96 



HANDBOOK OF PHYSIOLOGY. 



side of the heart 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 ca- 

 pillaries, and lastly into the veins, by which it is conveyed back again to 

 the heart, thus completing a revolution or circulation. 



As the right side of the heart, however, does not directly communi- 

 cate with the left, in order to complete the circulation it is necessary 

 that the blood should 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 (Fig, 87). 

 Thus there are two circulations by which the blood must pass ; the one. 



Pulmonary 

 Artery. 



Diaphragm. 



FIG. 88. 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 col- 

 lapsed. 



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 circu- 

 ation, which may be diagrammatically represented by a double loop, as 

 in the accompanying figure (Fig. 87). 



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 



