CHAP. XXViii.] SUCTION POWER OF THE AURICLE. 379 



escape of blood through the arteries, and the heart's action is 

 thereby much weakened and often depressed. 



Muscular movements likewise favour the venous circulation, as 

 is well shown in the operation of venesection, when the patient is 

 made to move his fingers freely, the flow of blood from the vein 

 being thereby immediately increased. It is the action of the valves 

 which determines the centripetal flow of the blood in the veins 

 under muscular pressure ; for, as the contracting muscles simply 

 compress the veins, the blood would be driven either or both 

 ways ; but the valves affording a direct impediment to the cen- 

 trifugal flow, it is forced to take the opposite course. This is 

 obviously one of the ways in which exercise favours the circulation 

 and promotes the general health. 



It has been supposed that the contraction of the auricles, by par- 

 tially emptying those cavities, calls into play an elastic force in their 

 walls, which favours the rush of blood into them, and that thus a 

 certain suction power of the auricle may be enumerated among the 

 forces which aid the venous circulation. The idea is illustrated by 

 exhausting an India-rubber bag, to which a glass tube is attached, 

 and then immersing the open extremity of the latter in a vessel of 

 water, when the water will pass freely into it under the influence 

 of the atmospheric pressure on the water. The principal fact in 

 favour of this view, is the experiment of Wedemeyer, which is thus 

 detailed by Muller. " Wedemeyer and Guenther having tied the 

 jugular vein of a horse, made an opening into it between the 

 ligature and the heart, and introduced a catheter, to which a bent 

 glass tube had been cemented. The longer descending branch of 

 the tube (two feet in length) was placed in a glass filled with 

 water. At first, the inspirations and the contractions of the heart 

 were nearly simultaneous, and of the same frequency, namely, 

 thirty in a minute, and the coloured water rose suddenly two or 

 more inches in the tube at the moment of each inspiration and pul- 

 sation of the heart, and sank again each time to its former level. 

 The inspirations gradually became twice as frequent as the pulsa- 

 tions of the heart, and Wedemeyer and Guenther now observed, 

 for a long period, that the rise of fluid did not take place at each 

 inspiration, but at every beat of the heart, and, consequently, 

 simultaneously with each dilation of the auricle. This experi- 

 ment," adds Muller, " seems to prove beyond doubt, that the heart 

 exerts a power of suction." It is most probable, however, that 

 this power is extremely small, and that it does no more than 

 counteract the obstructive influence which would otherwise arise 



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