MOTION OF THE BLOOD IN THE VEINS. 499 



causes accumulation in the veins, will assist the heart in propelling the blood 

 into the arteries ; and by the combined action of these two causes is produced, 

 among other effects, the rising and sinking of the Brain, synchronously with 

 expiration and inspiration, which are observed when a portion of the cranium 

 is removed. Several considerations, however, agree in pointing to the conclu- 

 sion that no great efficacy can be rightly attributed to the Respiratory move- 

 ments, as exerting any general influence over the Venous circulation. The 

 Pulmonary circulation, being entirely within the chest, cannot be affected by 

 variations in atmospheric pressure ; the entire venous circulation of the foetus, 

 also, is independent of any such agency. Again, it has been shown experi- 

 mentally by Dr. Arnott and others, that no suction-power exerted at the farther 

 end of a long tube, whose walls are so deficient in firmness as are those of the 

 Veins, can occasion any acceleration in a current of fluid transmitted through 

 it ; for the effect of the suction is destroyed, at no great distance from the point 

 at which it is applied, by the flapping together of the sides of the vessels. One 

 of the most powerful of the general causes which influence the Venous circula- 

 tion is doubtless the frequently-recurring pressure of the muscles upon their 

 trunks. In every instance that Muscular movement takes place, a portion of 

 the Veins of the part will undergo compression ; and as the blood is prevented, 

 by the valves in the veins, from being driven back into the small vessels, it is 

 necessarily forced on towards the heart. As each set of muscles is relaxed, 

 the veins compressed by it fill out again, to be again compressed by the renewal 

 of the force. That the general Muscular movement is an important agent in 

 maintaining the circulation, at a point above that at which it would be kept by 

 the action of the heart and vascular system alone, appears from several con- 

 siderations. The pulsations are diminished in frequency by rest, accelerated 

 by exertion, and very much quickened by violent effort ( 511, d^). In all kinds 

 of exercise, and in almost every sort of effort, there is that alternate contraction 

 and relaxation of particular groups of Muscles which has been just mentioned 

 as affecting the flow of blood through the veins ; and there can be little doubt 

 that the increased rapidity of the return of blood through them is of itself a 

 sufficient cause for the accelerated movements of the heart. When a large 

 number of muscles are put in action after repose, as is the case when we rise up 

 from a recumbent or a sitting posture, the blood is driven to the heart with a 

 very strong impetus; and if that organ should be diseased, it may arrive there 

 in a quantity larger than can be disposed of; so that sudden death may be the 

 result. Hence the necessity for the avoidance of all sudden and violent move- 

 ments, on the part of those who labor under either a functional or structural 

 disease of the centre of the circulation. 



532. The Venous circulation is much more liable than the Arterial, to be 

 influenced by the force of Gravity ; and this influence is particularly noticeable, 

 when the tonicity of the vessels is deficient. The following experiments per- 

 formed by Dr. C. J. B. Williams, 1 to elucidate the influence of deficient firm- 

 ness in the walls of the vessels, and of gravitation, over the movement of fluids 

 through tubes, *throw great light on the causes of venous congestion. A tube 

 with two equal arms having been fitted to a syringe, a brass tube two feet long, 

 having several right angles in its course, was adapted to one of them, whilst to 

 the other was tied a portion of a rabbit's intestine four feet long, and of caliber 

 double that of the brass tube, this being arranged in curves and coils, but with- 

 out angles and crossings. When the two tubes were raised to the same height, 

 the small metal tube discharged from two to five times the quantity of water dis- 

 charged in a given time by the larger but membranous tube; the difference 

 being greatest when the strokes of the piston were most forcible and sudden, by 



1 "Principles of Medicine," 3d Am. Ed., 156. 



