82 THE MECHANISM OF THE CIRCULATION. 



and contrary variation in another part, and thus, throughout the wear and 

 tear of life, the aortic pressure is kept at a constant mean height. Pawlow 1 

 possessed a dog so excellently trained that it lay still during any period of 

 observation. In this dog, by means of an insignificant skin cut, Pawlow 

 inserted a cannula into a superficial artery on the inner side of the knee. By 

 this means he was able to measure the arterial pressure on successive days, 

 connecting the cannula with the manometer on each occasion. As a rule, 

 the mean pressure did not vary more than 10 mm. Hg. Neither when the 

 animal was fed on dry food after twelve hours' abstention from water, nor 

 after the imbibing of large quantities of drink, did the pressure vary to 

 a greater extent. 



Pawlow observed the following pressures (in millimetres of mercury) in 

 his dog on different days in the course of a month's observation : — 



Third Day. Fourth Day. Sixth Day. Tenth Day. Twenty-fourth Day. 

 128 131 128 " 129 131 



The same constancy has been observed in man by means of the sphygmo- 

 manometer. 



The Velocity of Blood Flow. 



In order that the circulation may be constantly maintained through- 

 out the vascular system, an equal quantity of blood must on the average 

 pass through each section of the system in the same time. 



The output per second of the heart, divided by the total sectional 

 area of the vascular system at any point, gives the mean velocity per 

 second at that point. Thus the velocity is inversely proportional 

 to the sectional area; and since the total sectional area of the 

 arterial system increases from the aorta to the capillaries, the mean 

 velocity must steadily decrease towards the periphery. On the other 

 hand, since from the capillaries to the vena- cavse the total sectional 

 area decreases, the mean velocity in the veins must increase centri- 

 petally (see Fig. 46, p. 69). 



The velocity and the tension of the blood in the aorta are dependent on 

 the energy of the heart and the resistance in the peripheral vascular system. 

 In the living animal both of these factors are constantly varying, and we can 

 consider the effect of such variations in the following examples : — 



1. The energy of the heart is constant: (a) the peripheral resistance 

 increases. The tension in the aorta will become greater and the velocity less. 

 (b) The peripheral resistance decreases. The velocity will become greater and 

 the tension less. 



2. The resistance remains constant : (a) the energy of the heart increases. 

 Both the tension and the velocity become greater. (b) The energy of the 

 heart decreases. Both the tension and the velocity become less. 



3. The energy of the heart increases : the resistance at the same time 

 becomes greater, and in proportion to the increase of cardiac energy. The 

 tension rises and the velocity remains constant. 



4. The energy of the heart increases : the resistance at the same time 

 decreases, and in proportion to the increase of cardiac energy. The tension 

 remains constant and the velocity becomes greater. 



5. The energy of the heart decreases : the resistance increases, and in pro- 

 portion to the decrease of cardiac energy. The tension remains constant and 

 the velocity becomes less. 



6. The energy of the heart decreases : the resistance becomes less, and in 



1 Arch. f. d. gcs. Physiol, Bonn, 1878, Bd. xvi. S. 266 ; 1S70, Bd. xx. S. 213. 



