THE CAPILLARIES. 165 



at which the eye must be fixed on the outer contour of the 

 vessel, from which (to quote Professor Cohnheim's words) 

 here and there minute, colourless, button-shaped elevations 

 spring, just as if they were produced by budding out of the 

 wall of the vessel itself. The buds increase gradually and 

 slowly in size, until each assumes the form of a hemispherical 

 projection, of width corresponding to that of a leucocyte. 

 Eventually the hemisphere is converted into a pear-shaped 

 body, the small end of which is still attached to the surface 

 of the vein, while the round part projects freely. Gradu- 

 ally the little mass of protoplasm removes itself further and 

 further away, and, as it does so, begins to shoot out delicate 

 prongs of transparent protoplasm from its surface, in no- 

 wise differing in their aspect from the slender thread by 

 which it is still moored to the vessel. Finally the thread 

 is severed, and the process is complete. The observer has 

 before him an emigrant leucocyte, which in all appreciable 

 respects resembles those which have been already described 

 in the aqueous humour of the inflamed eye." 



Various explanations of these remarkable phenomena 

 have been suggested. Probably the nearest to the truth 

 are those which attribute the chief share in the process to 

 the vital endowments with respect to mobility and contrac- 

 tility of the parts concerned both of the corpuscles 

 (Bastian) and the capillary wall (Strieker). Dr. Sanderson 

 remarks, " the capillary is not a dead conduit, but a tube of 

 living protoplasm. There is no difficulty in understanding 

 how the membrane may open to allow the escape of leuco- 

 cytes, and close again after they have passed out ; for it is 

 one of the most striking peculiarities of contractile substance 

 that when two parts of the same mass are separated, and 

 again brought into contact, they melt together as if they 

 had not been severed." 



Hitherto, the escape of the corpuscles from the interior 

 of the blood-vessels into the surrounding tissues has been 

 studied chiefly in connection with pathology. But it is im- 



