HERMANN VON HELMHOLTZ 



latent stimulation. Edward Weber had endeavoured 

 to draw a distinction between the movements of in- 

 organic and animal matter by assuming that the latter 

 were instantaneous, but the discovery of the latent 

 period at once put this distinction out of the question. 

 Helmholtz next made use of the graphic method, 

 introduced about that time into physiological investi- 

 gation, and the muscle, by means of a myograph, was 

 caused to write the curve of its contraction on the 

 blackened surface of a rotating drum. About the 

 beginning of the century, Thomas Young showed 

 how time might be recorded by marks made on a 

 rotating cylinder, moving with a uniform velocity. 

 James Watt then applied the graphic method to 

 recording the movements of the indicator of his 

 engine on a cylinder rotated by the engine itself. 

 Thus he obtained a curve representing variations 

 of steam pressure at different times. This suggested 

 to Ludwig, then at the beginning of his important 

 investigations into the dynamics of the circulation, 

 the conception of the kymograph, by which varia- 

 tions of pressure in the blood vessels of a living animal 

 were recorded on a drum in a series of waves, the 

 smaller ones corresponding to the individual beats of 

 the heart, and the larger to the respiratory move- 

 ments. The myograph of Helmholtz, already re- 

 ferred to (p. 36), recorded the shortening of the 

 muscle with great exactitude, and thus Pouillet's 

 galvanometrical method was discarded. He con- 

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