288 FUNCTIONS OF NUTRITION. 



But this difference in time of the arterial pulsations, in different 

 parts of the body, is rather relative than absolute. The cardiac im- 

 pulse is communicated at the same instant to every part of the arterial 

 system, and the distension begins in all the arteries simultaneously ; 

 but it reaches its completion more rapidly in the neighborhood of the 

 heart, more slowly at a distance. The arterial pulse, as perceived by 

 the finger, marks the condition of maximum distension ; and this con- 

 dition occurs at a later period, according to the distance of the artery 

 from the heart. 



The contraction of the left ventricle is a brisk and sudden motion. 

 The blood driven into the arterial system, meeting with a certain 

 resistance from that already in the vessels, does not instantly displace 

 a quantity equal to its own, but a part of its force is expended in 

 stretching the vascular walls. The expansion of the nearer arteries 

 is therefore sudden and momentary, like the contraction of the heart. 

 But it still requires a certain expenditure of time ; so that, a little dis- 

 tance farther on, the vessel is not distended with the same rapidity, 

 and the arterial dilatation arrives more slowly at its maximum. 



On the other hand, at the moment of cardiac relaxation, the elastic 

 reaction of the larger arteries propels a portion of blood into the 

 smaller vessels beyond, and thus partially maintains their distension. 

 In the larger arteries, accordingly, there is a noticeable difference in 

 size between the periods of their expansion and collapse ; since they 

 are fully distended by the ventricular systole, and afterward emptied, 

 in great measure, by their own reaction. But in the smaller arterial 

 branches, this difference is not so marked. They are less fully dis- 

 tended at the time of the cardiac impulso, because this force is partly 

 expended on the large vessels ; and their subsequent reaction is less 

 complete, because they are then subjected to the elastic pressure from 

 the arterial trunks. This produces a gradual modification of the arte- 

 rial pulse, from the heart toward the periphery. 



The mechanism of this change is illustrated in the experiments of 

 Marey,* by means of an elastic tube attached to a forcing pump, and 

 open at its farther extremity. At different points on the tube are small 

 levers, which are lifted by its distension when water is driven into it 

 from the forcing pump. Each lever carries at its extremity a small 

 pencil, which marks upon a strip of paper, moving with uniform rapid- 

 ity, the curves of its elevation and depression. By these curves both 

 the extent and rapidity of the distension of different parts of the tube 

 may be registered, as shown in Fig. < 7. 



From this it appears that the distension produced by the stroke of 

 the forcing pump begins at the same moment throughout the tube, and 

 that the pulsation is everywhere of equal length. But near the com- 

 mencement of the tube, its expansion is wide and sudden, lasting for 

 only one-sixth part of the entire pulsation, while the remaining five- 



* Journal de la Physiologie. Paris, Avril, 1859. 



