154 



HANDBOOK OF PHYSIOLOGY. 



they move more slowly than before, rolling lazily along the side of the 

 vessel, and often adhering to its wall. Part of this slow movement of 

 the pale corpuscles and their occasional stoppage may be due to their 

 having a natural tendency to adhere to the walls of the vessels. Some- 

 times, indeed, when the motion of the blood is not strong, many of the 

 white corpuscles collect in a capillary vessel, and for a time entirely pre- 

 vent the passage of the red corpuscles. 



Intermittent flow in the Capillaries. When the peripheral re- 

 sistance is greatly diminished by the dilatation of the small arteries and 

 capillaries, so much blood passes on from the arteries 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 ventric- 

 ular systole is not converted into a continuous stream by the elasticity 

 of the arteries before the capillaries are reached; and so intermittency 

 of the flow occurs in capillaries and veins and a pulse is produced. The 

 same phenomenon may occur when the arteries become rigid from dis- 

 ease, 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 insuf- 

 ficient to properly distend the elastic arteries. 



Diapedesis of Blood-Corpuscles. 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-vessel was rup- 

 tured. It is true that an 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 con- 

 firming what had been stated a short time pre- 

 viously by Addison); and that, as no opening 

 could be seen before their escape, so none 

 could be observed afterwards so rapidly was 

 the part healed. But these observations did 

 not attract much notice until the phenomena 

 of escape of the blood-corpuscles from the capil- 

 laries and minute veins, apart from mechanical 

 injury, were re-discovered 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 man- 



Fia. 140. A large capillary 

 from 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; 6, some already 

 (Frey.) 



