THE CIRCULATION. 187 



While it lias been impossible to measure the exact rate 

 of flow in the vessels in man, repeated experiments have 

 been made on the lower animals, and certain inferences 

 have been drawn from them with reference to the condition 

 of things in the human body, which are probably not very 

 far from the truth. Such calculations show that the blood 

 in the human carotid flows about 400 millimetres, or six- 

 teen inches in a second. In the smaller arteries it is, of 

 course, less, while finally in the narrow capillaries the blood 

 moves through a distance of about -gV of an inch only in a 

 second. As capillaries in a very general way are about -V 

 of an inch long, it means that it takes the blood about one 

 second to traverse them. In the larger veins the rate 

 is from 100 to 125 millimetres, that is, from four to five 

 inches per second. This rate may be much modified by 

 variations in the energy of the respiratory movements. As 

 in the case of the smaller arteries, so with the smaller 

 veins, the rate is much slower than four inches. 



2. Time. These measurements of the rapidity of the flow 

 have been made on the lower animals by interpolating be- 

 tween the cut ends of the vessel some form of bent glass 

 tube through which the gradual streaming of the blood could 

 be visible throughout. As the rate of flow is different for 

 nearly all portions in the course, it is impossible by this 

 means to calculate how much time would be required for 

 the blood to make its round through lung or tissues. It is, 

 however, possible to make such determination and to dis- 

 cover very accurately the exact amount of time it takes a 

 drop of blood to pass through the routes of circulation. 

 Thus, if some substance which could easily be detected be 

 injected into the carotid artery of a horse and the re-appear- 

 ance of such injected substance watched for in the blood 

 returning from the jugular veins, the time to traverse the 

 circulation between these two points would be at once 

 given. Experiments of this kind on lower animals and cal- 

 culations from these to the human body show that on an 

 average the time required for the blood to make one com- 



