162 EXPERIMENTAL PHYSIOLOGY 



elastic tube some of the energy is at once imparted to the tube wall 

 and is represented by the stretching of the wall, which, being elastic, 

 can recoil and thus re-impart its store of energy to the fluid from 

 which it received it. This is the most important factor explaining 

 the conversion of the intermittent flow from the syringe into a con- 

 stant flow through the high resistance a ; for though the original 

 source of energy is intermittent in its action, the recoil of the tube 

 wall is constant and lasts as long as the tube is kept over-distended. 

 The elastic wall therefore acts as a means of temporarily storing 

 the energy produced by each forcible emptying of the syringe and 

 subsequently yielding it up again, not as a sudden discharge analogous 

 to the manner in which it received that energy, but as a steady 

 delivery extending over some time. 



There is a further phenomenon of the circulation which we can 

 also study upon the schema, namely, the pulse wave. At each 

 emptying of the syringe the fluid ejected at first only produces a 

 distension of the first part of the arterial tube, and as a consequence 

 that part is at a higher tension than that of the piece of tube 

 immediately following. It therefore contracts and forces some of its 

 contents into the next piece of tubing, which in its turn becomes 

 more stretched than the piece next following, and therefore contracts, 

 and so the series of phenomena are repeated along the whole tube. 

 Thus a wave of distension and contraction passes along the tube, 

 which wave is known as a pulse wave. As the pulse wave is in 

 reality a wave of pressure we can study it by recording the changes 

 in pressure at two points on the tube by means of manometers, as in 

 the previous experiment, or we can examine it by recording the 

 changes in the transverse diameter of the tube at two different points, 

 which is, of course, but another way of recording changes of pressure, 

 for the tube is distended in proportion to the pressure of the fluid 

 within it. We will employ this latter method, for it is the one 

 ordinarily employed in recording the pulse wave in man. 



Experiment 2. — For this purpose we may modify the apparatus of fig. 

 124 by omitting the manometers and in their place pass the tube over two 

 grooved metal supports, a and b, fig. 125, firmly fixed to collars held upon a 

 vertical bar. Attached to the two collars are recording levers l 1 l'-, which 

 are moved by two light vertical rods with grooved pads, e and F, at their 

 ends where they rest upon the tube. In this way an increase of the diameter 

 of the tube is recorded as a rise of the writing lever. Adjust the two points 

 to write vertically over one another, and record the movements of the two 

 levers caused by a single compression of the syringe, and secondly by a 

 series of regularly repeated compressions. 



In such an experiment it is found that there is an appreciable 

 time interval between the instants at which the tube begins to expand 



