96 AN AMERICAN TENT-BOOK OF PHYSIOLOGY. 



muscles, those of inspiration, regularly add their rhythmic contractions to the 

 continuous pull of the lungs, to reinforce the latter. Each time that the chest 

 expands there is an increased tendency for blood to be sucked into it through 

 the veins. At the beginning of each expiration this increase of suction 

 abruptly ceases. 



The Respiratory Pulse in the Veins near the Chest, and its Limita- 

 tion. — In quiet breathing the movements of the chest-wall produce no very 

 conspicuous effect. W, however, deep and infrequent breaths be taken, the 

 pro-ore within the veins close to the chest becomes at each inspiration much 

 more negative than before; and at each inspiration the area of negative 

 pressure may extend to a greater distance from the chest along the veins of 

 the neck, and perhaps of the axilla. As the venous pressure in these parts 

 now falls as the chest rises, and rises as the chest falls, a visible venous pulse 

 presents itself, coinciding, not with the heart-beats, but with the breathing. 

 At each inspiration the veins diminish in size, as their contents are sucked 

 into the chest faster than they are renewed. At each expiration the veins may 

 be seen to swell under the pressure of the blood coming from the periphery. 

 If the movements of the air in the windpipe be mechanically impeded, 

 these changes in the veins reach their highest pitch; for then the muscles of 

 expiration may actually compress the air within the lungs, and produce a 

 positive pressure within the vena cava and its branches, with resistance to the 

 return of venous blood during expiration, shown by the swelling of the veins. 

 These phenomena are suddenly succeeded by suction, and by collapse and 

 disappearance of the veins, as inspiration suddenly recurs. The respirators 

 venous pulse, when it occurs, diminishes progressively and rapidly as the 

 veins arc observed farther and farther from the root of the neck — a fact 

 which results from the ilaccidity of the venous wall. Were the walls of the 

 veins rigid, like glass, the successive inspirations would produce obvious 

 accelerations of the How throughout the whole venous system, and the con- 

 tractions of the muscles of inspiration would rank higher than they do among 

 the causes of the circulation. In fact, the walls of the veins are very soft 

 and thin. li\ therefore, near the chest, the pressure of the blood within the 

 vein- -inks below that of the atmosphere, the place of the blood sucked into 

 the chest is filled only partly by a heightened flow of blood from the periph- 

 ery, l>nt partly also by the soft venous wall, which promptly sinks under 

 the atmospheric pressure. This is shown by the visible flattening, perhaps 

 disappearance from view, of the vein. This process reduces the visible 

 venous pulse, when' it occur-, to a local phenomenon; for, at each inspira- 

 tion, the promptly resulting shrinkage of all the affected veins together is 

 marly equivalent to the loss of volume due to the sucking of blood into the 

 chest. Therefore the How in the more peripheral veins remains but slightly 

 affected, and the pressure within them continue- to be positive and without 

 a visible pulse. During expiration the swelling of the veins near the chest, 

 the return of positive pressure within them, may be simply from the return 

 of the ordinary balance of forces alter the effects of a deep inspiration have 



