154 THE CIRCULATION OF THE BLOOD 



coat is replaced for some distance from the heart by circularly arranged 

 striped muscular fibers continuous with those of the auricles. The internal 

 coat of veins consists of a fenestrated membrane lined by endothelium. The 

 fenestrated membrane may be absent in the smaller veins. The veins are 

 supplied with valves in certain regions of the body, especially in the veins of 

 the arms and legs. The general construction of these valves is similar to 

 that of the semilunar valves of the aorta and pulmonary artery already 

 described. Their free margins are turned in the direction toward the heart, 

 so as to prevent any movement of blood backward. They are commonly 

 placed in pairs, at various distances in different veins. In the smaller veins 

 single valves are often met with, and three or four are sometimes placed 

 together or near one another in the larger veins such as in the subclavians 

 at their junction with the jugular veins. During the period of their in- 

 action, when the venous blood is flowing in its proper direction, they lie by 

 the sides of the walls of the veins; but when in action, they come together 

 like valves of the arteries, figure 151. 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 151, 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, 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 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 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 



