no 



THE CIRCULATION. 



Larynx 



Aorta 



In the living body the heart and lungs are in constant 

 rhythmic movement, the result of which is an unceasing 

 stream of air through the trachea alternately into and out of 

 21.* the lungs, and an 



unceasing stream 

 of blood into and 

 out of the heart. 

 It is with this 



TracTiea lSSm& Mlllik last 6V6nt that W6 



are concerned es- 

 j^pecially in this 

 chapter, with 

 the means, that is 

 to say, by which 

 the blood which 

 at one moment is 

 forced -out of the 

 heart, is in a few 

 moments more re- 

 turned to it, again 

 to depart, and again pass through the body in course of 

 what is technically called the circulation. The purposes for 

 which this unceasing current is maintained, are indicated in 

 the uses of the blood enumerated in the preceding chapter. 

 The blood is conveyed away from the heart by the 

 arteries, and returned to it by the veins ; the arteries and 

 veins being continuous with each other, at one end by 

 means of the heart, and at the other by a fine network of 

 vessels called the capillaries. The blood, therefore, in its 

 passage from the heart passes first into the arteries, then 

 into the capillaries, and lastly into the veins, by which it 

 is conveyed back again to the heart, thus completing a 

 revolution, or circulation. 



As generally described there are two circulations by 



Fig. 31. View of heart and lungs in situ. The front portion of the 

 chest- wall, and the outer or parietal layers of the pleurae and pericardium 

 have been removed. The lungs are partly collapsed. 



