174 THE CIRCULATION OF THE BLOOD 



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 sinus spaces 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, peri-vascular 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 alter- 

 nate 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 quan- 

 tity of blood at each contraction into the great arteries. Owing to the inter- 

 action 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 contractions, then, 

 pumping against this high arterial tension, are sufficient to maintain a con- 

 stant 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, which 

 are muscular, 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 contraction. Each muscular cham- 

 ber of the heart may, therefore, be said to have its systole and diastole. The 

 whole series of events from the beginning of one contraction until the cor- 

 responding 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 

 filling with the blood flowing into them from the veins, and a portion of this 

 blood is passing at once through the auricles into the ventricles, the opening 



