PRESSURE CONDITIONS IN LUNGS AND THORAX. 663 



pressure in man may be taken by means of the sphygmomanometer 

 according to a principle first suggested by Erlanger. The principle 

 involved is to set the pressure in the cuff on the outside of the arm, 

 somewhat below systolic pressure. The pulsations in the man- 

 ometer will therefore be submaximal (see p. 504). If now the blood 

 pressure rises the amplitude of the pulsations will increase, since the 



Fig. 271. Respiratory waves of blood-pressure. Typical blood-pressure record as 

 taken with a mercury manometer: Bp the blood -pressure record, shows the separate 

 heart beats and the larger respiratory waves, each of which comprises six to seven heart 

 beats. 



intra-arterial diastolic pressure thereby approaches the extra- 

 arterial pressure. The reverse of course happens with a fall of 

 blood-pressure. Making use of this method it is found* that in 

 some cases in man the blood-pressure falls during inspiration and 

 rises during expiration, while in other cases the reverse relation- 

 ship holds. 



It is generally agreed that this effect of the respiratory 



* Erlanger and Festerling, "Journal of Experimental Medicine," 15, 370, 

 1912; also Snyder, "American Journal of Physiology," 36, 430, 1915. 



