CIRCULATION OF THE BLOOD. 



159 



be at rest; for if any of tne corpuscles happen to be forced within it, 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. Sometimes, in- 

 deed, when the motion of the blood is not strong, many of the white cor- 

 puscles collect in a capillary vessel, and for a time entirely prevent 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 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 

 to properly distend the elastic arteries. 



Diapedesis of Blood Corpuscles. Until with- 

 in the last few years it has been generally supposed 

 that the occurrence of any transudation from the in- 

 terior 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 were ruptured. It is true that an Eng- 

 lish 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 phenomena of escape of the blood-corpuscles from 

 the capillaries and minute veins, apart from mechanical injury, were re- 

 discovered by Professor 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- 



Fio. 141. A large cap- 

 illary from the frog^s 

 mesentery eight hours 

 after irritation had been 

 set up, showing emigra- 

 tion of leucocytes, a, 

 Cells in the act of trav- 

 ersing the capillary 

 6, some already 

 (Frey.) 



wall; 



