482 CIRCULATION OF BLOOD AND LYMPH. 



important peculiarities in the blood-flow may be observed directly. 

 If the field is properly chosen one may see at the same time the flow 

 in arteries, capillaries, and veins. It will be noticed that in the 

 arteries the flow is very rapid and somewhat intermittent, that is, 

 there is a slight acceleration of velocity, a pulse, with each heart 

 beat. In the capillaries, on the contrary, the flow is relatively very 

 slow; the change from the rushing arterial stream to the deliber- 

 ate current in the capillaries takes place, indeed, with some 

 suddenness. The capillary flow, as a rule, shows no pulses corre- 

 sponding with the heart beats, but it may be more or less irregular, 

 that is, the flow may nearly cease at times in some capillaries, 

 while again it becomes distinct and uniform. In the veins the 

 flow increases markedly in rapidity, and, indeed, it may be ob- 

 served that the larger the vein, the more rapid is the flow. There 

 is not, however, as a rule, any indication of an intermittence or 

 pulse in this flow the velocity is entirely uniform. In both 

 arteries and veins it will be noticed that the red corpuscles form a 

 solid column or core in the middle of the vessel, and that between 

 them and the inner wall there is a layer of plasma containing only, 

 under normal conditions, an occasional leucocyte. The accumula- 

 tion of corpuscles in the middle of the stream makes what is known 

 as the axial stream, while the clear layer of plasma is designated as 

 the inert layer. The phenomenon is readily explained by physical 

 causes. As the blood flows rapidly through the small vessels the 

 layers nearer the wall are slowed by adhesion, so that the greatest 

 velocity is attained in the middle or axis of the vessel. The cor- 

 puscles, being heavier than the plasma, are drawn into this rapid 

 part of the current. It has been shown by physical experiments 

 that, when particles of different specific gravities are present in a 

 liquid flowing rapidly through tubes, the heavier particles will be 

 found in the axis and the lighter ones toward the periphery. In 

 accordance with this fact, leucocytes, which are lighter than the 

 red corpuscles, may be found in the inert layer. When the con- 

 ditions become slightly abnormal (incipient inflammation) the 

 leucocytes increase in number in the inert layer sometimes to a 

 very great extent, owing apparently to some alteration in the 

 endothelial walls whereby the leucocytes are rendered more ad- 

 hesive. The agglutination of the leucocytes and their migration 

 through the walls into the surrounding tissues are described in 

 works on Pathology. 



The Velocity and Volume of the Blood-flow. The microscopical 

 observations described above show that the velocity of the blood- 

 current varies widely, being rapid in the arteries and veins and slow 

 in the capillaries. To ascertain the actual velocity in the larger 

 vessels and the variations in vessels of different sizes experimental 

 determinations are necessary. While the general principle involved 



