VENOUS CIRCULATION. 567 



seen to be partially emptied ; whilst during Expiration they become turgid, partly in con- 

 sequence of the accumulation from behind, and of the check in front ; and partly (it may 

 bo) in some cases, through an absolute reflux from the veins within the chest ( 723, c). 

 The fact that, in the immediate neighbourhood 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 Physiolo- 

 gists. On the other hand, the expiratory movement, while it directly causes accumulation 

 in the Veins, will assist the Heart in propelling the blood into the Arteries; and by the com- 

 bined action of these two causes are produced, among other effects, the rising and sinking of 

 the Brain, synchronously with expiration and inspiration, which are observed when a por- 

 tion of the cranium is removed. Several considerations, however, agree in pointing to the 

 conclusion, that no great efficacy can be rightly attributed to the Respiratory movements, 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 ; 

 and it may be further remarked, that the whole mechanism of respiration is so different in 

 Birds, from that which exists in Mammalia, that no vacuum can ever be said to exist in 

 their chests, although the venous circulation is performed as actively as usual. The Venous 

 circulation of the fcetus, also, is independent of any such agency. Again, it has been sho\vn 

 experimentally 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 oc- 

 casion 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 flap- 

 ping together of the sides of the vessel. 



c. One of the most powerful of the general causes which influence the Venous circula- 

 tion, is doubtless the frequently-recurring action 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 Capillaries alone, appears from several considerations. The pulsations 

 are diminished in frequency by rest, accelerated by exertion, and very much quickened by 

 violent effort. 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 effecting 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 

 movements, on the part of those who labour under either a functional or structural disease of 

 the centre of the circulation. 



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

 influenced by the force of Gravity ; and this influence is particularly notice- 

 able, when the tonicity of the vessels is deficient. 



a. The following experiments performed by Dr. Williams, to elucidate the influence of 

 deficient firmness 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 calibre double that of the brass tube, this being 

 arranged in curves and coils, but without angles and crossings. When the two tubes were 

 raised to the same height, the small metal tube discharged from two to five times the quan- 

 tity of water discharged 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 which the 

 intestine was much dilated at its syringe end, but conveyed very little more water. When 

 the discharging ends were raised a few inches higher, the difference increased considerably, 

 the amount of fluid discharged by the gut being much diminished ; and when the ends were 

 raised to the height of eight or ten inches, the gut ceased to discharge, each stroke only 

 moving the column of water in it, and this subsiding again, without rising high enough to 



