106 A MANUAL OF PHYSIOLOGY 



every section contains more blood while the pulse-wave is 

 passing over it than it contained immediately before. And 

 since there is always a fairly free passage for this blood 

 towards the periphery, there is a bodily transfer on the 

 whole of a certain quantity with every wave. 



The translation of the blood, instead of being entirely 

 intermittent, as it would be in a rigid tube or in an elastic 

 system with a slow action of the central pump, is to some 

 extent constantly going on ; for a portion of a blood-wave is 

 always passing through every section of the arterial channel. 

 Thus, we arrive at the same distinction as to the onward 

 movement of the blood itself as we previously reached in 

 regard to the blood-pressure, the distinction between the 

 constant or permanent factor of the velocity and the periodic 

 factor, which we may call the velocity-pulse. 



The Velocity of the Blood. By the velocity or rate of flow of a 

 river we should mean, if the flow were uniform throughout the whole 

 cross-section, the rate of movement of any given portion or particle 

 of the water. If we could identify a portion of the water, we could 

 determine the velocity by measuring the distance travelled over by 

 that portion in a given time. If the velocity was uniform over the 

 channel, we could predict the actual time which would be required 

 to traverse any fractional part of the measured distance. If, how- 

 ever, the velocity of the current changed from point to point, then 

 we could only deduce from our observation the mean rate of the river 

 for the measured distance. To determine the actual rate for any 

 given portion of this distance over which the rate was uniform, we 

 should have to make a separate observation for this portion alone. 



But as soon as we pass from an ideal frictionless river to an actual 

 stream, in which the water at the bottom and near the banks flows 

 more slowly than that in the middle and on the surface, we are in 

 every case restricted to the notion of mean velocity. We may 

 distinguish between the velocity of different parts of the current, 

 between that of the mid-stream and the side current, the bottom and 

 the surface layers ; but when we consider the river as a whole, we 

 take cognizance only of the mean or average velocity. And at any 

 cross-section this may be defined as the volume of water passing per 

 hour, or whatever the unit of time may be, divided by the cross- 

 section of the current. It is evident that this does not enable us to 

 determine the actual velocity of any given particle of the water at any 

 given moment within a measured interval ; nor does it tell us whether 

 or not the average velocity of the current has itself undergone 

 variations within the period of observation. 



We have dwelt upon this point because the measurement 

 of the velocity of the blood, to which we must now turn, 



