u6 THE CIRCULATION OF THE BLOOD AND LYMPH 



the rise is not maintained, owing to the dilatation of the cutaneous 

 vessels. In the anterior tibial artery of a boy whose leg was to be 

 amputated, the blood-pressure, measured by means of a manometer 

 connected directly with the artery, was found to vary from 100 to 

 1 60 mm., according to the position of the body and other circum- 

 stances. In a woman sixty years old, in good health, the following 

 readings were obtained with a sphygmomanometer : 



June 28 - 126 130 mm. of mercury. 



,, 29 ----- 126 136 



Aug. 3 132 144 



,,7 134 M 



12 136 144 



Such measurements on man show that the mean blood-pressure 

 under similar conditions in one and the same artery, and in one and 

 the same individual, may vary for a considerable time only within 

 comparatively narrow limits. 



This relative constancy of the general arterial pressure is the 

 result of a delicate adjustment between the work of the heart, the 

 resistance of the vessels, and the volume of the circulating liquid. 

 The quantity of the blood is tolerably steady in health, and con- 

 siderable changes may be artificially produced in it (p. 191) without 

 affecting the pressure in any great degree. On the other hand, the 

 work of the heart and the peripheral resistance are highly variable 

 and vastly influential. A narrowing of the arterioles throughout 

 the body or in some extensive vascular tract increases the peripheral 

 resistance ; and if the heart continues to beat as before, the pressure 

 must rise. If the arterioles are widened, while the heart's action 

 remains unchanged, the pressure must fall. In like manner an 

 increase or a decrease in the activity of the heart, in the absence of any 

 change in the peripheral resistance, will cause a rise or a fall in the 

 blood-pressure. But if a slowing of the heart is accompanied by an 

 increase in the peripheral resistance, or a dilatation of the arterioles 

 by an increase in the activity of the heart, the one change may be 

 partially or completely balanced by the other, and the pressure may 

 vary within narrow limits or not at all. 



Not only is the mean pressure, as measured in a large artery, 

 tolerably constant, but if recorded simultaneously in two arteries at 

 different distances from the heart, it is seen to decrease very gradu- 

 ally so long as the arteries remain large enough to hold a cannula. 

 It is nearly as high, for instance, in the crural artery of a dog as in 

 the carotid. It is easy to see that this must be so, for the resistance 

 of the arteries between the point where the arterioles are given off 

 and the heart is only a small fraction of the total resistance of the 

 vascular path; and we have said (p. 84) that the lateral pressure at 

 any cross-section of a system of tubes through which liquid is flow- 

 ing is proportional to the resistance still to be overcome. This is 

 also the reason why the pressure is always much lower in the pul- 

 monary artery and right ventricle than in the aorta and left ventricle 



