282 PHYSIOLOGY CHAP. 



capillaries to the venae cavae, where, however, it is lower than in 

 the aorta. The intra- and extra-thoracic veins nearest the heart 

 show gentle pulsatile oscillations, either of pressure or velocity, 

 which coincide with the presystole and systole of the right heart. 



If we consider the great velocity with which the wave move- 

 ments, produced by the intermittent outflow of blood from the 

 heart, are propagated through the arteries, it is evident that they 

 have no connection with the mean velocity of the circulation, i.e. 

 with the time required by a drop of blood or a corpuscle to 

 traverse the whole vascular circuit and return to its starting-point. 

 " Unda uon est materia progrediens,sed forma materiae progrediens." 

 In this expressive sentence E. H. Weber does not deny that the 

 passage of the sphygmic wave through the arteries is accompanied 

 by an acceleration of the now, and is thus an adjunct to the 

 circulatory movements ; he only intended to make a sharp dis- 

 tinction between the average velocity with which the blood streams 

 through the vessels and the rate of transmission of the pulse wave, 

 which is essentially a form propagated in a fluid. 



In order to measure (at least approximately) the duration of 

 the entire circulation, E. Hering (1829) invented a method which 

 consisted in the injection of a harmless and easily recognisable 

 substance into the jugular vein of an animal, after which a sample 

 of blood was taken from the jugular vein on the opposite side at 

 intervals of five seconds (as marked by a metronome), and tested 

 for the injected substance. He selected ferrocyanide of potash, 

 since its presence, even in minimal quantities, can be detected by 

 an iron salt, in the presence of which Prussian blue is formed. 

 The results which he obtained on the horse were confirmed at a 

 later time by Vierordt (1858), and applied to other animals, by a 

 method which was a little more exact in regard to measuring the 

 intervals between the extraction of the different samples of blood. 



It was found from the best of Bering's experiments on horses 

 that the blood required 26'2 sees, (on an average of nineteen 

 experiments) to pass through the entire circulation from one 

 jugular vein to the other. This path from one jugular to the 

 other is one of the shortest that a drop of blood can take, in order 

 to return to its starting-point after traversing the entire system. 

 If, on the other hand, the ferrocyanide is injected into a crural 

 vein, and the blood from the crural vein on the other side tested, the 

 path is considerably longer, and more time is required. Vierordt's 

 work shows, however, that the time difference between these two 

 paths is very small, so that the blood in the larger arterial and 

 venous vessels circulates very rapidly, any marked delay first 

 occurring in the capillaries and the smallest vessels. 



The discovery that the average time of circulation varies 

 among the different mammals, and that it is shorter in small than 

 in larger animals, is certainly among the most important results of 



