2 5 8 



HUMAN PHYSIOLOGY 



means of a rigid-walled tube filled with some fluid to pre- 

 vent coagulation. Before starting an observation the pres- 

 sure in this tube is raised to something like that expected 

 in the artery, and thus a rush of blood into the tube is 

 prevented. (Fig. 124, A.) 



In man it may be done by taking advantage of the fact 

 that, when the pressures inside and outside an artery are 

 equal, the pulse wave causes the greatest variation in the 

 size of the artery. This may be determined by Barnard and 

 Hill's Sphygmometer, which is made on the principle of an 

 anseroid barometer attached to an elastic sac placed over an 

 artery. As the pressure is increased in this system, the 

 pulse of the artery, as shown by the hand of the android, 

 becomes more and more marked, until it reaches a maximum, 

 when the pressure in the sac is equal to that in the artery. 

 (Fig 124, A) 



Arteries. If the pressure in the aorta, in the radial, in 

 the dorsalis pedis, and in one of the smallest arteries is 



measured, it is 

 V. found that 



while it is great 

 in the great 

 arteries about 

 160 mm. Hg in 

 the aorta it is 

 much less in the 

 small arteries. 

 This distribu- 

 tion of arterial 

 pressure might 

 . be plotted out as 



160 



Ar 



20 



FlG. 125. Diagram of the Distribution of Mean Blood Pres- in Fig. 125, Ar. 



sure throughout the Blood Vessels. Ar., the Arteries ; Vftins If 



C., the Capillaries ; V., the Veins. 



the pressure in 



any of the small veins, in a medium vein, and in a large 

 vein near the heart be measured, it will be found 



1st. That the venous pressure is less than the lowest 

 arterial pressure. 



2nd. That it is highest in the small veins, and becomes 

 lower in the larger veins. In the great veins entering the 



