THE CIRCULATION. 89 



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. 



FIG. 31. 



Lnrynx. 

 Trachea. 



Aorta. - 



Pnlmonar. 

 Artery. 



Diaphragm. Heart. 



View of heart and lungs in situ. The froct portion of the chest-wall, and the outer 

 or parietal layers of the pleurae and pericardium, have been removed. The lungs are 

 partly collapsed. 



As generally described there are two circulations by which 

 all the blood must pass ; the one, a shorter circuit from the 

 heart to the lungs and back again ; the other and larger cir- 

 cuit, from the heart to all parts of the body and back again ; 

 but more strictly speaking, there is only one complete circula- 

 tion, which may be diagrammatically represented by a double 

 loop, as in Fig. 32. 



On reference to this figure and noticing the direction of the 

 arrows which represent the course of the stream of blood, it 

 will be observed that while there is a smaller and a larger 

 circle, both of which pass through the heart, yet that these are 

 not distinct, one from the other, but are formed really by one 

 continuous stream, the whole of which must, at one part of its 

 course, pass through the lungs. Subordinate to the two prin- 

 cipal circulations, the pulmonary and systemic, as they are 



