THE CAPILLARY CIRCULATION 219 



tardatioii of the blood-stream. Such a rhythmical movement is 

 absent from the capillaries and veins in a normal condition; the 

 stream is continuous in both. In the arteries the core of red 

 corpuscles does not completely fill the lumen, but moves along the 

 axis of the stream. To the outside there lies a clear layer of plasma, 

 in which, when the stream moves slowly, white corpuscles roll. In 

 the veins there is also a similar peripheral plasmatic layer, in which 

 the white corpuscles roll slowly along, sticking now and again to the 

 wall of the vessels in their course. In the smallest capillaries the 

 plasma layer cannot be distinguished, the red corpuscles march in 

 single file, and often become distorted and bent as they pass. These 

 capillaries are invisible to the eye so soon as corpuscles cease 

 to pass through them. Thus, in the course of an observation, 

 capillaries may be seen to appear and vanish from view. In the 

 angles of the capillary network, red corpuscles may be seen to stick 

 and hang in the balance, bent round the angle, half in one branch 

 and half in another, until finally swept on into the rush and hurry 

 of the stream. 



The white corpuscles progress with a slow rolling motion in the 

 plasmatic layer. The axial stream travels with the greatest velocity, 

 and thus the side of the leucocyte which lies at any moment nearer 

 the axis is driven on with the greater speed; hence the rolling move- 

 ment. The white corpuscles travel in the peripheral layer, the red 

 in the axial layer, for the latter are the heavier. It is not, as has been 

 supposed, that the white are lighter and the red corpuscles heavier 

 than the plasma. Both forms are of a higher density than the plasma. 



If particles of graphite and carmine be circulated through glass 

 capillaries, the lighter carmine particles travel in the peripheral layer. 

 When resin is substituted for graphite, the carmine travels in the 

 axis. If pus corpuscles and milk globules are circulated, the cor- 

 puscles occupy the axial stream. 



If the resistance in the arterioles be lowered to a certain point, the 

 capillary circulation remains no longer pulseless. Thus, when the 

 chorda tympani nerve is stimulated, the blood may issue in pulses 

 from the vein of the submaxillary gland. By plunging the hand in 

 very hot water, the pulse may be seen to reach even the turgid veins 

 on the back of the hand. In cases of aortic insufficiency, a capillary 

 pulse is readily obtained in any area of congestion which is produced 

 by scratching the skin. 



The effect of vaso -dilatation can be observed under the microscope 

 e.g., in the tongue of a curarized frog. On brushing the tongue 

 an appearance of intense redness shows that arterial congestion has 

 set in. All the vessels, arteries, capillaries, and veins, are wide and 

 strongly distended with blood. Innumerable capillaries are perceptible 

 at a glance, where previously a few red-coloured threads were toil- 

 somely sought for ; and in all these vessels, small and large, the blood 

 rushes on with the greatest rapidity so rapidly that even in the 

 capillaries the eye in vain strives to catch the outline of a single 

 corpuscle. 



