GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 79 



more work than during a single contraction, the weight in each case being 

 the same. In the former condition the height of contraction through sum- 

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

 work is done, however, i.e., the continuous liberation of energy, as shown by 

 an increase in the temperature. 



When a weight which is lifted by a muscle during a single contraction 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 

 ork is done, as shown by the fact that the muscle becomes fatigued. 



Work Done Daily. The muscle system in its entirety is to be regarded 

 as a machine for the transformation of potential into kinetic energy, and in 

 so doing accomplishes work. Through the intermediations of the bones of 

 the skeleton which play the part of levers the individual not only changes his 

 position in space, but overcomes to some extent the resistances offered by 

 the environment. The employment of artificial levers, tools, as distinguished 

 from natural levers, bones, materially adds to the effectiveness of the muscle 

 machine. The amount of work which a man of average physical develop- 

 ment weighing 72 kilos can perform in eight hours has been variously esti- 

 mated. It will naturally vary according to the character of the occupation. 

 If the work done be calculated from the number of kilograms raised one 

 meter, the average laboring-man performs about 300,000 kilogrammeters of 

 work. 



ELECTRIC PHENOMENA 



Electric Currents from Injured Muscles. The energy liberated as 

 the result of the action of a nerve impulse 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 electrodes connected by wires with a sensitive 

 galvanometer or capillary electrometer. When such electrodes are brought 

 in contact with a muscle properly prepared, there is at once developed and 

 conducted to the galvanometer an electric current the intensity and direction 

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

 existence of this current is most conveniently demonstrated with Dingle 

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

 branosus 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 longitudinal 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. 



When the natural longitudinal and artificial transverse surfaces are 

 connected with the wires of a galvanometer the terminals of which are pro- 

 vided with non-polarizable electrodes, an electric current is at once de- 

 veloped. In all instances the current, as shown by the deflection of the 

 needle, originates at the transverse surface, passes through the muscle to 



