THE CIRCULATION. 107 



of the blood, or of any other part, flows out of the har- 

 monious action of all the parts. 



16. Change of Color. The blood undergoes a variety 

 af changes in its journey through the system. As it visits 

 the different organs it both gives out and takes up mate- 

 rials. In one place it is enriched, in another it is impover- 

 ished. By reason of these alterations in its composition, 

 the blood also changes its color. In one part of the body 

 it is bright red, or arterial ; in another it is dark blue, or 

 venous. In the former case it is pure and fit for the sup- 

 port of the tissues ; in the latter, it is impure and charged 

 with effete materials. (The details of the change from 

 dark to bright will be given in the chapter on Respira- 

 tion.) 



17. Circulation. The blood is in constant motion 

 during life. From the heart, as a centre, a current is 

 always setting toward the different organs ; and from these 

 organs a current is constantly returning to the heart. In 

 this way a ceaseless circular movement is kept up, which is 

 called the Circulation of the Blood. This stream of the 

 vital fluid is confined to certain fixed channels, the blood- 

 vessels. Those branching from the heart are the arteries ; 

 those converging to it are the veins. The true course of 

 the blood was unknown before the beginning of the seven- 

 teenth century. In 1619 it was discovered by the illustri- 

 ous William Harvey. Like many other great discoverers, 

 he suffered persecution and loss, but unlike some of them, 

 he was fortunate enough to conquer and survive opposi- 

 tion. He lived long enough to see his discovery universally 

 accepted, and himself honored as a benefactor of mankind. 



18. The Heart. The heart is the central engine of 

 the circulation. In this wonderful little organ, hardly 



1 G. Changes in the blood ? What further is stated ? 



17. Motion of the blood? What is meant by the circulation of the blood* 

 How confined ? Discovery made by Harvey ? 



18. Office of the heart? Location of the heart ? Its beat? Its shape? Pro- 

 tection to the heart ? What else i* t-n\d in relation to the heart ? 



