230 



ANIMAL MECHANISM. 



the pectoral muscles, a small instrument is slipped, which is 

 intended to show the dilatation of the muscles, and is constructed 

 in the following manner : 



A little metal pan (fig. 93), containing within it a spiral 

 spring, is closed by a membrane of india-rubber. This closed 

 pan communicates with a tube transmitting air. 



Fig. 94. — Experiment to determine by the electrical and myographical 

 methods, at the same time, the frequency of the movements of the wing 

 and the relative durations of its elevation and depression. 



Each pressure on the india-rubber membrane depresses 

 it, and the spring gives way ; the air is driven out of the pan, 

 and escapes by the tube. When the pressure ceases, the air 

 is returned to the instrument by the elasticity of the spring 



