GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 89 



Mechanic Work. If the muscle is permitted to shorten and 

 raise a weight, some of the energy liberated takes the form of mechanic 

 motion. If the weight is removed at the height of the contraction, 

 external work is accomplished. The greater the weight raised, 

 within limits, the greater is the percentage of energy which takes the 

 direction of mechanic motion. In accordance with the law of the 

 conservation of energy, the heat produced, stated in calories, plus 

 the energy required in the raising of the weight, expressed in kilogram- 

 meters of work, must equal the potential energy transformed. 



A muscle during a tetanic contraction of short duration accom- 

 plishes more work than during a single contraction; the weight in 

 each case being the same. In the former condition the height of 

 contraction through summation, and hence the work done, is greater 

 than in the latter. The work done by a short tetanic contraction may 

 be two or three times that of a single contraction, but after the 

 muscle reaches its maximum degree of shortening and then con- 

 tinues in a state of tetanus, no further work is done. Internal 

 work is done, however, as shown by an increase in the temperature. 



When a weight which is lifted by a muscle during a single con- 

 traction is allowed to act on the muscle during the relaxation, no 

 external work is accomplished. All the energy set free manifests 

 itself as heat. Internal work is done, as shown by the fact that the 

 muscle becomes fatigued. 



ELECTRIC PHENOMENA. 



Electric Currents from Injured Muscles. The energy liber- 

 ated during a muscle contraction is not only transformed into 

 heat and mechanic motion, but to some extent also into electric 

 energy. The presence of points of different potential on the 

 surface of the muscle, the necessary condition for the development 

 of electric currents, is tested by means of non-polarizable elec- 

 trodes connected by wires with a sensitive galvanometer or capil- 

 lary electrometer. When such electrodes are brought in* contact 

 with a muscle properly prepared, there is at once developed and con- 

 ducted to the galvanometer an electric current the intensity and direc- 

 tion of which are indicated by the deflection of the galvanometer needle. 

 The existence of this current is most conveniently demonstrated with 

 single muscles the fibers of which are parallel e. g., the sartorius, or 

 the semimembranosus of the frog. If the tendinous ends of either of 

 these muscles be removed by a section made at right angles to the long 

 axis, a muscle prism is obtained which presents a natural longitudi- 

 nal surface and two artificial transverse surfaces. A line drawn around 

 the surface of such a muscle prism at a point midway between the 

 two transverse sections constitutes the equator. 



