THE CIRCULATION OF THE BLOOD. 



When the peripheral resistance is greatly diminished by the dilata- 

 tion 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 ventricular systole is not converted 

 into a continuous stream by the elasticity of the 

 arteries before the capillaries are reached; and so in- 

 lermittency of the flow occurs both in capillaries 

 and veins and a pulse is produced. The same phe- 

 nomenon 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 sys- 

 tole has time to pass on to the capillaries before the 

 next stroke occurs; the amount of blood sent at each 

 stroke being insufficient to properly distend the 

 elastic arteries. 



It was formerly supposed that the occurrence 

 of any transudation from the interior of the capil- 

 laries 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 ruptured. 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 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 phenome- 

 non of escape of the blood-corpuscles from the capillaries 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- 

 ner. A frog is urarized, that is to say, paralysis is produced by ejecting 

 under the skin a minute quantity of the poison called urari; 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. 



Fig. 188. A large ca- 

 pillary 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.) 



