RESPIRATOKY UNDULATIONS. 395 



the quantity of blood ejected at each stroke will increase, and each stroke 

 will ( 148) be increased in vigor, in consequence of which the arterial 

 pressure will rise. Conversely, during the decrease of flow into the ventricle 

 the arterial pressure will fall. Hence the general effect of the movements 

 of the chest on the pulmonary vessels will be during the beginning of inspira- 

 tion to continue the lowering of arterial pressure which was taking place 

 during expiration, but subsequently to raise the arterial pressure; and con- 

 versely, at the beginning of expiration to continue the rise of arterial pres- 

 sure which was taking place during inspiration, but subsequently to lower 

 the arterial pressure. In ordinary breathing, as we have seen, what may be 

 considered as the normal relations of blood-pressure to the respiratory move- 

 ments are precisely of this kind. 



327. Effects of the respiratory movements, however, are seen not only 

 in natural but also in artificial respiration. When, for instance, in an animal 

 under urari, artificial is substituted for natural respiration, undulations of 

 the blood-pressure curve, synchronous with the respiratory movements, are 

 still observed (Fig. 108), though generally less in extent than those seen 

 under natural conditions. 



Now in artificial respiration, the mechanical conditions under which the 

 thoracic viscera are placed as regards pressure, are the exact opposite of 

 those existing during natural respiration, for when air is blown into the 

 trachea to distend the lungs, the pressure within the chest is increased 

 instead of diminished. Under these circumstances, applying the consid- 

 erations laid down in the preceding paragraph with regard to natural 

 respiration, we should expect to find that while the first effect of an arti- 

 ficial inspiration would be to drive an additional quantity of blood out 

 of the lungs into the left ventricle, and thus to raise arterial pressure, 

 this would be in turn followed by a fall of arterial pressure due to the 

 increased resistance offered both to the passage of blood through the 

 lungs and to the entrance of blood through the vense cavse into the 

 right auricle. Conversely, the effect of the succeeding expiration would 

 be an initial continuance of the fall of arterial pressure succeeded by a 

 rise. In other words, we should expect to find in artificial respiration 

 effects exactly the reverse of those which we find in normal respiration ; 

 and, indeed, in many curves of blood-pressure taken during artificial 

 respiration this is the case. 



Both in natural and in artificial respiration, however, the features of the 

 blood-pressure curve vary according as the breathing is hurried or slow, 

 shallow or deep, and according to the facility with which air enters the 

 chest, so much so that at times the blood-pressure curves of natural and 

 artificial respiration may closely resemble each other. And a little con- 

 sideration would lead us to expect this. 



We have seen that the rise in arterial pressure which marks the respir- 

 atory undulation is, in the main, due to a temporary greater amount of 

 blood thrown into the aorta by the left ventricle, and that, correspondingly, 

 the fall of pressure completing the undulation is in the main due to a tem- 

 porary lessening of the amount so thrown. Though the causes discussed in 

 325 undoubtedly make themselves prominent in labored and violent re- 

 spiratory movements, we may conclude that in ordinary respiration, both 

 natural and artificial, the main events producing the respiratory undula- 

 tions are those discussed in 326. We may restate the conclusions of 

 that discussion by saying that the respiratory movements affect the amount 

 of flow of blood into the left ventricle, and so the discharge of blood from 

 the left ventricle into the aorta, in two main ways. In the first place, 

 through the widening or narrowing of the pulmonary vessels they alter 



