VELOCITY AND PRESSURE OF BLOOD-FLOW. 435 



bladder of the same animal. With a good preparation many 

 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 maintains a constant flow. In the veins the flow 

 increases markedly in rapidity, and indeed it may be observed 

 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 accumulation of cor- 

 puscles 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 can not be considered 

 in this connection. 



The Velocity of the Blood-flow. The microscopical observa- 

 tions 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 



