434 THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



excursion of the needle is seen which indicates the presence of a current in the 

 portion led off. This current is in the same direction as the current applied to 

 the nerve (called the polarizing current), and is spoken of as an electrotonic 

 current. The strength of this current depends upon many different circum- 

 stances; it is stronger, the less the distance from the portion of the nerve trav- 

 ersed by the polarizing current, and the stronger the latter is; moreover, the 

 change (in the frog) is greater in the region of the anode than in the region 

 of the cathode. 



The electrotonic currents are branches of the polarizing current. According 

 to Griinhagen, they arise because the inner parts of the nerve fibers, the axis 

 cylinders, are better conductors than the medullary sheaths. Consequently the 

 current (E in Fig. 171) spreads out over great lengths of the nerve, and when 

 connection is made from these extrapolar parts with the galvanometer (G, G") 

 the threads of current break through to the surface. It may be fairly doubted 

 now whether, as Hermann imagined, a polarization between the inner and outer 

 parts of the nerve plays any part in producing these electrotonic currents. 



B. THE MUSCLE TONE 



If a person sticks his finger in his ear and then contracts his arm vigor- 

 ously, he hears a dull sound, the pitch of which has been determined by Wallasten 

 and others to be about thirty-two to thirty-six vibrations a second. Helmholtz 

 observed that the same sound is heard very clearly if the ears (best at night) be 

 stopped with drops of sealing wax and the masseter muscles be powerfully con- 

 tracted. So long as the muscles remain at a uniform tension, one hears a dull, 

 roaring sound, whose fundamental tone is not changed materially by increasing 

 the tension, whereas the accompanying roar becomes both stronger and higher. 



Helmholtz demonstrated further that the vibrations of voluntary muscles 

 which produce the muscular sound do not occur so regularly as those of a 

 musical tone, nor so rapidly as thirty-two to thirty-six per second. He found 

 on the average only about nineteen per second. The muscular sound is there- 

 fore an overtone of the true muscle vibrations. Since the pitch of this sound 

 changes also with the condition of the ear drum, it follows that the sound experi- 

 enced is a resonance tone of the tympanic membrane, produced by the irregular 

 concussions of the muscles. From these facts it is not difficult to understand 

 why the simple contraction of a muscle produced by a single stimulus, and the 

 systole of the heart as well, is accompanied by a muscular sound (cf. page 168). 



C. THE CHEMICAL ALTERATIONS IN MUSCLE DUE TO ITS ACTIVITY 



Active muscle acquires an acid reaction. This is probably due in part to 

 an increased percentage of monophosphates, and in part to the formation of 

 lactic acid. According to Helmholtz, working muscle contains less substance 

 soluble in water and more substance soluble in alcohol than resting muscle. 

 Again it is stated that in work the total quantity of creatin and creatinin 

 increases and that of the xanthin bases decreases. Finally, the percentage of 

 glycogen in the muscle diminishes. 



An acid reaction has been observed in the neighborhood of the electrodes 

 when nerves are stimulated. Since however no such change can be demon- 

 strated at points of the nerve which have not been touched by the stimulating 

 current, this acid reaction must be regarded as a direct effect of the current 

 i. e., electrolysis. Waller concludes from certain phenomena with the action cur- 

 rent that the nerve forms carbon dioxide during its activity. 



