250 PATHOLOGY OF THE BLOOD AND CIRCULATION. 



of the leukocytes. The acceleration of the blood-current, 

 which may be only transitory, is followed by its progressive 

 retardation until the vessels are engorged with blood. The 

 current becomes slower and slower, until finally each individual 

 red blood-cell can be recognized, and every heart-beat causes 

 sudden jerks in the flow. 



But soon one's attention is fixed upon tine veins. Normally 

 there are only a few leukocytes to be seen in the periphery of 

 the current, where they are occasionally driven from the axial 

 stream of red blood-cells by reason of their lower specific 

 gravity. But now the number of leukocytes in this peripheral 

 stream is rapidly increasing. They lag behind the still quite 

 rapidly moving axial stream of z*ed blood-cells, momentarily 

 clinging from place to place as they are carried on. This same 

 tendency is also seen in the capillaries. 



In the arteries, with each cardiac diastole a number of leu- 

 kocytes roll into the peripheral stream, only to be swept back 

 into the axial stream with the next systole. Finally the inner 

 surface of the walls of the veins is almost lined by a layer 

 of leukocytes. This phenomenon of margination was described 

 by all the earlier observers (Figs. 95, 96, and 97). 



Third stage: Diapedesis; interstitial exudation. Margina- 

 tion is followed, sometimes immediately, sometimes only after 

 an hour or more, by diapedesis or migration of the leukocytes. 

 There appears on the external surface of the vessel- wall, 

 usually on a vein, but sometimes on a capillary, a small but- 

 ton-like elevation or hump, which little by little grows gradu- 

 ally larger. This colorless protuberance is seen to undergo 

 manifold variations in form, to throw out and retract little 

 finger- like processes; it finally becomes pear-shaped with its 

 point toward the vessel. The tapering end is gradually en- 

 larged into a slender pedicle, which finally separates from the 

 vessel-wall and is retracted into the body of the leukocyte, 

 for this separated mass of protoplasm is now readily recog- 

 nized as such. The migration of a leukocyte takes some- 

 times two hours. The same process is repeated at a large 

 number of points around the veins and capillaries, so that 

 by the end of six or eight hours an immense number of 

 leukocytes have accumulated along the external surface of the 

 vessel-walls. In the arteries this phenomenon is not noted. 



