xo 4 TEXT-BOOK OF PHYSIOLOGY 



nerve-muscle preparation (the gastrocnemius muscle and sciatic nerve) and 

 to use as a stimulus the induced electric current. (See Fig. 49.) When 

 kept moist, this preparation is extremely sensitive to either the galvanic 

 or the induced current 



Though the development and conduction of a nerve impulse may be 

 demonstrated by the deflection of the galvanometer needle or the move- 

 ment of the mercury in the capillary electrometer, it is more conveniently 

 demonstrated by the contraction of a muscle, the extent of which, may 

 be taken as a measure of the number of nerve-fibers stimulated. 



It has heretofore been assumed that the passage of a weak, a^ medium 

 or a strong induced electric current through a nerve, developed in all the 

 fibers, nerve-impulses of a minimal, medium or maximal intensity, which 

 when transmitted to a muscle called forth a correspond- 

 ing contraction of all its fibers. This explanation is 

 however not in accord with facts. Here as in the case 

 of muscle it is believed that the law "all or none " 

 also holds true. If the nerve-fiber responds at all to a 

 stimulus the resulting nerve-impulse is maximal in its 

 intensity. The effect of nerve stimulation will there- 

 fore depend on the number of fibers stimulated. 



Tetanization of a muscle indicates that the nerve 

 impulses arrive at the muscle with a frequency so great 

 that the muscle does not succeed in relaxing from the 

 effect of one stimulus before the next arrives. Complete 

 as well as incomplete tetanus may be developed by gradu- 

 ally increasing the frequency of the stimulus. The 

 character of the contraction caused by indirect stimula- 

 tion i.e., through the nerve does not differ in any es- 

 sential respect from that due to direct stimulation. 



FIG. 49. NERVE- 

 MUSCLE PREPARA- 

 TION OF A FROG. F. 

 Femur. S. Sciatic 

 nerve. I. Tendo 

 Achillis. (Landois 

 and Sterling.) 



ELECTRIC PHENOMENA OF NERVES 



Electric Currents from Injured Nerves. It was discovered by du- 

 Bois Reymond that electric currents can be obtained from nerves as well as 

 from muscles, and that the electric properties of the former correspond in 

 most respects to those of the latter. The laws governing the development 

 and mode of action of the currents derived from muscles are equally 

 applicable to the currents derived from nerves. 



A nerve-cylinder obtained by making two transverse sections of any 

 given nerve presents, as in the case of muscles, a natural and two artificial 

 transverse surfaces. A line drawn around the cylinder at a point lying 

 midway between the two end surfaces constitutes the equator. From such 

 a cylinder strong currents are obtained when the natural longitudinal sur- 

 face and the transverse surface are connected with the electrodes of the 

 galvanometer circuit. The strength of the current thus obtained will 

 diminish or increase according as the electrode on the longitudinal surface 

 is removed from or brought near to the equator. If two symmetric points 

 on the longitudinal surface equidistant from the equator are united, no 

 current is obtainable. When asymmetric points on the longitudinal sur- 

 face are connected, weak currents are obtained, in which case the point 



