328 TEXT-BOOK OF PHYSIOLOGY. 



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 

 intermittency in their 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 

 known as the expansion wave. 



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 afterwards again 

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

 capillaries, venules, and veins may therefore be schematically arranged 

 (Fig. 152) in a manner identical with the schematic arrangement of tubes 

 represented on page 325. The heart, with which they are in connection, 

 when filled with blood may be compared with the reservoir filled with water, 

 and the in tra- ventricular pressure developed during the contraction, to 

 the downward pressure of the water when the stopcock at is A opened . 



The Stream-bed. The stream-bed, the path along which the blood 

 flows, varies widely in its total sectional area in different parts of its course, 

 being least in the aorta and venae cavae, and greatest in the capillaries. In 

 passing from the base of the aorta toward the capillaries the sectional area of 

 individual arteries, in consequence of repeated branching, diminishes, 

 though their total sectional area increases and in direct proportion to their 

 distance from the heart. In the capillary system the sectional area of an 

 individual capillary attains its minimal value, though the total sectional 

 area attains its maximal value. Comparing one with the other, it has been 

 estimated that the total sectional area of the aortic bed is to the total sectional 

 area of the capillary bed as i is to 600 or 800. In passing from the capillary 

 into the venous system the sectional area of individual veins increases, 

 though the total sectional area decreases and in direct proportion to their 

 distance from the capillaries. 



