224 HANDBOOK OF PHYSIOLOGY. 



The white corpuscles pass through the capillary wall chiefly by the 

 amosboid movement with which they are endowed. This migration oc- 

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

 much increased. 



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

 circumstances 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 them, work their way through by amoeboid movement. 



Various explanations of tl:~3e remarkable phenomena have been 

 suggested. Some believe that pseudo-stomata between contiguous endo- 

 thelial cells provide the means of escape for the blood-corpuscles. But 

 the chief share in the process is probably to be found in the vital en- 

 dowments with respect to mobility and contraction of the parts con- 

 cerned both of the corpuscles and of the capillary wall itself. 



The circulation through the capillaries must, of necessity, be largely 

 influenced by that which occurs in the vessels on either side of them 

 in the arteries or the veins; their intermediate position causing them to 

 feel at once, so to speak, any alteration in the size or rate of the arterial 

 or venous blood-stream. Thus, the apparent contraction of the capilla- 

 ries, on the application of certain irritating substances, and during fear, 

 and their dilatation in blushing may be referred primarily to the action 

 of the small arteries. 



The Venous Flow. 



The blood-current in the veins is maintained (a) primarily by the vis 

 a tergo of the contraction 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, as well as (c) by the suction action of 

 the heart, and (e) aspiration of the thorax. 



The effect of muscular pressure upon the circulation may be thus 

 explained. 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 pres- 

 sure becomes swollen and distended as far back as the next pair of 

 valves, which are in consequence closed. Thus, whatever force is exer- 

 cised by the 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. 



The circulation might lose as much as it gains by such an action, if 

 it were not for the numerous communications, one with another; for 

 through these, the closing up of the venous channel by the backward 



