86 



THE MECHANISM OF THE CIRCULATION. 



increase of velocity which is produced by the dicrotic wave, is far more 

 evident on the carotid than on the facial curve. 



These results show how the intermittent energy of the heart is stored 

 up by the elasticity of the large arteries, and expended in maintaining 

 a continuous How through the small arteries. In the root of the ascend- 

 ing aorta, the mean velocity during systole, according to the calcula- 

 tions of Nicolls, 1 is ten times greater than the mean velocity in the 

 abdominal aorta (see pp. 42, 43). 



Another method of recording variations in velocity depends on the 

 principle of Pitot's tubes. Suppose that a current is flowing constantly 

 through the uniformly wide tube (BC) in the direction of the arrow (Fig. 42), 

 then the lateral pressure in the piezometer (M) will be considerably higher 

 than that in X. The pressure head in the piezometer (M) is an expression of 

 the velocity head and the effective head, in the piezometer (N) of the effective 

 head, minus the velocity head. 



When an alteration of resistance arises in the outflow, the lateral pressures 

 at points nearer the periphery vary by larger fractions than those at points 

 further up the stream. As the velocity in an artery varies inversely as the 

 peripheral resistance, any change in the velocity may be estimated by noting 

 the changes in the lateral pressures in two such manometers placed in connection 

 with the same artery at two different points. In Marey's apparatus the two 

 lateral tubes are connected with two tambours. The buttons of these tambours 

 are arranged so as to press on either side of a lever which works upon a third 

 tambour. The pressure in this tambour varies in relation to the proportionate 

 pressures in the other two tambours, and the variations are registered by a 

 recording tambour. 2 



Cybulski contrived an arrangement, by which the two lateral Pitot-tubes 

 are prolonged into two manometric tubes. These, connected together at the 

 top, are filled in the lower part with fluid, in the upper part with air. The 

 meniscus of the fluid in each tube is photographed on to a moving sensitised 

 plate, and thus the difference in the pressures recorded. 



In the following experiments, Cybulski determined the velocity to be — 





Owing to the cardiac acceleration, and the consequent increase of the output 

 of the heart, the velocity is seen to be greatly increased by section of the vagi. 



From the study of the variations of the velocity of the blood, we may 

 conclude that the following general statements, first formulated by 

 Marey, are true : — 



1 Journ. Physiol., Cambridge and London, 1896, vol. xx. 

 '-' Marey, "Circulation du sang," p. 310. 



