IO4 



TEXT-BOOK OF PHYSIOLOGY 



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 vigor of which, within 

 limits, may be taken as a measure of the intensity of the impulse. The 

 preparation should be enclosed in a moist chamber and the nerve con- 

 nected with the inductorium through the intervention 

 of non-polarizable electrodes. The muscle may be at- 

 tached to the muscle-lever and its contractions recorded. 

 A single shock of an induced current develops, it is 

 believed, a single nerve impulse followed by a single 

 muscle contraction. A minimal contraction following a 

 minimal electric stimulus presupposes the development 

 of a nerve impulse of low intensity. Within certain 

 limits a maximal contraction following a maximal elec- 

 tric 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 grad- 

 ually increasing the frequency of the stimulus. The 

 character of the contraction caused by indirect stimu- 

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

 that due to direct stimulation. 



FIG. 50. NERVE* 

 MUSCLE PREPARA- 

 TION OF A FROG. F. 

 Femur. S. Sciatic 

 nerve. I. T e n d o 

 Achillis. (Landois 

 and Stirling.} 



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 

 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 Ion- 



