68 



CIRCULATION OF THE BLOOD IN THE VESSELS. 



the vessel and its changes in caliber. The first physiologist who put this in 

 practice was Vierordt, who constructed quite a complex instrument, so 



FIG. 25. Sphygmograph of Marey applied to the arm. 



arranged that the impulse from an accessible artery, like the radial, was 

 conveyed to a lever, which marked the movement upon a revolving cylin- 

 der of paper. This instrument was called a sphygmograph. The traces 

 made by it were perfectly regular and simply marked the extremes of dilata- 

 tion exaggerated, of 

 course, by the length of 

 the lever and the num- 

 ber of pulsations in a giv- 

 en time. The latter can 

 be easily estimated by 

 more simple means; and 

 as the former did not con- 

 vey any very definite physiological idea, the apparatus was regarded rather 

 as a curiosity than an instrument for accurate research. 



The principle on which the instrument of Vierordt was constructed was 

 correct; and it remained only to devise one which would be easy of ap- 



FIG. 26. Trace of Vierordt. 



FIG. 27. Trace of Marey. 

 Portions of four traces taken in different conditions of the pulse. 



plication and produce a trace representing the shades of dilatation and 

 contraction of the vessels, in order to lead to important practical results. 

 These conditions are realized in the sphygmographs now in use, which differ 

 from each other mainly in the convenience with which they are applied, 

 the principle of all being substantially that of the sphygmograph of Marey, 

 which is shown in Figs. 24 and 25. The modern sphygmographs simply 

 amplify the changes in the caliber of the artery incident to the pulse ; and 

 although their application is, perhaps, not so easy as to make these instru- 

 ments generally useful in the practice of medicine, in the hands of Marey 

 and other physiologists, they have led to a definite knowledge of the 

 physiological characters of the pulse and its modifications in certain diseases, 

 information which could hardly be arrived at by other means of investigation. 



