418 



THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



human motor nerves has been estimated at 33 m. per second. Lately much 

 higher figures, up to 66 m. per second, have been given. 



The stimulus passes from the motor nerve to its muscles through the 

 motor end plates. Here a delay is experienced which with a maximal stimulus 

 amounts to about 0.002-0.003 of a second (Bernstein). 



C. MECHANICAL STIMULATION OF NERVES 



All kinds of mechanical disturbances, provided they take place with suffi- 

 cient abruptness, have a stimulating effect on a nerve. 



A light hammer let fall from different heights upon the nerve, resting upon 

 a solid support, is commonly used for demonstrating the mechanical stimula- 

 tion. If the nerve be subjected to a slowly increasing pressure or tension, its 

 excitability at first increases, then as the pressure or tension becomes still greater 

 it falls. Beyond a certain limit pressure applied to a nerve entirely abolishes 

 its power of conducting impulses (see page 411). According to Kiihne and 

 Uxkull, stimulation may occur on releasing a nerve from pressure. 



D. ELECTRICAL STIMULATION OF MUSCLE AND NERVE 



1. Method. The kinds of electrical stimuli the effects of which have been 

 most fully studied are the constant and induction currents. 



In applying the electric current to a muscle or nerve, or in leading off elec- 

 trical currents generated by animal tissues to a galvanometer, nonpolarizable 

 electrodes are used wherever it is practicable. Metal electrodes e. g., of plati- 

 num are not well adapted to such a purpose, partly because it is difficult to 

 find two pieces of metal between which there would be no differences of poten- 



FIG. 156. Schema of the rheocord of Du Bois-Reymond. The battery wires and the electrodes 

 are connected with the rheocord by means of the binding posts O and P. The current 

 coming to the binding post P splits into two lesser currents, one going through the rheocord, 

 the other through the electrode. The strength of current which will pass through the 

 electrodes will depend on the amount of resistance in the rheocord. This resistance is increased 

 by moving the slide 2, from left to right, also by throwing into the circuit other coils of wire 

 by means of the metal connections 1, 2, 3, 4, etc. 



tial, and partly because the contact of such electrodes with moist animal tissues 

 may very easily set up a difference of potential. In either case the nerve would 

 be subjected to an extraneous current generated by the electrodes themselves, 

 which often perhaps would make no essential difference in the results of the 

 experiment, since such a current would necessarily be very weak; but in many 

 investigations, especially when exact determinations of potential differences aris- 



