162 



CIRCULATION OF THE BLOOD. 



From this room, the water runs immediately through 

 a little hole into a second room, called the forcing 

 room, where it, is forced out in a stream by simply 

 applying a powerful pressure upon it. The water 

 cannot run back again from the forcing room into the 

 receiving room, when the pressure is applied, because 

 there is a little valve or door placed over the hole. 

 The door, swinging inward into the forcing room, is 

 immediately crowded to and shut by the water, when 

 it would endeavor to escape, as the forcing com- 

 mences ; and the water is therefore compelled to fly 

 out into the hose or pipe provided for it. But when 

 the forcing ceases, the door opens and lets in more 

 water, — and so on continually. Such is a descrip- 

 tion of a fire engine of the simplest kind. There are 

 two of these engines in the heart ; each having its 

 receiving and forcing rooms, with its little door be- 

 tween them ; and each having its hoses to receive 

 the blood and to convey it where it is required : 

 making four rooms in the whole, and the heart being 

 divided off into four apartments for that purpose. 

 With one engine, the blood is received from every 

 part of the body by two hoses of veins which termi- 

 nate each in a single pipe, where they enter the 

 heart. By another hose, which is termed an artery, 

 the same engine forces all this blood into the lungs, 

 where it has to undergo a certain change from the 

 air. The second engine, in its turn, now receives 

 the blood from the lungs by other hoses of veins ; 

 and again by another arterial hose distributes it over 

 the whole system ; whence it is returned to the heart, 



