PRESSURE CONDITIONS IN LUNGS AND THORAX. 667 



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. The cause for this difference is explained below. 



It is generally agreed that the effect of the respiratory 

 movements on the arterial pressure is due mainly to mechanical 

 factors which influence the amount of blood discharged into the 

 aorta. The matter is difficult to analyze successfully,! but the 



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

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



fFor the older literature, see de Jager, "Journal of Physiology," 7, 130. 



