234 



PHYSIOLOGICAL PHYSICS. [Chap. xxi. 



against it a toothed wheel, to which is attached the lever 

 c. Every movement of the knob is communicated by 

 the screw to the wheel, and consequently to the lever. 



The movements of 

 A 



} ever are writ- 

 ten, by its point, on 

 a piece of smoked 

 glass or card d, 

 carried towards c by 



Fig. 112. Marey's Sphygmograph applied, the clockwork below 



d. The instrument 



is secured to the arm by side pieces and straps. 

 Sphygmographs have also been constructed on the 

 tambour principle, arrangements being also made to 

 determine by weights the pressure exerted on the 

 artery, and to vary it at pleasure. 



The spliygmoscope of Marey (Fig. 113) con- 

 sists of a small glass cylinder 2, which 

 has two openings, one of which is closed 

 by an indiarubber tube, leading to a 

 registering tambour, whose lever is in 



o O * 



contact with a recording surface. Into 

 the other opening is tightly fitted a 

 tube, carrying at its end a little india- 

 rubber bag. The bag and tube are con- 



o o 



nected with a receiving tambour, whose 



. 



membrane has attached to it a knob 

 which is placed over an artery. The 

 variations of pressure in the receiving tambour are 

 communicated to the indiarubber bag. The expansion 

 of the indiarubber bag compresses the air outside of 

 it, i.e. in the glass cylinder, and consequently affects 

 the recording lever. Similarly, the diminution of the 

 air in the bag rarefies the air in the cylinder. For 

 use with Lortet's dromograph, the indiarubber from 

 the bag is connected directly with the tube projecting 

 upwards from the instrument tied in the blood-vessel. 



