338 TEXT-BOOK OF PHYSIOLOGY 



distention of the tube. This latter effect continues until the elastic recoil of the 

 walls of the tube exactly counterbalances the pressure of the water from within. 

 When this condition is established the tube becomes practically a tube with rigid 

 walls, and hence so long as the primary pressure is uniform, the velocity and lateral 

 pressure will obey the laws which hold true for rigid tubes. 



If, however, the primary pressure be intermittently applied or alternately 

 increased or decreased, and the water forced into the tube, previously filled with 

 water but under no particular pressure, it will be forced out of the peripheral end 

 of the tube more rapidly during the period of the increase of pressure and 

 less rapidly during the period of the decrease of pressure or it may cease entirely. 

 The extent to which the outflow becomes merely remittent, or entirely intermittent 

 will depend on the amount of resistance, whether this be due to length of tube or 

 a narrowed outlet, and the degree of elasticity. 



When these factors are of such a nature that the resistance is very high and the 

 elasticity slight, the outflow will be intermittent. But if they are made to change 

 gradually, and this is especially the case with the resistance, from a slight to a 

 greater value, the outflow gradually changes from an intermittent to a remittent 

 and finally to a continuous outflow and for the following reasons: 



With a given resistance and elasticity, the fluid which is driven into the tube by 

 the action of the primary pressure exerts more or less lateral pressure, gives rise to 

 a distention of the tube, and acquires a certain velocity of outflow. In consequence 

 of the distention, a portion of the fluid accumulates. With the cessation in the 

 action of the primary pressure, the elastic walls recoil and force the accumulated 

 fluid forward and so maintain more or less effectively the same velocity of outflow 

 until there is a return of the pressure. If the resistance be great and the elasticity 

 slight, this is impossible and the outflow will be entirely intermittent. But if they 

 are made to increase in value, the proportionate amount of the fluid which accumu- 

 lates during the action of the primary pressure will also increase in amount and 

 hence there will be an increase in the distention of the tube. The elastic recoil 

 will therefore be greater in amount and longer in duration, and hence the outflow 

 will change to a remittent and finally to a continuous outflow. 



Coincident with the action and cessation of action of the primary pressure 

 there is a corresponding increase and decrease of the lateral pressure and when the 

 intermittent action is sufficiently rapid, the excess of fluid entering the tube over 

 that discharged becomes sufficiently great to maintain a certain average or mean 

 pressure, which, however, undergoes an alternate increase and decrease with each 

 variation in the primary pressure. 



The temporary increase and decrease of the pressure and the consequent 

 expansion and recoil of the tube in the neighborhood of the pressure vessel, give 

 rise to a wave on the surface of the tube which is propagated with more or less 

 rapidity though with decreasing amplitude from the beginning to the end of 

 the tube and causing in each section a corresponding expansion and recoil, and 

 which is known as the expansion wave. 



If a system of branching and again uniting tubes with elastic walls arranged 

 in a manner similar to that represented in Fig. 152 is connected with a reservoir, 

 and the primary pressure is made to act intermittently, or is alternately increased 

 or decreased, phenomena will be presented which closely resemble the phenomena 

 presented by the circulatory apparatus at the time of the heart beat. 



THE APPLICATION OF THE FOREGOING FACTS TO THE 

 VASCULAR APPARATUS 



The systemic vascular apparatus may be conceived of as a system of tubes 

 which have symmetrically divided and subdivided and afterward again 

 united and reunited in a corresponding manner. The arteries, arterioles, 



