226 TIME OF THE ENTIRE CIRCUIT. [BOOK i. 



travelling from the left ventricle back to the left ventricle, would 

 spend a large portion of its time in the capillaries, minute arteries 

 and veins. The entire time taken up in the whole circuit has 

 been approximately estimated by measuring the time it takes for 

 an easily recognized chemical substance after injection into the 

 jugular vein of one side to appear in the blood of the jugular vein 

 of the other side. 



While small quantities of blood are being drawn at frequently 

 repeated intervals from the jugular vein of one side, or while the blood 

 from the vein is being allowed to fall in a minute stream on an absor- 

 bent paper covering some travelling surface, an iron salt such as potas- 

 sium ferrocyanide (or preferably sodium ferrocyanide as being more 

 innocuous) is injected into the jugular vein of the other side. If the 

 time of the injection be noted, and the time after the injection into one 

 side at which evidence of the presence of the iron salt can be detected 

 in the sample of blood from the vein of the other side be noted, this 

 gives the time it has taken the salt to perform the circuit ; and on the 

 supposition that mere diffusion does not materially affect the result, the 

 time which it takes the blood to perform the same circuit is thereby 

 given. 



In the horse this time has been experimentally determined at 

 about 30 sees, and in the dog at about 15 sees. In man it is 

 probably from 20 to 25 sees. 



Taking the rate of flow through the capillaries at about 1 mm. 

 a sec. it would take a corpuscle as long a time to get through 

 about 20 mm. of capillaries as to perform the whole circuit. 

 Hence, if any corpuscle had in its circuit to pass through 10 mm. 

 of capillaries, half the whole time of its journey would be spent in 

 the narrow channels of the capillaries. Inasmuch as the purposes 

 served by the blood are chiefly carried out in the capillaries, it is 

 obviously of advantage that its stay in them should be prolonged. 

 Since, however, the average length of a capillary is about *5 mm., 

 about half a second is spent in the capillaries of the tissues and 

 another half second in the capillaries of the lungs. 



125. We may now briefly summarise the broad features of 

 the circulation, which we have seen may be explained on purely 

 physical principles, it being assumed that the ventricle delivers 

 a certain quantity of blood with a certain force into the aorta 

 at regular intervals, and that the physical properties of the blood 

 vessels remain the same. 



We have seen that owing to the peripheral resistance offered 

 by the capillaries and small vessels the direct effect of the 

 ventricular stroke is to establish in the arteries a mean arterial 

 pressure which is greatest at the root of the aorta and diminishes 

 towards the small arteries, some of it being used up to drive the 

 blood from the aorta to the small arteries, but which retains at the 

 region of the small arteries sufficient power to drive through the 



