THE VENOUS FLOW 223 



occurs to a limited extent in health, but in inflammatory conditions is 

 much increased. 



The process of diapedesis of the red corpuscles, which occurs under cir- 

 cumstances of impeded venous circulation, and consequently increased 

 blood pressure, resembles closely the migration of the leucocytes, with the 

 exception that they are squeezed through the wall of the vessel, and do not, 

 like the leucocytes, work their way through by ameboid movement. 



Various explanations of these remarkable phenomena have been 

 suggested. Some believe that pseudo-stomata between contiguous 

 endothelial cells provide the means of escape for the blood corpuscles. 

 But the chief share in the process is probably due to mobility and con- 

 traction of the parts concerned, both of the corpuscles and of the capillary 

 wall itself. 



The Speed of the Blood in the Capillaries. The velocity of the 

 blood through the capillaries must, of necessity, be largely influenced by 

 that which occurs in the vessels on both sides of them, in the arteries and 

 the veins. Their intermediate position causes them to respond at once to any 

 alteration in the size or rate of the arterial or venous blood stream. Thus, 

 the apparent contraction of the capillaries, on the application of certain 

 irritating substances or during certain mental states, and their dilatation in 

 blushing may be referred primarily to the corresponding action of the small 

 arteries. 



The Measurement of Velocity in the Capillaries. The observation of 

 Hales, E. H. Weber, and Valentin agree very closely as to the rate of the 

 blood current in the capillaries of the frog. The mean of their estimates 

 gives the velocity of the systemic capillary circulation at about o . 5 mm. per 

 second. The velocity in the capillaries of warm-blooded animals is greater, 

 in the dog 0.5 to 0.75 mm. per second. This may seem inconsistent with 

 the facts, which show that the whole circulation is accomplished in about half 

 a minute. But the whole length of capillary vessels, through which any 

 given portion of blood has to pass, probably does not exceed o . 5 mm. There- 

 fore the time required for each quantity of blood to traverse its own appointed 

 portion of the general capillary system will scarcely amount to more than a 

 second. This comparatively slow velocity is evidently favorable to the 

 nutritive interchanges that go on through these thin-walled vessels between 

 the blood within the capillaries and the outside active tissues. 



The Venous Flow. The blood current in the veins is maintained, 

 a, primarily by the contractions of the left ventricle; but very effectual assist- 

 ance to the flow is afforded, b, by the action of the muscles capable of pressing 

 on the veins with valves, and, c y by the aspiration of the thorax and possibly, 

 d, by the aspiration of the heart itself. 



The effect of muscular pressure upon the circulation may be thus ex- 

 plained: When pressure is applied to any part of vein, and the current of 



