386 OF THE CIRCULATION OF BLOOD. 



within the chest ( 489). It was maintained by Sir D. Barry, that, the suction 

 of the blood towards the chest in Inspiration, is one of the most important 

 causes of the maintenance of the Venous circulation ; but several considera- 

 tions agree in pointing to the conclusion, that no great influence can be rightly 

 attributed to it. 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 fetus, also, is independent of any such 

 agency. Again, it has been shown experimentally by Dr. Arnott and others, 

 that no suction-power exerted at the farther end of a long tube, whose w r alls 

 are so deficient in firmness as are those of the Veins, can occasion any accele- 

 ration 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 vessel. There can be no question about 

 the fact, however, that in the immediate neighbourhood of the chest, the flow 

 of blood towards the heart is aided by Inspiration and impeded by Expiration ; 

 for 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 expira- 

 tion, has been repeated and confirmed by several physiologists. It is 

 evident, that the suction of blood into the chest will aid the flow through the 

 Veins, by removing the obstacle to it in front ; although it does not exercise 

 any more direct influence over the current at a distance. 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 

 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. 



516. One of the most powerful of the general causes which influence the 

 Venous circulation, 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 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 movements on the part of those who labour under either a func- 

 tional or structural disease of the centre of the circulation. That Gravity has 

 an influence in modifying the Circulation in particular parts, is a fact well 



