RAPIDITY OF THE CIRCULATION 85 



mata) or by a kind of filtration through the substance which unites the 

 borders of the endothelial cells. This phenomenon was described by 

 Waller, in 1841, but has attracted much attention since the more recent 

 researches of Cohnheim. In this process it is observed that the leuco- 

 cytes, which first adhere to the vascular walls, send out little projections 

 which penetrate the membrane, so that a point appears outside of the 

 vessel. This point becomes larger and larger, until the corpuscle has 

 passed through. The corpuscles then migrate a certain distance by 

 means of the movements known as ameboid, which have already been 

 described. It was supposed by Cohnheim that this was one of the early 

 phenomena of inflammation, the migrating corpuscles afterward multi- 

 plying by division, constituting the so-called pus-corpuscles. Following 

 stasis of blood in the small vessels, the red corpuscles, it is supposed, 

 may pass out in the same way. It is not certain that these are normal 

 processes or that they take place in the human subject. According to 

 Hering, red corpuscles pass through the walls of the vessels (diapedesis), 

 only when the blood-pressure is sufficient to produce 'transudation of 

 the plasma. 



RAPIDITY OF THE CIRCULATION 



Questions of considerable physiological importance arise in connec- 

 tion with the general rapidity of the circulation : 



1. What length of time is occupied 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 number of pulsations of the heart on the 

 general rapidity of the circulation ? 



The first of these questions is the one that has been most satis- 

 factorily answered by experiments on living animals. In 1827 Hering 

 made the experiment of injecting into the jugular vein of a living ani- 

 mal 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 intervals of five seconds after beginning the 

 injection, and thus, by repeated observations, ascertained pretty nearly 

 the rapidity of a circuit of blood in the animals on which he experi- 

 mented. 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 in these two ex- 

 periments were nearly identical. 



The only reasonable objection to these experiments is that a saline 



