230 CIRCULATION OF THE BLOOD. 



(particularly the scaphoid) by a plate of brass, the under sur- 

 face of which is covered with ebonite. In the middle of the 

 upper surface of this plate is a socket for the reception of the 

 point of a finely-cut screw, which revolves in it freely. Above, 

 the screw ends in a milled head (Y), between which and its 

 point it passes, first loosely through a guide, which is of the 

 same piece with the brass plate; and, secondly, through a hole 

 in the end of the brass frame of the sphygrnograph (F), in 

 which it fits closely. This being the construction, it is scarcely 

 necessary to explain that, by turning the milled head, the dis- 

 tance between the ebonite surface and the frame is varied 

 according to the direction of revolution, and that in this way 

 the pressure on the artery may be readily modified when the 

 instrument is in use. The extent of the modifications thus 

 produced, however, still remains undetermined, for they vary 

 according to the form of the limb and the relative position of 

 the arm and forearm at the time of observation. To measure 

 them, we must have recourse to another method which is at 

 once simple and accurate. It is obvious that, provided that 

 the spring is firmly and immovably fixed in its place, the press- 

 ure which it makes against any object pushed against it from 

 below is determinate by the force which is exerted in pushing 

 it. If, for example, I turn the instrument upside down, and 

 place a weight of 200 grammes on what was before the under 

 surface, now the upper surface, of the spring, I push it back 

 some fraction of an inch from its position of equilibrium ; I 

 learn that, whenever it is pushed back to this extent, the press- 

 ure it exerts on the surface opposed to it is that of 200 

 grammes' weight. Repeating the experiment with a series of 

 other weights, I can in a similar way obtain other measure- 

 ments of distance corresponding to them, and thus, by com- 

 bining the results, accomplish the graduation of the spring in 

 such a waj r that the pressure made by it can be alwa3 r s known 

 from the extent of its deflexion. The most convenient way of 

 determining this deflexion is either to measure the distance 

 between the head of the steel screw, the point of which rests 

 on the upper surface of the spring, and the surface of the brass 

 lever, with a scale (as shown in Fig. 210); or, better still, to 

 have the screw itself graduated. In either case, care must be 

 taken to fix the writing lever in the proper position i.e., in a 

 direction which coincides with the direction of movement of 

 the writing surface before making the measurements. 



40. The Artificial Artery or Arterial Schema. The' 

 phenomena of arterial pulsation can be best studied in a well- 

 constructed schema or artificial artery, consisting in an elastic 

 tube through which water is propelled by an artificial heart, 

 i. <;., by a pump of such construction that it discharges its 

 contents into the tube in a manner which mechanically 



