THE CELLULAR MECHANISMS OF DEFENCE 1147 



remove the skin without causing haemorrhage. The first effect 

 noticed in the immediate neighbourhood of the injury is a dilatation 

 of the vessels, especially of the venules, with acceleration of the blood- 

 flow. In the course of an hour the capillaries also become dilated, 



Fiu. 482. Inflamed mesentery of frog, to show margination of leucocytes 

 in the inflamed capillaries, a ; migration of leucocytes, I ; escape of red 

 corpuscles, c; accumulation of leucocytes outside the capillaries, d. 

 (From AD AMI after RIBBERT.) 



and many capillary channels, previously invisible, are now occupied 

 with blood. Through the dilated capillaries there is a rapid blood- 

 stream, the corpuscles occupying the axis of the vessel, so that there 

 is a periaxial layer of plasma. A little later this acceleration gives 

 place to a slowing of the blood-stream, and simultaneously the leuco- 

 cytes of the blood are seen to be 

 adherent to the capillary wall. 

 Apparently the latter becomes what 

 we may call ' sticky,' the effect of 

 the stickiness being to increase the 

 resistance to the passage of the blood 

 through the vessel and also to cause 

 the adhesion of the leucocytes to 

 the wall. As the current becomes still slower the distinction between 

 axial and peripheral streams disappears. The corpuscles are closely 

 packed together, the white corpuscles being predominant at the 

 margins of the capillary, where they form a lining to the vessel (Fig. 

 482). The next stage is the emigration of the leucocytes. These may be 

 observed to thrust a process through the vessel-wall (according to 

 Arnold this process of emigration always occurs through the stigmata, 

 i.e. the points where the endothelial cells come in contact Fig. 483). 

 The prolongation enlarges on the outer side of the vessel, while the por- 

 tion of the leucocyte within the vessel becomes smaller, so that finally 



FIG. 483. Emigration of leucocytes 

 through capillary wall. (ARNOLD.) 



