THE CIRCULATION OF THE BLOOD AND LYMPH 107 



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 160 mm., according to the position of the 

 body and other circumstances. In a woman sixty years old, in 

 good health, the following readings were obtained with a sphygmo- 

 manometer : 



June 28 126 1 30 mm. of mercury. 



29 - - 126136 



Aug. 3 132144 



7 134140 



I2 - 136144 



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 considerable changes may be artificially produced in it 

 (p. 174) without affecting the pressure in any great degree. On 

 the other hand, the work of the heart and the peripheral resist- 

 ance 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 un- 

 changed, 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 

 gradually 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. 76) that the lateral pressure at any cross-section of a system 

 of tubes through which liquid is flowing is proportional to the 

 resistance still to be overcome. This is also the reason why 

 the pressure is always much lower in the pulmonary artery and 



