BY DR. BURDON-SANDERSON. 233 



the figure. Its purpose is to write on the plate of the sphyg- 

 raogrnph the duration of the injection of liquid. It consists of 

 a cylinder of box-wood (Fig. 211, H), the steel axis of which rests 

 horizontally on bearings so placed that the cylinder revolves in a 

 direction at right angles to that of the movement of the plate at 

 a short distance from it. From one side of the cylinder a steel 

 needle projects, which, when the cylinder turns, makes a mark 

 on the smoked surface. Round one side of the cylinder runs 

 a cord of spun silk, the two ends of which stretch, one from 

 either side of it, to the point of a vertical arm (L) ; this arm 

 springs from the wooden lever already described, by which the 

 valves are opened and shut. Of the two cords, the upper one 

 is rendered partly elastic by the interposition of a short length 

 of caoutchouc. So long as the aortic valve is closed, the needle 

 remains in contact, but the moment the valve is opened, it is 

 withdrawn, and we obtain, first, an upper horizontal line, broken 

 at regular intervals which are, of course, limited in time by 

 the opening and closing of the aortic valve and, secondly, 

 a pulse-tracing (Fig. 212, 6), which may be compared with 

 it. This exact correspondence between the length of time the 

 heart is acting and the time which elapses between the begin- 

 ning of the expansion and the commencement of the contrac- 

 tion, affords evidence that the latter is dependent on the 

 former. 



3. Lastly, it can be shown that the second expansion is not, 

 as might be supposed, connected with the closure of the com- 

 munication between the pump and the elastic tube (the shut- 

 ting of the aortic valve), but is a consequence of the disturb- 

 ance of equilibrium produced in the tube itself by the act of 

 distension. To demonstrate this, the second expansion must 

 be studied under various conditions and by various methods; 

 among the best is the following: A narrow tube, closed at one 

 end, and containing air, is connected by means of a T-piece 

 with the experimental tube or arterj 7 . The volume of air con- 

 tained in the tube varies with the pressure, indicating its varia- 

 tions with great sensitiveness. If the surface of the liquid in 

 the tube is watched during the action of the pump, it is very 

 easy to see that the volume of air is diminished as the valve D 

 opens, enlarges for a moment, and again contracts after the 

 injection has ceased. If now the action of the pump is so modi- 

 lied that, after opening the valve D, the discharge of liquid is 

 continued for some seconds (both valves remaining open), we 

 learn that the first expansion is followed by a second just as 

 before. If the same experiment is made with the sphygmo- 

 graph, a tracing is obtained in which the ascent due to the open- 

 ing of the valve is succeeded by a momentary descent, then a 

 second ascent, the lever finally assuming a position correspond- 



