102 



TEXT-BOOK OF PHYSIOLOGY. 



A single shock of an induced current develops, it is believed, a single 

 nerve impulse followed by a single muscle contraction. A minimal con- 

 traction following a minimal electric stimulus presupposes the development 

 of a nerve impulse of low intensity. Within certain limits a maximal con- 

 traction following a maximal electric stimulus presupposes the development 

 of a nerve impulse of high intensity. Intermediate contractions indicate 

 nerve impulses of corresponding intensity. 



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 gradually increasing the frequency 

 of the stimulus. The character of the contraction caused by indirect 

 stimulation i.e., through the nerve does not differ in any essential respect 

 from that due to direct stimulation. 



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 cur- 

 rents are obtained when the natural longitud- 

 inal surface and the transverse surface are 

 connected with the electrodes of the galvano- 

 meter circuit. The strength of the current 

 thus obtained will diminish or increase accord- 

 ing as the electrode on the longitudinal sur- 

 face 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 

 surface are connected, weak currents are ob- 

 tained, in which case the point lying nearer the equator becomes positive 

 to the point more distant, which becomes negative. From these facts it is 

 evident that all points on the longitudinal surface are electrically positive to 

 the transverse surface and that the point of greatest positive tension is sit- 

 uated near the equator (Fig. 51). 



FIG. 51. DIAGRAM TO ILLUS- 

 TRATE THE CURRENTS IN NERVES. 

 The arrowheads indicate the direc- 

 tion; the thickness of the lines in- 

 dicates the strength of the currents. 

 (Landois and Stirling.) 



