THE CIRCULATION OF THK BLOOD. 



243 



the nature of arterial tracings, it is important that the circum- 

 stances under which they are obtained should be known. 



FiQ. 206.— Marey's improved sphygmograph arranged for taking a tracing. A, steel 

 spring; B, first lever; C, vpriting lever; C'^ its free writing end; 2), screw for 

 bringing B in contact with C; G, slide with smoked paper; H, clock-work; i, 

 screw for increasing the pressure; M^ dial indicating the amount of pressure, 

 K, K, straps for fixmg the instrument to the ai-m, and the latter to the double- 

 inclined plane or support {Byrom Bramwell). 



The movements of the vessel wall in most mammals suitable 

 for experiment and in man is so slight that it becomes necessary 

 to exaggerate them in the tracing, hence long levers are used to 

 accomplish this. 



The sphygmograph is the usual form of instrument em- 

 ployed for the purpose. It consists, essentially, of a clock-work 

 for moving a smoked surface 

 (mica plate commonly) on 

 which the movements of a 

 lever-tip, answering to those 

 of a button placed on the 

 artery, are recorded. 



We shall do well to in- 

 quire whether there are any 

 features in common in trac- 

 ings obtained jn various 

 ways, and which have there- 



FiQ. 207. — Diagrammatic schema showing the 

 essential part of the instrument when in 

 working order. The knife-edge B" of 

 the short lever is in contact with the 

 writing-lever C. Every movement of the 

 steel spring at A", communicated by the 

 arteries, will be imparted to the writing- 

 lever (Byrom Bramwell). 



fore in all probability a real foundation in nature. 



An inspection of a large number of pulse-tracings, taken un- 

 der diverse conditions, seems to show that in all of them there 

 occurs, more or less marked, the following : 1. An upward 



