126 



THE CIRCULATION OF THE BLOOD AND LYMPH 



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 cor- 

 puscle, 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 average flow through the hands of a healthy young man, as 

 determined in eighteen experiments on different dates ranging over 

 two years, at room temperatures varying from 19 to 27 C., was 

 12-8 grammes per 100 c.c. of hand per minute for the right hand, and 

 12-3 grammes for the left. Ten of the observations on this man are con- 

 densed in the table. 



Since the great function of the circulation in the skin is the regulation 

 of the temperature of the body (see Chapter XII.), the blood-flow in 

 the hands is, of course, much influenced by the external temperature. 

 Thus, by far the greatest flow in the above table corresponds to the 

 high room temperature of 27 C. With a given external temperature, 

 the degree of humidity of the air also affects the flow. Under similar 

 conditions of external temperature and daily routine, including diet, 

 the hand flow in one and the same individual does not vary greatly 

 when measured at about the same hour on different days. Different 

 individuals, when tested under apparently similar conditions, show a 

 greater range in the blood-flow. Some normal persons know and say 

 that their hands are habitually cool or cold; others, like the man on 

 whom the above results were obtained, that their hands are habitually 

 warm. The former may be expected to show a relatively small, and 

 the latter a relatively large, flow of blood through the hands. It is 



