298 



THE CIRCULATION IN THE BLOOD-VESSELS [CH. XXII. 



he had seen blood-corpuscles, both red and white, pass bodily through 

 the wall of the capillary vessel in which they were contained ; and 

 that, as no opening could be seen before their escape, so none could 

 be observed afterwards so rapidly was the part healed. But these 

 observations did not attract much notice until the phenomenon was 

 rediscovered by Cohnheim in 1867. 



Cohnheim's experiment was performed in the following manner : 

 A frog is anaesthetised ; and the abdomen having been opened, a portion 

 of small intestine is drawn out, and its transparent mesentery spread 

 out under a microscope. After a variable time, occupied by dilatation, 

 following contraction of the minute vessels and accompanying 

 quickening of the blood-stream, there ensues a 

 retardation of the current, and blood-corpuscles 

 begin to make their way through the capillaries 

 and small vessels. 



Diapedesis, or emigration of the white cor- 

 puscles, occurs to a small extent in health. 

 But it is much increased in inflammation, and 

 may go on so as to form a large collection of 

 leucocytes (i.e. white corpuscles) outside the 

 vessels. 



The emigration of red corpuscles is only 

 seen in inflammation, and is a passive process ; 

 it occurs when the holes made by the emigrat- 

 ing leucocytes do not close up immediately, 

 and so the red corpuscles escape too. 



The real meaning of the process of inflam- 

 mation is a subject which is being much dis- 

 cussed now, but it may be interesting to state 

 briefly the views of Metschnikoff, who has in 

 recent years been a prominent investigator of 

 the subject. Even if these views do not repre- 

 sent the whole truth, it can hardly be doubted 

 that the phenomena described play a very 

 important part in the process. Metschnikoff teaches that the 

 vascular phenomena of inflammation have for their object an in- 

 crease in the emigration of leucocytes, which have the power of 

 devouring the irritant substance, and removing the tissues killed by 

 the lesion. They are therefore called phagocytes (devouring or 

 scavenging corpuscles). It may be that the microbic influence, or 

 the influence of the chemical poisons they produce, is too powerful 

 for the leucocytes; then they are destroyed, and the dead leuco- 

 cytes become pus corpuscles; but if the leucocytes are successful 

 in destroying the foreign body, micro-organisms, and disintegrated 

 tissues, they disappear, wandering back to the blood-vessels, and 



FIG. 265. A large capillary 

 from the frog's mesentery 

 eight hours after irrita- 

 tion had been set up, 

 showing emigration of 

 leucocytes, a, Cells in 

 the act of traversing the 

 capillary wall; &, some 

 already escaped. (Frey.) 



