300 HISTOLOGY OF THE MORBID PROCESSES. 



current slackens, it is these which first make their way into the clear 

 serum at the periphery of the stream and soon come in contact with 

 the vascular wall (Figs. 265 and 266). Here, by virtue of their 

 adhesiveness, they cling to the endothelium, and must materially 

 increase the difficulty with which the blood is forced forward and 

 promote stasis. 



While the blood is circulating freely in the vessels the leucocytes 

 it contains are subjected to repeated mechanical shocks through 

 contact with other corpuscles or with the walls of the vessels 

 where these branch or form sharp curves. These blows cause the 

 cytoplasm to contract, maintaining the globular form of the cor- 

 puscle ; but when they come to rest upon the surface of the vascular 

 wall, as may occasionally happen under normal circumstances, and 

 is always the case in acute inflammations, the leucocytes have an op- 

 portunity to execute the movements which have been called " amoe- 

 boid/ 7 from their resemblance to those displayed by the amoeba. 

 The leucocytes send out pseudopodial processes and creep along the 

 surface of the vessel-wall. We must bear in mind that at this 

 time the capillary vessels are dilated, and that the cement between the 

 endothelial cells is somewhat stretched and thinned. The passage 

 of the pseudopodia of the leucocytes through the cement is facilitated 

 by these circumstances, so that soon after the circulation has become 

 slowed there is a passage of leucocytes through the walls of the ves- 

 sels into the spaces in the surrounding tissues. This escape of the 

 leucocytes is called their "emigration" (Fig. 267). The number 



FIG. 267. 



5 



Emigration of leucocytes through a capillary wall. (Engelmann.) a. leucocyte just leaving 

 one of the pseudostomata between the endothelial cells of the capillary wall ; 6, leucocyte 1 

 partly within and partly outside of the capillary ; c, nucleus of an endothelial cell of the 

 capillary wall. 



of leucocytes that escape from the blood in the manner described is 

 variable. In some varieties of inflammation the tissues outside of 

 the vessels contain substances that have an attraction for the leuco- 



