222 



THE CIRCULATION OF THE BLOOD 



into the capillaries at each stroke of the heart that there is not sufficient 

 remaining in the arteries to distend them. Thus, the intermittent current 

 of the ventricular systole is not always converted into a continuous stream 

 by the elasticity of the arteries before the capillaries are reached; and so 

 intermittency of the flow occurs both in capillaries 

 and veins and a venous pulse is produced. The 

 same phenomenon may occur when the arteries 

 become rigid from disease, and when the beat of the 

 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 being insufficient 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 con- 

 fined, 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 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). 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 

 corpuscles from the capillaries and minute veins, apart from mechanical 

 injury, was rediscovered by Cohnheim in 1867. 



Cohnheim's experiment demonstrating the passage 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 trans- 

 parent 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 



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.) 



