ACTION OF THE AURICLES 179 



they lie by the sides of the walls of the veins; but when in action they come 

 together like valves of the arteries, figure 150. Their situation in the 

 superficial veins of the forearm is readily discovered by pressing along its 

 surface, in a direction opposite to the venous current, i.e., from the elbow 

 toward the wrist, when little swellings, figure 150, B, will appear in the 

 position of each pair of valves. 



Lymphatic spaces are present in the coats of both arteries and veins; 

 but in the tunica adventitia or external coat of the large vessels they form 

 a distinct plexus of more or less tubular vessels. In smaller vessels they 

 appear as sinuses lined by endothelium. Sometimes, as in the arteries 

 of the omentum, mesentery, and membranes of the brain, the pulmonary, 

 hepatic, and splenic arteries, the spaces are continuous with vessels which 

 distinctly ensheath them, perivascular lymphatic sheaths. Lymph channels 

 are said to be present also in the tunica media. 



THE ACTION OF THE HEART. 



The heart's action in propelling the blood consists in the successive 

 alternate contraction, systole, and relaxation, diastole, of the muscular walls 

 of the auricles and the ventricles. This activity furnishes the power which 

 keeps the blood moving through the arteries, capillaries, and veins. The 

 heart in its activity is like a great force pump in that it injects a certain 

 quantity of blood at each contraction into the great arteries. Owing to 

 the interaction between this heart-beat and the peripheral resistance to the 

 flow of blood, together with the elasticity of the vessels themselves, a high 

 pressure in the arteries is maintained all the time. The heart's contrac- 

 tions pumping against this high arterial tension, are sufficient to maintain 

 a constant flow of blood through the capillaries, and therefore through the 

 active tissues. 



The heart beats at an average rate of about 72 times per minute during 

 life. Each successive contraction really begins in the great veins, the 

 superior vena cava and extends over the auricles and ventricles in regular 

 sequence. The contraction of each successive part is called its systole 

 and the relaxation its diastole. The diastole covers the period of active 

 relaxation of the muscle and the pause before beginning its next con- 

 traction. Each muscular chamber of the heart may, therefore, be said 

 to have its own systole and diastole. The whole series of events from the 

 beginning of one contraction until the corresponding event in the next 

 contraction is described as the cardiac cycle. 



Action of the Auricles. The description of the action of the heart 

 may be commenced at that period in each cycle in which the whole heart is 

 at rest. The heart is then in a passive state. The auricles are gradually 



