The progressive motion of the venous blood takes place with little force, and is therefore subject to considerable variations from external pressure. Thus the flow of the blood may be much accelerated by raising a limb, or retarded by keeping it in the depending posture from the mere effect of gravitation, and the common practice of making a person who is bled in the arm call the muscles of the arm into action during the operation, is a sufficient proof that the pressure of the muscles may be the means of accelerating in a considerable degree the venous circulation,—an effect obviously depen dent on the disposition of the valves. Gravita tion or muscular action are, however, only occa sional causes of the acceleration of the flow of blood in the veins, and both, but particularly gravitation, may in some instances offer an ob stacle to its progress.
There are some physiologists who believe the blood to be drawn through the veins to wards the heart by a power of suction which operates from the side of the heart or chest.
The remarks we have already made in treating of the arterial and capillary circulations render it unnecessary for us to revert in this place to the arguments employed by those who have supported the above view, merely on account of their belief in the inadequacy of the heart's force to maintain the complete circulation ; we shall only now state the direct experiments or reasonings by which it has been attempted to be proved that a vis afronte or suction power draws the blood towards the centre of the cir culation. We have already, in a former part of this article, stated our reasons for believing that the elastic power of the heart itself is not attended with any production of an appreciable force sufficient to draw the blood into its inte rior.
The facts which relate to the supposition that the chest or lungs become, during their motions in respiration, the source of a suction power which acts on the venous blood may be suitably considered under the first part of the fourth division of this article, viz.