MECHANICS OF THE CIRCULATION IN THE VESSELS 117 



(only one-fifth to one-sixth as great), for the total resistance of the 

 vascular path through the lungs is much less than that of the 

 systemic circuit. In dogs with natural respiration the pressure in 

 the pulmonary artery was found to vary between 14 and 26 mm. of 

 mercury, averaging about 20 mm. 



The Velocity-Pulse. We have seen that the blood is propelled 

 through the arteries in a series of waves that travel from the heart 

 towards the periphery. The particles in the front of the pulse-wave 

 are constantly changing, but since every section of the arterial tree 

 is successively distended, 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, however, 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 dis- 

 tance. 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. 



