VENOUS CIRCULATION. 385 



the Arteries ; but the fibrous tissue, of which their middle coat is /made up, 

 bears more resemblance to the areolar tissue of the skin than it does either to 

 muscular fibre, or to ,the true elastic tissue. The Elasticity of the Veins, 

 however, is shown by the jet of blood which at first spouts out in ordinary 

 venesection; when, by means of the ligature, a distension has been occasioned 

 in the tubes below it. A slight Contractility on the application of stimuli has 

 been observed; but this is not so decided as in the Arteries. The whole 

 capacity of the Venous system is considerably greater than that of the Arte- 

 rial ; the former is usually estimated to contain from 2 to 3 times as much 

 blood as the latter, in the ordinary condition of the circulation ; and when we 

 consider the great proportion which the Veins, in almost every part of the body, 

 bear to the arteries, we shall scarcely regard even the larger of these ratios as 

 exaggerated. Of course the rapidity of the movement of the blood in the two 

 systems, will bear an inverse ratio to their respective capacities ; thus if, in a 

 given length ; the Veins contain three times as much blood as the Arteries, the 

 fluid will move with only one-third of the velocity. Even at their origins in 

 the Capillary plexus, the Veins are larger than are the Arteries which termi- 

 nate in the same plexus ; so that, wherever the arterial and venous networks 

 form distinct strata, they are readily distinguished from each other. The 

 Veins are remarkable for the number of valves which they contain, formed of 

 duplicatures or loose folds of the internal tunic, between the component laminae 

 of which contractile fibres are interposed ; arid for the dilatations behind 

 these, which, when distended, give them a varicose appearance. The valves 

 are single in the small veins, the free edge of the flap closing against the 

 opposite wall of the vein ; in the larger trunks they are double ; and in a few 

 instances they are composed of three flaps. The object of these valves is evi- 

 dently to prevent the reflux of blood ; and we shall presently see that they 

 are of important use in assisting in the maintenance of the venous circulation. 

 They are most numerous in those Veins which run among parts affected by 

 muscular movement ; and they are not found in the veins of the lungs, of the 

 abdominal viscera, or of the brain. 



515. The movement of the blood through the Veins is, without doubt, chiefly 

 effected by the vis a tergo or propulsive force ; which results from the action 

 of the Heart and Arteries, and from the additional power generated in the 

 Capillary vessels. This is shown by the immediate arrestment of it, which 

 takes place when these forces are suspended. There are some concurrent 

 causes, however, which are supposed by some to have much influence upon 

 it, and of which the consideration must not be neglected. One of these, is 

 the suction power attributed to the Heart ; acting as a vis a fronte, in draw- 

 ing the blood towards it. It is very doubtful how far the Auricles have such 

 a power of active dilatation as that which would be required for this purpose ; 

 and no sufficient evidence has been given that the current of blood at any dis- 

 tance from the Heart is affected by it. Indeed, for a reason to be presently 

 given, this may be regarded as impossible. Another important agency has 

 been found by some physiologists, in the Inspiratory movement ; this is sup- 

 posed to draw the blood of the Veins into the chest, in order to supply the 

 vacuum which is created there, at the moment of the descent of the Dia- 

 phragm. That the movement in question has some influence on the flow of 

 Venous blood into the chest, is evident from the occurrence of the respiratory 

 pulse, long ago described by Haller ; which may be seen in the veins of the 

 neck and shoulder in thin persons, and in those especially who are suffering 

 from pulmonary diseases. During Inspiration, the Veins are seen to be 

 partially emptied : whilst during Expiration they become turgid, partly i 

 consequence of the accumulation from behind, and of the check in front ; and 

 partly (it may be) in some cases, through an absolute reflux from the veins 

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