102 



HAND-BOOK OF PHYSIOLOGY. 



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 con- 

 veyed back again to the heart, thus completing a revolution or circulation. 

 The right side of the heart does not directly communicate with the 

 left to complete the entire circulation, but the blood has to pass from the 

 right side to the lungs, through the pulmonary artery, then through the 

 pulmonary capillary-vessels and through the pulmonary veins to the left 

 side of the heart. Thus there are two circulations by which the blood 

 must pass; the one, a shorter circuit from the right side of the heart to 

 the lungs and back again to the left side of the heart; the other and 

 larger circuit, from the left side of the heart to all parts of the body and 

 back again to the right side; but more strictly speaking, there is only one 

 complete circulation, which may be diagrammatically represented by a 

 double loop, as in the accompanying figure (Fig. 90). 



R ; ght Lung. 



Pulmonary 

 Artery. 



Left Lung. 





Diaphragm. 



FIG. 91. 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. 



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 

 principal circulations, the Pulmonary and Systemic, as they are named, 

 it will be noticed also in the same figure that there is another, by which 

 a portion of the stream of blood having been diverted once into the cap- 

 illaries of the intestinal canal, and some other organs, and gathered up 

 again into a single stream, is a second time divided in its passage through 



