EAPIDITY OF THE CIRCULATION. 



105 



FIG. 38. Small blood-vessel of the mesentery of the 

 frog, showing diapedesis of leucocytes (Landoisj. 



w, w, walls of the vessel ; A, A, still layer ; a, R, red 

 blood-corpuscles ; L, L, leucocytes in contact with 

 the wall, c, c, in different stages of diapedesis ; 

 F, F, leucocytes that have passed out of the vessel. 



pass through the walls of the vessels, only when the pressure is sufficient 



to produce transudation of the 



blood-plasma. 



RAPIDITY OF THE CIKCULATION. 



Several questions of considerable 

 physiological importance arie in 

 connection with the general rapidity 

 of the circulation : 



1. What length of time is occu- 

 pied in the passage of the blood 

 through both the lesser and the 

 greater circulations ? 



2. What is the time required for 

 the passage of the entire mass of 

 blood through the heart? 



3. What influence has the num- 

 ber of pulsations of the heart on the general rapidity of the circulation ? 



The first of these questions is the one which has been most satisfactorily 

 answered by experiments on living animals. In 1827, Hering made the 

 experiment of injecting into the jugular vein of a living animal a solution 

 of potassium ferrocyanide, noting the time which elapsed before it could be 

 detected in the blood of the vein of the opposite side. This gave the first 

 correct idea of the rapidity of the circulation. He drew the blood at inter- 

 vals of five seconds after beginning the injection, and thus, by repeated ob- 

 servations, ascertained pretty nearly the rapidity of a circuit of blood in the 

 animals upon which he experimented. Vierordt (1858) collected the blood 

 as it flowed, in little vessels fixed on a disk revolving at a known rate, which 

 gave more exactness to the observations. The results obtained- by these two 

 observers were nearly identical. 



The only objection which could be made to these experiments is that a 

 saline solution, introduced into the circulation, would have a tendency to 

 diffuse itself throughout the whole mass of blood, it might be, with consider- 

 able rapidity. This certainly is an element which should be taken into ac- 

 count ; but from the definite data which have been obtained concerning the 

 rapidity of the arterial circulation and the inferences which are unavoidable 

 with regard to the rapidity of the venous circulation, it would seem that the 

 saline solution must be carried on by the mere rapidity of the arterial flow to 

 the capillaries, which are very short, taken up from them, and carried on by 

 the veins, and thus through the entire circuit, before it has had time to diffuse 

 itself to any considerable extent. It is not apparent how this objection can 

 be overcome, for a substance must be used which will mix with the blood ; 

 otherwise it could not pass through the capillaries. 



There seems no reason why, with the above restrictions, the results obtained 

 by Hering should not be accepted and their application be made to the human 

 subject. 



