275 



the blood from passing, and then the pressure is slowly lowered 

 till the blood passes and the finger again flushes. The height 

 of the column of mercury at that moment gives the systolic 

 pressure in the small arteries 

 of the finger. 



The diastolie pressure 

 may be measured by taking 

 advantage of the fact that 

 when the pressures inside 

 and outside an artery are 

 equal the pulse wave is 

 best marked. A bag and 

 band are applied to a limb 

 upon an artery, and the 

 movements of the pulse 

 are observed in the column 

 of mercury or on an ane- 

 roid barometer in the Hill 

 Barnard instrument, or by a 

 record taken with a small 

 tambour on a revolving 

 cylinder in Erlauger's 



strument (fig. 129). 

 ger's instrument may 



m- 



Erlan- 

 also 



FIG. 129. Scheme ot Erlanger's apparatus 

 for determining the arterial blood pressure 

 without opening a blood vessel, a, elastic 

 bag for arm ; b, mercurial manometer ; 

 c, pump ; /, glass bulb ; I, rubber ball ; 

 h, tambour with lever. 



be used as the Eiva Eocci for 



systolic pressures, and it is therefore the most valuable instru- 

 ment for the study of the arterial pressure in the man. 

 (Practical Physiology.) 



B. Results 



By these methods it has been found that the systolic pressure 

 in the brachial artery of man is about 110 mm. of mercury, 

 while the diastolie pressure is only about 65 mm. The difference 

 between these, of course, gives the pulse pressure or tension. 



The force of the heart and the degree of peripheral re- 

 sistance both modify the arterial pressure, and normally 

 these so act together that disturbance of one is compensated 

 for by changes in the other. Thus, if the heart's action 

 becomes increased and tends to raise the arterial pressure, the 

 peripheral resistance falls and prevents any marked rise. 

 Similarly, if the peripheral resistance is increased, the 



