CHAP, ii.] EESPIRATIOK 503 



spiration to continue the lowering of arterial pressure which 

 was taking place during expiration but subsequently to raise 

 the arterial pressure; and conversely at the beginning of ex- 

 piration to continue the rise 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-pres- 

 sure to the respiratory movements are precisely of this kind. 



313. 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 sub- 

 stituted for natural respiration, undulations of the blood-pres- 

 sure curve, synchronous with the respiratory movements, are 

 still observed (Fig. 100), 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 respi- 

 ration, 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 vense cavse into the right auricle. Con- 

 versely, 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-pres- 

 sure taken during artificial respiration this is the case. 



According to the explanation given above, the total effect 

 of each respiratory movement, both of inspiration and expira- 

 tion, whether natural or artificial, being the result of two factors 

 acting in contrary directions, one an initial one acting only at 

 the mere establishment of inspiration or expiration, the other 

 sequent and acting during the continuance of the inspiratory 

 or expiratory phase, ought to differ according to the character 

 of the respiratory movement. If, for instance, the respiration 

 is rapid, and each movement brief, the first factor will be more 

 prominent than the second ; on the contrary the second factor 

 will be prominent if the respiration be slow and each phase be 

 prolonged ; and the total effects will differ in the two cases. 

 We should expect therefore to find, what we do find, that both 



