OF THE VEINS. 301 



alternate relaxation of the heart, which produces a kind of at- 

 traction; inspiration, which produces a still more powerful 

 one, and the compression of the surrounding muscles. The 

 valves, by dividing the column of the blood, render these di- 

 verse powers more efficacious. The form of the venous sys- 

 tem is the cause that the movement of the blood, instead of 

 gradually diminishing, as in the arteries, is, indeed, slower 

 than these latter, the capacity of which is less than that of the 

 veins, but however, go on accelerating as it approaches the 

 heart. The venous circulation is much more dependent than 

 the arterial on the effects of gravity and pressure. 



450. The course of the blood in the veins is continuous, 

 and these vessels do not present any pulsations; in some 

 places and under certain circumstances, however, they present 

 something analogous to an arterial pulse, which for this reason 

 is called venous pulse. In the neighbourhood of the heart, 

 the venous trunks, which are deprived of valves, experience 

 alternately, during the contraction of the auricles, a reflux of 

 blood which makes them swell out, and during the relaxation 

 of the auricles there occurs a rapid flux, which causes the veins 

 to be depressed. In the ordinary and regular state of func- 

 tions, this double movement is confined to the vicinity of the 

 heart and is not sensible; but when the circulation is em- 

 harassed it extends into the abdomen, and becomes visible in 

 the neck. It is the same with the influence of the motions of 

 respiration: inspiration accelerates the entrance of the blood 

 into the venae cavae and their auricle; active expiration, dif- 

 ficulty or suspension of respiration, and efforts, on the contra- 

 ry, slacken or suspend it; in the ordinary state, these effects are 

 little appreciable or extended; but they become very much 

 so in the opposite cases. The efforts, in which the effects of 

 active expiration are carried to the highest degree, determine, 

 in a very sensible manner, the stasis of the venous blood in the 

 head, the abdomen, and gradually even as far as the limbs; 

 while it is to the contrary effects of inspiration on venous cir- 

 culation, that we must ascribe death by the introduction of air 

 into the heart. When, in fact, by an operation or accident, a 

 large vein is opened at the base of the neck or in the subcla- 



