Cir. xxi.] DIAPEDESIS. 2/5 



sequently increased blood-pressure, resembles closely the migration 

 of the white ones, with the exception that they are squeezed 

 through the wall of the vessel, and do not, like them, work their 

 way through by amoeboid movement. 



Various explanations of these phenomena have been sug- 

 gested. 



Some believe that pseudo-stomata between contiguous endothe- 

 lial cells provide the means of escape for the blood-corpuscles. 

 But the chief share in the process is to be found in the vital 

 endowments with respect to mobility and contraction of the 

 parts concerned both of the corpuscles and of the capillary wall 

 itself. 



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 corpuscles) outside the vessels. Such a collection is called 

 an abscess, and the corpuscles are called pus corpuscles ; they are, 

 however, mostly dead leucocytes, and show a considerable amount 

 of fatty degeneration in their protoplasm. 



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 inflammation is a subject 

 which is being much discussed now, but it may be interesting to 

 state briefly the views of Metschnikoff, who has in recent years 

 been one of the most prominent investigators of the subject. 

 Even if these views do not represent 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 

 increase 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 leucocytes 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 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 



T 2 



