CHAP. ii. J RESPIRATION. 619 



of arterial pressure which was taking place during inspiration but 

 subsequently to lower arterial pressure. In ordinary breathing, as 

 we have seen, what may be considered as the normal relations of 

 blood-pressure to the respiratory movements are precisely of this 

 kind. 



384. 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, syn- 

 chronous with the respiratory movements, are still observed 

 (Fig. 86), 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 considerations laid down 

 in the preceding paragraph with regard to natural respiration, we 

 should expect to find that while the first effect of an artificial 

 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 venae cavae into the right auricle. Conversely, the 

 effect of the succeeding expiration would be an initial continu- 

 ance 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 consideration would 

 lead us to expect this. 



We have seen that the rise in arterial pressure which marks 

 the respiratory 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 undu- 

 lation is in the main due to a temporary lessening of the amount so 

 thrown. Though the causes discussed in 382 undoubtedly make 

 themselves prominent in laboured and violent respiratory move- 

 ments, we may conclude that in ordinary respiration, both natural 

 and artificial, the main events producing the respiratory undulations 



