432 PHYSIOLOGY CHAP. 



upon the veins. It should, however, be remembered that the 

 arteries are not subject in the same degree as the veins to the 

 effects of the oscillations of intrathoracic and intra- abdominal 

 pressure. The walls of the arteries are in fact more robust, less 

 yielding, and are under high pressure. They are more liable to 

 the effects of the functional modifications of the heart induced by 

 the respiratory movements than to the direct consequence of these 

 movements. 



Generally speaking, physiologists in investigating the re- 

 spiratory waves of arterial blood - pressure have arrived at 

 sufficiently disparate results. This appears to us to be due less to 

 fallacies in the observations or to the method employed, than to 

 the varying effects of the respiratory mechanism upon arterial 

 pressure, according to the form and intensity of respiratory 

 rhythm. This may be extremely frequent and superficial, or 

 extremely infrequent and deep ; and between these two extremes 



FIG. 197. Respiratory oscillations of intrathoracic pressure (T) and pressure in carotia artery 

 (A) in a chloroformed dog. (Luciani.) Tracing A was taken with a Cham-pan and Marey's 

 sphygmoscope. 



many gradations of form may be observed, between which the 

 normal type represents the centre of the scale. 



With extreme frequency of respiration, arterial pressure does 

 not undergo any sensible modification, because the effects of 

 inspiration are obliterated by those of expiration, which rapidly 

 succeed them. But when the respiratory rhythm is not exces- 

 sively frequent, and is very intense, the respiratory undulations 

 do appear on the tracings of arterial pressure, and may suffer the 

 same delay, and coincide approximately with the waves of 

 intrathoracic pressure. This is apparent in the tracings of 

 Fig. 197, registered on a chloroformed dog, which in ten seconds 

 gave nine profound respirations and thirty-nine cardiac beats. 

 It will be seen that arterial pressure rises at each expiration, and 

 falls with each inspiration. It is highly probable that these 

 results depend essentially upon exaggerated expiratory activity of 

 the abdominal muscles, which obstructs the arterial blood-stream 

 flowing to the abdomen by compression of the capillaries, thus 

 producing indirect rise of 'pressure in the intrathoracic arteries, 



