1 08 A MANUAL OF PHYSIOLOGY 



right ventricle than in the aorta and left ventricle (only one- 

 third to one-sixth as great), for the total resistance of the vas- 

 cular 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 pro- 

 pelled 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 con- 

 tains 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 inter- 

 mittent, 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 con- 

 stantly 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 



