PRESSURE CONDITIONS IN LUNGS AND THORAX. 



6i 



at the beginning of inspiration, but rises during the rest of the 

 act. At the beginning of expiration the pressure continues to 

 rise for a time and then falls during most of this phase. On the 

 whole, therefore, the effect of inspiration, its final effect, is to cause 

 a rise of arterial pressure, while the effect of expiration is to cause 

 a fall. The relationship of the two curves varies in other animals, 

 so that a general statement regarcUng the relationship between the 

 phase of respiration and the change in blood-pressure cannot be 

 made. In the case of man various methods have been employed 

 to determine this relationship. A continuous record of blood- 

 pressiire in man may be taken by means of the sphygmomanometer 



Fig. 271. — Respiratory waves ot 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. 



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. 500). If now the blood 



