THE CAPILLARY FLOW 



233 



heart is so slow or so feeble that the blood at each cardiac systole has time 

 to pass on to the capillaries before the next stroke occurs. The amount of 

 blood sent at each stroke is not sufficient properly to distend the elastic 

 arteries. 



It was formerly supposed that the occurrence of any transudation 

 from the interior of the capillaries into the midst of the surrounding tissues 

 was confined, in the absence of injury, strictly to 

 the fluid part of the blood; in other words, that 

 the corpuscles could not escape from the circulating 

 stream, unless the wall of the containing blood ves- 

 sel 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 Addi- 

 son). He said that no opening could be seen 

 before their escape and that 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 cor- 

 puscles from the capillaries and minute veins, apart 

 from mechanical injury, was rediscovered by Cohn- 

 heim 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 pro- 

 duced by injecting under the skin a minute quantity of the poison called 

 curare. 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 accompanying 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 

 ameboid 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. 



FIG. 194. A Large 

 Capillaryfrom the Frog's 

 Mesentery Eight Hours 

 after Irritation had been 

 set up, Showing Emi- 

 gration of Leucocytes. 

 a, Cells in the act of 

 traversing the capillary 

 wall; b, some already 

 escaped. (Frey.) 



