BY DR. BURDON-SANDERSOtf. 235 



with the aid of a schema in which the heart is represented by 

 an elastic bag of such size that it can be squeezed with the hand. 

 This bag communicates at one end with a long elastic tube 

 representing the arterial system, at the other with a vessel con- 

 taining water, the apertures being furnished with valves which 

 open in directions corresponding to those of the heart. If 

 three levers, like those we have just been using, are so arranged 

 as to receive the successive expansion-waves produced by 

 repeatedly squeezing the bag at different distances from their 

 origin, the three tracings are obtained which are represented 

 in Fig. 214. It is instructive to observe that these tracings have 

 no resemblance to those of the arterial pulse. The reason is, 

 that the contracting hand is entirely unlike the contracting 

 heart. The real heart, like the schematic heart used in the pre- 

 vious experiments, contracts suddenly, exerting its greatest 

 vigor at the commencement. The hand contracts gradually, 

 and is, moreover, incomparably weaker, as compared with the 

 resistance to be overcome, than the heart. Hence the expan- 

 sion of the tube is slow, lasts a long time, and is followed by 

 no rebound. This very slowness of the process enables one to 

 see the steps of it better. In the distal part of the tube, to 

 which the upper tracing corresponds, the expansion culminates 

 later than in the proximal part, because the motion commu- 

 nicated to its contents by the grip of the hand at the outset 

 does not begin to tell on the former (distal) until the latter is 

 fully expanded. 



In the pulse tracings obtained with the schema arranged as 

 in Fig. 211, so as to imitate the natural pulse, the two effects 

 produced in the preceding experiments separately, are combined 

 with each other. Thus in Fig. 212 a, the abrupt initial ascent 

 of the lever is the first of a series of vibratory movements of 

 the same kind as those shown in Fig. 213, and is instantly fol- 

 lowed by a recoil. In the same tracing, the more gradual ac- 

 cumulation of arterial pressure manifests itself in the fact that 

 the lever jerked up 1 by the vibration does not (as in Fig. 213) 

 descend to its previous position, but remains elevated for a 

 period, which, as already seen, depends on the duration of the 

 injection of liquid. 



This combination of effects is seen with equal distinctness in 

 the natural radial pulse. The abrupt line of ascent with which 



1 In the sphygmographs, lately made by Bregnet, the movement of 

 the spring is communicated to the writing lever by a mechanism shown 

 in Fig. 209 &, more simple and effectual than that described on p. 227. 

 The screw is hinged to the upper surface of the spring in such ;\ way 

 that it presses gently against the axis of the lever, and acts upon it as a 

 rack on its pinion. In this way the lever follows the movements of the 

 screw muck more exactly, and the jerk is diminished. (See Garrod on 

 Sphygmography. Journ. of Anat. and Phvs., May, 1872, p. 399.) 



