234 CIRCULATION OF THE BLOOD. 



ing to the increased pressure produced by the continuous cur- 

 rent which is now passing through the tube. 



From this experiment we learn, as regards the artificial 

 artery, first, that the second beat of the pulse is not, as has 

 been sometimes imagined, a mere product of the instrumental 

 method we employ to demonstrate it, for it can be shown quite 

 as distinctly in other ways ; and secondly, that it is a result 

 of the disturbance produced in the tube by the sudden disten- 

 sion of its proximal end, independently of any subsequent move- 

 ment or action of the pump. 



41. Experiments with the Schema relating to the 

 Form of the Arterial Pulse. In the schema, the injection 

 of liquid by the artificial heart into the proximal end of the 

 elastic tube produces two effects, which can not only be dis- 

 tinguished in the tracing, but can be proved experimentally to 

 be independent of each other. One of these consists in the 

 transmission of a series of vibratory movements of the liquid 

 (i. e., movements in alternately opposite directions) from the 

 proximal to the distal end ; the other, in the communication 

 of the pressure existing in the artificial heart at the moment 

 that the valve D is opened to the contents of the arterial tube. 

 The first of these effects can be readily demonstrated on the 

 schema. 



If 3-011 take an elastic tube, distended with water, and closed 

 at both ends, and give it a smart rap with a hammer at one end, 

 an effect is transmitted along the tube which, although of an 

 entirely different nature to that which constitutes the pulse, yet 

 mixes itself up with it under certain conditions. This effect is 

 called, from its mode of origin, a percussion-wave. To produce 

 it, close the communication between the schematic heart and 

 artery, and arrange the lever (Fig. 211) in such a manner that, 

 by striking on it with a hammer (at D), the required percus- 

 sion may be produced. The tube being placed under the spring 

 of the sphj'gmograph (at o), in such a position that the length 

 of tubing between the point of percussion (r>) and the spring 

 (<;) is equal to two metres, a succession of percussion-waves is 

 produced, and a tracing obtained similar to those shown in Fig. 

 213, in which the interruptions in the upper line indicate the 

 moment of percussion, the vertical ascents in the lower line the 

 effects. In the figure, the interval of time between cause and 

 effect corresponds to the portion of the horizontal line (in the 

 lower tracing) which lies between the short vertical scratch and 

 the commencement of the ascent. The rate of movement of 

 the clock-work during the experiment being 8 centimetres per 

 minute, this distance corresponds to about a fifteenth of a 

 second. 



The other effect, the communication of pressure from the 

 artificial heart to the elastic tube, may be readily illustrated 



