234 THE CIRCULATION OF THE BLOOD 



Various explanations of these remarkable phenomena have been 

 suggested. It is no longer believed that pseudo-stomata between contigu- 

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

 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 0.5 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 incon- 

 sistent with the facts, which show that the whole circulation is accom- 

 plished 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 0.5 mm. Therefore 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 

 assistance to the flow is afforded, &, by the action of the muscles capable of 

 pressing on the veins with valves, and c, 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 a vein, and the current of 

 blood in it is obstructed, the portion behind the seat of pressure becomes 

 swollen and distended as far back as the next pair of valves, which are in 

 consequence closed. Thus, whatever force is exercised by the external 

 pressure of the muscles on the veins, is distributed partly in pressing the 

 blood onward in the proper course of the circulation, and partly in pressing 

 it backward and closing the valves behind. 



