THE CAPILLARY FLOW 



201 



other words, that the corpuscles could not escape from the circulating stream, 

 unless the wall of the containing blood-vessel was ruptured. It is true that 

 the English physiologist Augustus Waller affirmed in 1846 that he had seen 

 blood-corpuscles, both red and white, pass bodily 

 through the wall of the capillary vessel in which 

 they were contained (thus confirming what had 

 been stated a short time previously by Addison), 

 and that as no opening could be seen before their 

 escape, so none could be observed afterward, so 

 rapidly was the part healed. But these observations 

 did not attract much notice until the phenomenon 

 of escape of the blood-corpuscles from the capil- 

 laries and minute veins, apart from mechanical injury, 

 was rediscovered by Cohnheim in 1867. 



Cohnheim's experiment demonstrating the. pas- 

 sage of the corpuscles through the wall of the blood- 

 vessel is performed in the following manner: A frog 

 is curarized, that is to say paralysis is produced by 

 injecting under the skin a minute quantity of the 

 poison called curari. The abdomen is then opened, 

 a portion of the small intestine is drawn out, and its 

 transparent mesentery spread out under a microscope. 

 After a variable time, occupied by dilatation following 

 contraction of the minute vessels and the accom- 

 panying quickening of the blood stream, there ensues a retardation of the 

 current and the red and white blood-corpuscles begin to make their way 

 through the capillaries and small veins. 



The white corpuscles pass through the capillary wall chiefly by the ame- 

 boid movement with which they are endowed. This migration occurs to a 

 limited extent in health, but in inflammatory conditions is much increased. 



The process of diapedesis of the red corpuscles, which occurs under cir- 

 cumstances of impeded venous circulation, and consequently increased 

 blood pressure, resembles closely the migration of the leucocytes, with the 

 exception that they are squeezed through the wall of the vessel, and do not, 

 like the leucocytes, work their way through by ameboid movement. 



Various explanations of these remarkable phenomena have been sug- 

 gested. Some believe that pseudo-stomata between contiguous endothelial 

 cells provide the means of escape for the blood-corpuscles. But the chief 

 share in the process is probably due to mobility and contraction of the parts 

 concerned, both of the corpuscles and of the capillary wall itself. 



The Speed of the Blood in the Capillaries. The velocity of the 

 blood through the capillaries must, of necessity, be largely influenced by 

 that which occurs in the vessels on both sides of them, in the arteries and 



FIG. 194. A Large Cap- 

 illary from the Frog's 

 Mesentery Eight Hours 

 after Irritation had been 

 set up, Showing Emigra- 

 tion of Leucocytes. a, 

 Cells in the act of travers- 

 ing the capillary wall; b, 

 some already escaped. 

 (Frey.) 



