As the size of the veins is generally greater than that of the corresponding arteries at the same distance from the heart, and as they are also more numerous, the velocity of blood is less in these parts of the veins than of the arte ries ; and as the whole venous system contains considerably more blood than the arterial, the velocity of the blood taken as a whole must be less in the veins than in the arteries. The same quantity of blood must be brought by the vence cavw to the right auricle as issues from the left ventricle, (making allowance for the expendi ture by secretions, &c.) and consequently the velocity of the blood entering and of that issuing from the heart must be equal. Again, the ve locity of the blood must be gradually on the increase in its progress from the small to the larger veins, because the capacity of the vessels into which it flows is gradually becoming less.
In the systemic veins, exceptino. the venw portm, the direction of the flow of blood is de termined by the structure of the valves, which permit of the return of blood from the extremi ties of the veins towards the heart, but oppose, by the filling of their pouches and the apposi tion of their free edges, a complete obstacle to the reflux of the blood in another direction.
The principal cause of the progressive flow of the blood in the veins is unquestionably the force of itnpulsion of the heart continued through the arteries and small vessels, as ap pears from the flow from the remote part of an opened vein and the simple experiments of Hales, Magendie, and Poiseuille already re ferred to. Hales ascertained, by introducing tubes into the larger veins of the horse, that the pressure on the blood from behind, or vis a tergo, is sufficient to raise the blood in the tube to a considerable height above the level of the heart, and is consequently more than sufficient to re turn the blood to the auricle of the heart. The blood did not, in Hales' experiments, in ge neral at first rise in the tube connected with a vein more than six inches, but this he shewed to proceed from the easy escape of the blood by lateral communicating vessels, for when the other large veins were tied, or when they became fully distended with blood, that fluid sometimes rose in the tube connected with a large vein to a height of three or four feet. M. Poiseuille* dernonstrated, in a still more satisfactory manner, the action of the pressure of the heart on the blood in the veins by means of the bent tube with which he mea sured the pressure of the arterial blood : and this fact is proved in an equally convincing manner by Magendie's experiment of isolating the principal artery and vein from the other parts of the limb of an animal, in which it was found that the flow of blood from the vein is immediately stopped by pressure or ligature of the artery. It is scarcely necessary, in order to
obtain a proof of this fact, to have recourse to the vivisection of animals, for in common bleeding from the arm, the flow of blood from the vein will be found to be immediately influ enced by the state of the artery, and even with out the division of a vein, it is easy to observe the action of this force of impulsion which drives the blood onwards towards the heart in any of the superficial veins of the arm by the application of external pressure, a mode of illustration successfully adopted by Harvey in his explanation of the course of the blood. These very simple experiments are looked upon by some as quite sufficient to demonstrate the proposition that the blood is moved in the veins by an impulsion from behind, and that that impulsion is derived from the action of the heart; while others, not satisfied with this ex planation, have endeavoured to point out addi tional forces as contributing to the progressive motion of the blood in the veins.
The larger veins are, like the arteries, highly elastic, and they are generally reg-arded as stronger proportionally to the thickness of their coats than the arteries. This elasticity belongs chiefly to the external cellular coat, for a mid dle fibrous coat is not apparent in most of the larger healthy veins, and in those rarer in stances in which it is apparent, it is very much thinner than in the arteries. The smaller or capillary veins appear also to be possessed of some degree of irritability, for they have been seen to contract on the application of a stimu lus in the web of the frog's foot by Drs. Thom son and Hastings. This, however, occurs much more rarely than the contraction of the small arteries. It has been remarked that in some animals muscular fibres are prolonged from the auricle upon the adjoining part of the vena cava; and Spallanzani, and others have recorded the fact of the rythmic contraction of parts of the great veins adjoining the auricles. But, excepting in these situations and in the caudal heart, observed by M. Hall in the Eel, muscularity of the veins cannot be considered as having a.ny effect in promoting the flow of the blood in these vessels.