330 OF THE CIRCULATION OF THE BLOOD. 



small plate of ivory R capable of elevation or depression by the screw v. 

 The long arm of the wooden lever L terminates in a fine point touching the 

 plate of smoked glass, or glazed card P, which is made to move gradually 

 forwards by clockwork in the box H. The immediate descent of the lever 

 after its elevation is accomplished by the very delicate spring at r. 1 [Mr. 

 Keyt, of Cincinnati, has substituted for the elastic spring of Marey, an elastic 

 membrane and water to communicate the movements of the artery to the trac- 

 ing lever. This instrument can also be used as a cardiograph. 2 ] A modi- 



FIG. 130. 



Marey 's Sphygmograph. 



fication of Marey's instrument is shown in Fig. 131, by which the pressure 

 exerted on the artery can be approximatively measured. The tracings made 

 by either of these instruments show that under ordinary circumstances the 

 period of ascent or of increase in the tension of the arterial walls is rapid ; 

 whilst the period of decrease, during which the elasticity of the larger arte- 

 ries is acting, is comparatively slow and prolonged. The line representing 

 the period of ascent is straight, but that of descent presents several secon- 

 dary undulations, which are not invariably present, vary much in their form 

 and amplitude, and are partly natural and partly artificial, that is to say, due 

 to defects of the instrument. The first of these has been termed by Mahomed 

 and Galabin the tidal wave, though they interpret it differently ; Mr. Ma- 

 homed considering it to be a wave of distension of the arterial walls, follow- 

 ing the primary wave, which he regards as a vibration of the blood column, 

 travelling with great rapidity, whilst Mr. Galabin, probably more correctly, 

 holds it to be due to an oscillation of the Sphygraograph, as a whole, caused 

 by the inertia of the instrument. For when the spring employed in some 

 forms of the Sphygmograph to depress the long lever is absent, no such 

 waves are seen ; whilst if it be present the tidal wave is sometimes broken 

 into two, and if the spring has a short period of vibration a scries of undula- 



1 Much, Sitzungshcricht d. k. Akad. zu Wion , 18C8, p. 5">, Baker, of JJolhorn, Dr. 

 Foster, of Birmingham, Journal of Anal, ami 1'hys., ser. ii, vol. i, p. 02, Burdon- 

 Sanderson, The Sphygmograph (pamphlet), Mahomed, Med. Times and (Jazette, 

 ]S72, have all suggested modifications in the construction of the spring and lever. 

 F<>r various explanations of tracings sec in addition to works quoted above, A. H. 

 (iarrod, in Juiirn. of Anat and Physiol., vol. vii, 1873, p. 08, Galabin, in idem, vol. 

 viii, p. i, 1874; and Halthax.ar Foster, in Aitkon's Practice of Medicine. 



- [New York Med. Journal, January, 1870.] 



