498 t OP THE CIRCULATION OF THE BLOOD. 



The whole capacity of the Venous system is considerably greater than that of 

 the arterial; 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 the arteries which terminate 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; and also 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 evidently 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 move- 

 ment ; and they are not found in the veins of the lungs, of the abdominal 

 viscera, or of the brain. 



531. The movement of the blood through the Veins is, without doubt, chief- 

 ly effected by the vis d tergo or propulsive force, which results from the action 

 of the heart and arteries ; this, as already shown ( 519), is very greatly dimin- 

 ished by the time that it acts on the blood in the veins; but the resistance to 

 the onward movement of the blood is now so slight that a very feeble power is 

 adequate to overcome it. There are some ^concurrent causes, however, which 

 are supposed by some to have much influence upon it, and of which the consider- 

 ation must not be neglected. One of these is the suction-power attributed to 

 the Heart ; acting as a vis d fronte, in drawing the blood towards it. It is 

 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 distance. from the Heart is affected by it. 

 Indeed, for a reason to be presently stated, this may be regarded as impossible. 

 Another important agency has been found by some Physiologists, in the in- 

 spiratory movement ; this is supposed 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 diaphragm. That the movement in question has some influ- 

 ence 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 in 

 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 

 within the chest ( 504). The fact that, in the immediate neighborhood of the 

 chest, the flow of blood towards the heart is aided by inspiration and impeded 

 by expiration, is further proved by Sir D. Barry's experiment, which consisted 

 in introducing one extremity of a tube into the jugular vein of a Horse, and the 

 other into water, which exhibited an alternate elevation and depression with 

 inspiration and expiration ; this has been repeated and confirmed by several 

 Physiologists. On the other hand, the expiratory movement, while it directly 



