156 THE VASCULAR MECHANISM. 



113. Time of the entire circuit. It is obvious from the foregoing that a 

 red corpuscle in performing the whole circuit, iii 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 absorbent paper covering some travelling surface, 

 an iron salt such as potassium 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 

 seconds and in the dog at about 15 seconds. In man it is probably from 20 

 to 25 seconds. 



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

 would take a corpuscle as long a time to get through about 20 mm. of capil- 

 laries 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 0.5 mm., about 

 half a second is spent in the capillaries of the tissues and another half sec- 

 ond in the capillaries of the lungs. 



114. We may now briefly summarize the broad features of the circu- 

 lation, 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 bloodvessels 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 toward 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 

 small arteries, capillaries, and veins just as much blood as is being thrown 

 into the aorta by the ventricular stroke. We have seen, further, that in the 

 large arteries at each stroke the pressure rises and falls a little above and 

 below the mean, thus constituting the pulse, but that this extra distention 

 with its subsequent recoil diminishes along the arterial tract and finally 

 vanishes ; it diminishes and vanishes because it too, like the whole force of 

 the ventricular stroke, of a fraction of which it is the expression, is used up 

 in establishing the mean pressure ; we shall, however, consider again later 

 on the special features of this pulse. We have seen, further, that the task 

 of driving the blood through the peripheral resistance of the small arteries 

 and capillaries consumes much of this mean pressure, which consequently is 

 much less in the small veins than in the corresponding small arteries, but 



