io8 A MANUAL OF PHYSIOLOGY 



energy that disappears. The stock of energy imparted by 

 the contraction of the heart to a given mass of blood con- 

 stantly diminishes as it passes round from the aorta to the 

 right side of the heart, for friction is constantly being over- 

 come and heat generated. This energy, as we have seen, 

 exists in a moving liquid in two forms, potential and kinetic, 

 the former being measured by the lateral pressure, the latter 

 varying directly as the square of the velocity. Whenever 

 the velocity, and therefore the kinetic energy, of a given mass 

 of the blood is diminished without a corresponding increase 

 in the potential energy, some of the total stock of energy 

 must have been used up to overcome resistance (p. 73). 



In a uniform, rigid, horizontal tube, as has been already remarked, 

 the velocity (and consequently the kinetic energy) is the same at 

 every cross-section of the tube, while the potential energy, represented 

 by the lateral pressure, diminishes regularly along the tube. When 

 the calibre of the tube varies, it is different. Suppose, for instance, 

 that the liquid passes from a narrower to a wider part, the velocity 

 must diminish in the latter. The kinetic energy of visible motion 

 which has disappeared must have left something in its room. Here 

 there are three possibilities : (i) The kinetic energy that has dis- 

 appeared may be just enough to overcome the extra friction in the 

 wider part of the tube due to eddies and consequent change of 

 direction of the lines of flow ; in this case the potential energy of a 

 given mass of the liquid will be the same at the beginning of the 

 wider part as in the narrower part. The lost kinetic energy will 

 have been transformed into heat. (2) The kinetic energy which has 

 disappeared may be greater than is enough to overcome the extra 

 resistance ; a portion of it must, therefore, have gone to increase the 

 potential energy, and the lateral pressure will be greater in the wide 

 than in the narrow part. (3) The lost kinetic energy may be less 

 than enough to overcome the extra resistance ; in this case both the 

 lateral pressure and the velocity will be less in the wide than in the 

 narrow part. It has been experimentally shown that when a narrow 

 portion of a tube is succeeded by a considerably wider portion, 

 and this again by a narrow part, case (2) holds; and the liquid may, 

 under these conditions, actually flow from a place of lower to a place 

 of higher lateral pressure. 



In the vascular system the conditions are not the same. 

 The widening of the bed which takes place as we proceed in 

 the direction of the arterial current is not due to the widen- 

 ing of a single trunk, but to the branching of the channel 

 into smaller and smaller tubes. In the larger arteries the 

 increase of resistance is so gradual that both the potential 



