CIRCULATION. 375 



kind to the core, even when the nature and form of the particles employed 

 are varied. 1 



Emigration of Leucocytes. It has been said that a leucocyte may often 

 adhere for a time to the wall of the capillary, or of the arteriole or venule 

 in which it is. Sometimes the leucocyte not only adheres to the wall, but 

 passes through it into the tissue without by a process which has received the 

 name of u emigration." : A minute projection from the protoplasm of the 

 leucocyte is thrust into the wall, usually where this consists of the soft cement- 

 substance between the endothelial cells. The delicate pseudopod is seen pres- 

 ently to have pierced the wall, to have grown at the expense of the main body 

 of the cell, and to have become knobbed at the free end which is in the tissue. 

 Later, the flowing of the protoplasm will have caused the leucocyte to assume 

 something of a dumb-bell form, with one end within the blood-vessel and the 

 other without. Then, by converse changes, the flowing protoplasm comes to 

 lie mainly within the lymph-space, with a small knob only within the vessel ; 

 and, lastly, this knob too flows out ; what had been the neck of the dumb-bell 

 shrinks and is withdrawn into the cell-body, and the leucocyte now lies wholly 

 without the blood-vessel, while the minute breach in the soft wall has closed 

 behind the retiring pseudopod. This phenomenon has been seen in capillaries, 

 venules, and arterioles, but mainly in the two former. It seems to be due to 

 the amoeboid properties of the leucocytes as well as to purely physical 

 causes. Emigration, although it may probably occur in normal vessels, is 

 strikingly seen in inflammation, in which there seems to be an increased 

 adhesiveness between the vascular wall and the various corpuscles of the 

 blood. 



Speed of the Blood in the Minute Vessels. As a measure of the speed 

 of the blood in a vessel, we may fairly take the speed of the red corpuscles. 

 It must, however, be remembered that as the friction increases toward the wall, 

 the speed of the red corpuscles is least in the outer layers of blood, and in- 

 creases rapidly toward the long axis of the tube. At the core of the stream the 

 speed may be twice as great as near the wall. As we have seen, the stream of red 

 corpuscles in an arteriole is rapid and pulsating. In the corresponding venule, 

 which is commonly a wider vessel, the stream is less swift, and its pulse has dis- 

 appeared. In the capillary network between the two vessels the speed of the red 

 corpuscles is evidently slower than in either arteriole or veuule ; and here, as in 

 the veins, no pulse is to be seen ; the pulse comes to an end with the artery 

 which exhibits it. In one capillary of the network under observation the 

 movement may be more active than in another; and even in a given capillary 

 irregular variations of speed at different moments may be observed. Where 

 two capillaries in which the pressure is nearly the- same are connected by a 

 cross-branch, the red corpuscles in this last may sometimes even be seen to 



1 A. Sohklarewsky : " Ueber das Blut und die Suspensionsfliissigkeiten," Pftuger's Archivfiir 

 die gesammte Physiologic, 1868, Bd. i. p. 603. 



2 For the literature of emigration see R. Thoma : Text-book of General Pathology and Patho- 

 logical Anatomy, translated by A. Bruce, 1896, vol. i. p. 344. 



