LECT. XVII. VELOCITY OF THE CIRCULATION. 319 



accurately ascertained by the before-mentioned experiments 

 of Hering, and the numbers given by him are very gene- 

 rally adopted. But it appears to me easy to show that his 

 method is liable to several errors, and that his results are 

 far from expressing the duration of the circulation. If, in 

 place of introducing into the vein a liquid solution, the 

 presence of which we must afterwards detect in the opposite 

 vessel, we could cause a small body, of a density equal to 

 that of the blood, to pass there, and which would accom- 

 pany the blood when traversing the capillaries and the whole 

 circulatory system^ the period which elapsed, from the 

 moment of its introduction into one jugular to its appear- 

 ance in the other, would be precisely the time required. 

 But we must bear in mind that, if a solution susceptible of 

 mixing with another be poured into any part of the latter, 

 we soon find it in the entire mass, even supposing that it be 

 much more considerable than that of the first, and without 

 any motion. Two solutions, capable of mixing, diffuse 

 themselves, and rapidly mix in consequence of the effects 

 of chemical action, aided by the physical properties of the 

 liquids. Hence, it is not necessary, for the kind of diffusion 

 of which we are now speaking, that the ferrocyanide should 

 have traversed the entire circulatory circle. It must be 

 especially observed, that it is impossible to introduce a 

 liquid solution into the veins without propelling it by a 

 pump, and employing a force which, it is obvious, is con- 

 siderable, since it must overcome the pressure exercised by 

 the blood, so that the mixture of this solution with the san- 

 guineous mass is promoted, more or less, according to the 

 degree of force employed. 



In consequence of these objections, I cannot admit the 

 accuracy of the numbers given to show the time in which 

 the blood accomplishes its entire circulation; and this 



