70 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



of the usable material in the muscle, and the heat thus produced 

 acts upon the doubly refractive material as upon a piece of machin- 

 ery and causes it to shorten by imbibition. Contraction, in a word, 

 is a phenomenon of thermic imbibition. Engelmann has given an 

 appearance of verisimilitude to this hypothesis by constructing 

 an artificial muscle from a piece of violin string. The apparatus 

 used is illustrated in Fig. 26. A catgut string (ra) is surrounded 

 by a coil of platinum wire (w) through which an electrical current 

 may be sent. The object of this arrangement is to heat the catgut 

 suddenly. The platinum coil should not actually touch the catgut. 

 The catgut is attached to a lever, as shown in the figure. The 



Fig. 27. Curve of simple contraction obtained from an artificial muscle. The dura- 

 tion of the stimulus (heating effect caused by the current) is shown by the break in the 

 line beneath the curve. 



catgut is thoroughly soaked by immersing it in a beaker of water 

 and the temperature is then raised to 50 to 55 C. If then a 

 current is turned into the coil the slight but somewhat rapid heating 

 of the catgut will cause it to shorten, owing to the imbibition of 

 more water. When the current is broken the catgut cools and 

 relaxes slowly. Records may be obtained in this way which are 

 altogether similar or identical with those given by a strip of plain 

 muscle when stimulated (see Figs. 27 and 28). The model may be 

 used to show the effect of temperature upon the extent and dur^ 

 tion of the contractions, the effect of variations in strength of 



