 

 nor could they perform their appropriate functions. The 

 moment any part of the body is deprived of blood, from that 

 time it ceases to grow ; it withers and decays, and soon be- 

 comes a mass of dead matter. If the arteries which supply 

 any of the limbs with blood are tied, the limb soon grows 

 black, and mortifies. In the same way, if we stop the cir- 

 culation of sap in a tree by girding or dividing the vessels 

 which convey this fluid to the branches, the tree immediate- 

 ly dies. Blades of grass and corn are often destroyed by 

 worms severing the same vessels with their teeth ; and 

 peach and other fruit trees, are generally short lived from 

 the same cause. 



4. Some animals are destitute of a circulation, such as in- 

 sects, worms, &c. Air and food are essential to the existence 



