ELECTRICAL PHENOMENA 143 



The efficiency of muscular contraction is found from the formula 



where a ^actual work done (in cals.) per unit of time. 



b =total energy used (in cals.) ,, ,, ,, 



This gives the gross efficiency. The net efficiency is obtained 

 by correcting for the energy expended during complete inactivity 

 during a similar unit of time. 



Net efficiency E n=JT <100 ' 



u C 



where c^energy expended during inactivity (in cals.) per unit 

 of time. 



As no external work is done during inactivity it is difficult to 

 assess the value of the efficiency of this phase. 



The temperature co-efficient for a complete muscle cycle is 

 1 -8, which means that the rate of the physico-chemical reactions 

 involved is almost doubled by an increase of 10 C. As we have 

 seen this rate is a compromise between the decrease in the rate 

 of the physical reactions of the contraction phases and the increases 

 in the physical and chemical reactions of the restitution phase. 



Character of the Electrical Variations in Muscle. 



The existence of electrical currents in tissues did not find 

 direct proof until the year 1824, when Nobili devised the galvano- 

 meter and showed that "natural currents" occur in the frog. 

 Other investigators examined this current of rest and found that 

 in a muscle removed from the body the current in the muscle 

 passed from the ends to the middle and in the external (galvano- 

 meter} circuit from the middle to the ends. It has been con- 

 clusively proved that these " natural " currents are not natural 

 at all but are an indication of injury to the muscle. Slight injury 

 is unavoidable in the process of removing the muscle from the 

 body the less the preparation is injured the smaller is the current 

 obtained from it. In other words, normal resting muscle is iso- 

 electric and therefore currentless. 



Current of Injury or Demarcation Current. 



The injured part of a muscle is like the injured part of any 

 cell (p. 134), " zincative " or electropositive to the uninjured part. 

 That is, if non-polarisable electrodes are led off from injured and 

 non-injured parts to a current-detecting device, it will indicate 



