BURDEN AND OVER-BURDEN. 65 



mum in about one-tenth of a second. The maximum 

 having been reached, the muscle is unable to contract 

 further. The energy diminishes, and finally disappears, 

 the muscle returning to its original condition. 



5. In the experiments described above, weights were 

 connected with the muscle v»hich the latter necessarily 

 raised as soon as it strove to contract; but these weights 

 did not act upon the muscle aa long as it remained 

 quiescent. It was, therefore, not weighted in the sense 

 which has already been described ; for the weights at- 

 tached were unable to extend the muscle. The com- 

 paratively slight weight of the lever alone extended the 

 muscle, and could be regarded as burden in the ordinary 

 sense. In order to distinguish these weights, which 

 are without effect until the muscle strives to contract 

 from weight in the ordinary sense, we will apply the 

 term ' over-burden ' to them. The burden of a muscle 

 may be great or small. In the experiments described 

 above it was equal to the weight of the lever. Greater 

 weights may be selected, a weight being placed upon the 

 scale-plate and the muscle being then raised by means 

 of the screw at the top of the apparatus, so long as the 

 platinum point p still rests on the platinum plate. The 

 muscle is then extended by the weight applied. If 

 additional weight is added to that already on the scale- 

 plate, the former acts as burden, the latter as over- 

 burden. When a muscle thus circumstanced contracts, 

 it has to lift both weights. Let us return to our first 

 series of experiments, in which the weight = 0, or was 

 at least very small. If more and more over-burden 

 is gradually added, it is evident that a point will be 

 reached at which the muscle will no longer be able to 

 lift the weight. This point may be very accurately 



