274 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



systole is not converted into a continuous stream by the elasticity 

 of the arteries before the capillaries are reached ; and so intermit- 

 tency of the flow occurs both in capillaries and veins and a 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 out at each stroke 

 being insufficient to properly distend the 

 elastic arteries. 



It was formerly supposed that the occur- 

 rence of any transudation from the interior 

 of the capillaries into the midst of the sur- 

 rounding 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 ruptured. 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 after- 

 wards 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 capillaries and minute 

 veins, apart from mechanical injury, were 

 rediscovered by Cohnheim in 1867. 



Cohnheim's experiment, demonstrating the passage of the cor- 

 puscles through the wall of the blood-vessel, is performed in the 

 following manner : A frog is curarized, and the abdomen having 

 been opened a portion of 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 accompanying quickening of the blood-stream, 

 there ensues a retardation of the current, and blood-corpuscles, 

 both red and -white, begin to make their way through the 

 capillaries and small veins. 



The process of diapedesis of the red corpuscles, which occurs 

 under circumstances of impeded venous circulation, and con- 



Fig. 269. A large capil- 

 lary from the frog's 

 mesentery eight hours 

 after irritation had 

 been set up, showing 

 emigration of leuco- 

 cytes, a, Cells in the 

 act of traversing the 

 capillary wall ; b, some 

 already escaped. 



(Frey. 





