CH. XXL] 



DIAPEDESIS 



295 



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 corpuscles, 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 cor- 

 puscles) 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 emigrating 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 inflam- 

 mation have for their object an increase in the 

 emigration of leucocytes, which have the power 

 of devouring the irritant substance, and re- 

 moving 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 leucocytes become pus corpuscles; but if the leuco- 

 cytes are successful in destroying the foreign body, micro-organisms, 

 and disintegrated tissues, they disappear, wandering back to the 

 blood-vessels, and the lost tissue is replaced by a regeneration of 

 the surrounding tissues.* 



The circulation through the capillaries must, of necessity, be 

 largely influenced by that which occurs in the vessels on either side 

 of them in the arteries or the veins; their intermediate position 

 causes them to feel at once any alteration in the size, rate, or pres- 



* This question is closely related to that of immunity, which is discussed in the 

 chapter on the Blood (Chapter XXVI). 



FIG. 296. 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; b, some 

 already escaped. (Frey.) 



