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A MANUAL OF PHYSIOLOGY 



over 200 mm. to under 100 mm. per second in the carotid of the 

 rabbit, and from over 500 mm. to less than 250 mm. in the 

 carotid of the dog. Chauveau, with the dromograph, found the 

 velocity in the carotid of a horse to be 520 mm. per second during 

 systole, 150 mm. during the pause, 220 mm. during the period 

 of the dicrotic wave. 



It is probable, however, that if these numbers are at all accu- 

 rate for bloodvessels .in the immediate neighbourhood of the 

 heart, there must be a rapid diminution in the velocity even 

 while the arteries are still of considerable calibre. For it has 

 been found by the electrical method that, in anaesthetized dogs 

 at any rate, as is shown in the following table, the mean velocity 

 between the origin of the aorta and the crural artery in the 

 middle of the thigh is usually less than 100 mm. per second. 



In I. the injecting cannula was in the descending part of the 

 thoracic aorta, in V. at the very origin of the aorta, and in II., III., 

 and IV. in the left ventricle. 



As to the speed of the blood in the arteries of man, our data 

 are insufficient for more than a loose estimate. But it does not 

 seem likely that the mean velocity of a particle of blood in 

 moving from the heart to the femoral artery can exceed 150 mm. 

 per second for the whole of its path. This would correspond to 

 rather more than a third of a mile per hour. In the arch of the 

 aorta the average speed may be twice as great. ' The rivers of 

 the blood ' are, even at their fastest, no more rapid than a 

 sluggish stream. A red corpuscle, even if it continued to move 

 with the velocity with which it set out through the aorta, would 

 only cover about 15 miles in twenty-four hours, and would 

 require five years to go round the world. 



The Volume-pulse. When the pulse-wave reaches a part it 

 distends its arteries, increases its volume, and gives rise to what 

 may be called the volume-pulse. 



This may be readily recorded by means of a plethysmograph, an 

 instrument consisting essentially of a chamber with rigid walls which 



