228 TIME OF THE ENTIRE CIRCUIT. [BOOK i. 



see, may be brought about in various ways. Hence any numerical 

 statement as to the rate of flow in these vessels must be regarded 

 as a general statement only. 



Moreover, it must be remembered that though we speak of the 

 flow past a point of a large artery as being of a certain rapidity, 

 say 300 mm. a second, that rapidity is continually varying. The 

 cause of the flow through the whole system is the pressure of the 

 ventricular systole manifested as what we have called blood 

 pressure. At each point along the system nearer the left ventricle, 

 and therefore further from the right auricle, the pressure is greater 

 than at a point further from the left ventricle and so nearer the 

 right auricle ; it is this difference of pressure which is the real 

 cause of the flow from the one point to the other; and other 

 things being equal the rapidity of the flow will depend on the 

 amount of the difference of pressure. But the pressure exerted 

 by the ventricle is not constant ; it is intermittent, rhythmically 

 rising and falling. Hence at every point along the arterial system 

 the flow is increased in rapidity during the temporary increase of 

 pressure due to the ventricular systole, and diminished during the 

 subsequent temporary decrease, the increase and decrease being 

 the more marked the nearer the point to the heart ; this is shewn 

 in observations made by means of Chauveau and Lortet's instru- 

 ment or by the plethysmographic method ( 122). 



124. Time of the entire circuit. It is obvious from the fore- 

 going that a red corpuscle in performing the whole circuit, in 

 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 absorb- 

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

 sium ferrocyanide (or preferably sodium ferrocyanide as being less 

 injurious) 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. 



A modification of this method, doing away with the necessity of 

 withdrawing blood, is based on the fact that the electrical conductivity 

 of the blood may be changed by altering the saline constituents. Two 



