498 



RESPIRATORY UNDULATIONS. [Book ii. 



c 



Fig. 99. Comparison of Blood-Pressure Curve with Curve of 

 Intra-thoracic Pressure. (Dog.) 



a is the blood-pressure curve taken by means of a mercury manometer ; it 

 shews the respiratory undulations, the slower beats on the descent being very 

 marked, b is the curve of intra-thoracic pressure obtained by connecting one 

 limb of a manometer with the pleural cavity. Inspiration begins at i, expiration 

 at e. With the beginning of inspiration (i) the expansion of the chest causes a 

 marked fall of the mercury in the intra-thoracic manometer ; but the effect soon 

 diminishes, since the lessening of intra-thoracic pressure does not bear on the 

 manometer alone but on the lungs also ; and as the lungs expand more and more 

 the fall in the mercury becomes less and less until towards the end of inspiration 

 the curve becomes very nearly a straight line. Conversely, the return of the 

 chest at the beginning of expiration (e) produces at first a marked rise of the 

 mercury in the manometer ; but this soon ceases as the air leaves the chest and 

 the lungs shrink, whereupon the mercury falls slowly. 



When these undulations of the blood-pressure curve are 

 compared carefully with the respiratory movements or with the 

 variations of intra-thoracic pressure, what is most commonly 

 observed is that while the blood-pressure, on the whole, rises 

 during inspiration and falls during expiration neither the rise 

 nor the fall is exactly synchronous with either inspiration or 

 expiration. Fig. 99 shews two tracings from a dog taken at 

 the same time, one, a, being the ordinary blood-pressure curve 

 from the carotid, and the other, b, representing the condition of 

 the intra-thoracic pressure as obtained by carefully bringing a 

 manometer into connection with the pleural cavity. On com- 

 paring the two curves it is evident that neither the rise nor the 

 fall of arterial pressure coincides exactly either with inspiration 

 or with expiration. At the beginning of inspiration (i) the 

 arterial pressure is seen to be falling ; it soon however begins 

 to rise, but does not reach the maximum until some time after 

 expiration (<?) has begun ; the fall continues during the 

 remainder of expiration, and passes on into the succeeding 

 inspiration. This suggests the idea that, while inspiration 

 tends to increase and expiration to diminish the blood-pressure, 

 there are causes at work which in each case delay the effect. 



Extended observations however shew that such a relation as 



