CHAPTER XIII. 

 THE CIRCULATION OF THE BLOOD. 



1. The Circulation. Blood is not allowed to lie at rest 

 in any part of the body. It is kept all the time moving 

 round and round, from organ to organ, through a set of 

 tubes, the blood-vessels. This regular flow of the blood 

 is named the circulation. 



2. The Use of the Circulation. If blood which had 

 been enriched by the absorption of nourishment from 

 the alimentary canal should remain stationary, the mus- 

 cles and brain would be starved. If blood in the skin 

 and that in deeper parts did not change places, the skin 

 would become too cold, and the inside of the body too 

 hot (p. 78). If blood in the muscles were not kept 

 moving on, and fresh blood taking its place, it would 

 soon become so loaded with waste matters from the 

 working muscles that it would poison them. It has to 

 be carried off to organs (the lungs, Chap. XV., and 

 kidneys, Chap. XVII.) in which these injurious matters 

 are separated from it, and it is thus made again ready 

 for use. By means of the circulation, then, the blood 

 flows through every organ in turn; here becoming rich 



1. What is the circulation of the blood ? 



2. How might blood have nourishment in it and yet brain and 

 muscles be starved ? What would happen, as regards the heat of the 

 body, if the blood were not kept flowing through the skin ? How 

 might the blood in a muscle poison it ? What organs separate waste 

 substances from the blood ? What is accomplished by the circulation ? 



