566 OF THE CIRCULATION OF BLOOD. 



4. Of the Venous Circulation. 



743. The Venous system takes its origin in the small trunks that are 

 formed by the re-union of the Capillaries ; and it returns the blood from these 

 to the Heart. The structure of the Veins is essentially the same with that of 

 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, 

 and on irritation of the Sympathetic nervous fibres, 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 Arterial ; the former is usually 

 estimated to contain from 2 or 3 times as much blood as the latter, in the 

 ordinary condition of the circulation ; and when we consider the great pro- 

 portion, 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 net-works form 

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

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

 catures or loose folds of the internal tunic, between the component lamina? 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 circu- 

 lation. They are most numerous in those Veins which run among parts 

 afl'ected by muscular movement ; and they are not found in the veins of the 

 lungs of the abdominal viscera or of the brain. 



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

 chiefly effected by the vis d 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 con- 

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

 upon it, and of which the consideration must not be neglected. 



a. One of these is the suction-power attributed to the Heart; acting as a vis a fronte, in 

 drawing the blood towards it. It is very doubtful how far the Auricles have Mich a power 

 of active dilatation, a.s that which would be required for this purpose; and no sufficient 

 evidence has been given, that the current of blond at any distance from the Heart is affected 

 by it. Indeed, lor a rra.-on if) be presently stated, this may be regarded as impo.-sible. 



b. Another important agency has been loimd by sonic Physiologists in the luspiratory 

 movement; this is supposed to draw the blood of the Veins into the chest, in order to sup- 

 ply the vacuum which is created there, at the moment of the descent of tho Diaphragm. 

 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 Halter ; 

 which may be seen in the veins of the neck and shoulder in thin persons, ami in those 

 especially who are suffering from pulmonary diseases. During Inspiration, the Veins are 



