388 AN AMERICAN TEXT-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. If, however, deep and infrequent breaths be taken, the 

 pressure 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 exaggerated swelling 

 of the veins. These phenomena are suddenly succeeded by suction, and by 

 collapse and disappearance of the veins from view, as inspiration suddenly re- 

 curs. The respiratory venous pulse, when it occurs, diminishes progressively 

 and rapidly as the veins are observed farther and farther from the root of the 

 neck, a fact which results from the flaccidity of the venous wall. Were the 

 walls of the veins rigid, like glass, the successive inspirations would produce 

 rhythmic accelerations of the flow throughout the whole venous system, and 

 the contractions 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. If, therefore, near the chest, the pressure of the blood within 

 the veins sink below that of the atmosphere without, the place of the blood 

 sucked into the chest is filled only partly by a heightened flow of blood from 

 the periphery, but 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 

 venous pulse, where it occurs, to a local phenomenon ; for, at each inspira- 

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

 equivalent to the loss of volume due to the sucking of blood into the chest. 

 Therefore the flow in the more peripheral veins remains unaffected, and the 

 pressure within them continues to be pulseless and positive. During expira- 

 tion 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 

 after the effects of a deep inspiration have disappeared. But, if expiration be 



