218 CIRCULATION OF THE BLOOD 



The pulse, as a rule, can be detected only in the arteries. This is ex- 

 plained by the fact that in any elastic tube a wave tends to be obliterated by 

 friction. In an unbranched tube the length necessary for this obliteration 

 would be very great, but in one composed of many branches like the vascular 

 system, the obliteration is favored by every bifurcation, since thereby the 

 total wall becomes greater and the active force of the wave is consumed the 

 more rapidly. The wave is reflected also at every point where the vascular 

 wall changes direction, and on this account it is consumed sooner than 

 otherwise. 



5. FINAL SURVEY OF THE MOVEMENTS OF THE BLOOD IN 



THE ARTERIES 



Now that we have learned the details of the blood flow in the arteries, it 

 remains for us to reconstruct these details into a connected whole. We shall 

 follow for this purpose the description of E. H. Weber. 



Let us suppose that the heart consists of only one ventricle, also that to 

 begin with the blood in all divisions of the vascular system is under the same 

 pressure. When the ventricle contracts the atrio-ventricular valves close and 

 prevent the blood from flowing backward. All the blood must therefore take 

 the same direction into the arteries. If these were rigid tubes no blood could 

 be pressed into them without at the same time setting in motion the entire 

 column of blood in all its parts. In this case no wave would be produced, 

 but only a stream of blood which would last as long as the contraction of 

 the ventricle continued. But since the arteries are elastic tubes, propulsion 

 of the different segments of the blood column takes place successively. The 

 mass of blood discharged from the ventricle can find a place for itself only 

 by distending a portion of the arterial system, and thus producing a positive 

 wave of high pressure which spreads through the vessels. 



If there were no semilunar valves, and if the ventricular contractions 

 stopped immediately after the discharge of the blood, the distended arteries 

 would at once drive a part of the blood back into the ventricle. But since 

 this is prevented by the semilunar valves, the successive parts of the blood 

 column are moved a little farther forward in the vascular system by each 

 ventricular systole. 



As soon as the heart relaxes at the end of systole and the atrio-ventricular 

 valves are open, the blood flows from the veins into the heart and produces 

 a negative wave which is propagated along the veins. The valves connected 

 with the heart are so arranged that with the systole and diastole of the heart 

 periodically alternating, positive waves pass out only along the arteries, nega- 

 tive waves only along the veins. 



If the vascular system were composed of a single continuous tube of uni- 

 form diameter every wave would run through the entire system with great 

 velocity, and would produce a state of equilibrium in the entire circuit before 

 the next ventricular systole could follow. But because of the resistance in 

 the smallest arteries, veins and capillaries, matters are quite otherwise. On 

 account of the friction in the smaller vessels the blood cannot pass through 

 as rapidly as would be necessary for the propagation of a positive wave all 



