i GENERAL PHYSIOLOGY OF MUSCLE 73 



indicate the strength of current, but the electromotive force or difference of 

 potential between the two electrodes. It is thus an electrical manometer, 

 the sensitiveness of which is so great that it reacts to T^^HV volt. The dis- 

 placements of the mercury surface can be photographed. 



Einthoven (1905-6) introduced the string galvanometer which has distinct 

 advantages over its predecessors. 



This instrument has a fine thread of silvered quartz or platinum stretched 

 between the two poles of a strong magnet. On passing a weak current 

 through the string, it moves laterally in proportion to the strength of the 

 current. The poles of the magnet are pierced by holes so that the thread may 

 be illuminated by an electric light from the one side, and observed from the 

 other by means of a microscope ; or a magnified image may be thrown on a 

 screen, or moving sensitive surface on which it is photographed. 



Einthoven devised this apparatus for the special purpose of studying 

 the electrical variations of the human heart. But it may be substituted 

 advantageously for all purposes instead of the apparatus described above. 



We will briefly consider the principal electromotive phenomena 

 in muscle, keeping distinct the electrical manifestations of the 

 resting and the active states. 



When a muscle with parallel fibres, e.g. the frog's sartorius, is 

 dissected out, and the tendinous end trimmed neatly with a razor, 

 a regular cylinder of muscle substance is obtained, with a natural 

 longitudinal surface and two artificial cross-sections. If any two 

 points of this muscle are connected with the galvanometer by 

 unpolarisable electrodes there is nearly always a deflection of the 

 galvanometer needle, showing that the two points led off are not 

 isoelectric, but that there is a difference in potential. 



If the electrodes are applied to points on the natural 

 longitudinal and the artificial transverse surfaces, the former is 

 found to be " positive " in relation to the latter, which is " negative." 

 Du Bois-Keymond made a minute study of the different degrees 

 to which the galvanometer needle was deflected by altering the 

 position of the electrodes upon the muscle cylinders, and drew up 

 the following laws of the muscle current : 



(a) Strong currents appear on leading off to the galvano- 

 meter from the natural longitudinal surface and artificial cross- 

 section of the muscle. The current is strongest when a point in 

 the equatorial median line of the longitudinal surface is connected 

 with the axial point of an artificial cross-section, and decreases 

 regularly with increased distance from these points. 



(5) Weak currents are obtained when two points at unequal 

 distance from the equator of the longitudinal section are united ; 

 still weaker currents when two points of the cross-section at un- 

 equal distances from the ends of its axis are connected. 



(c) No current is obtained on connecting two points of the 

 equator or any two points at equal distance from the same ; nor on 

 connecting the two axial points of the cross-section, or any two 

 points of the cross - section equidistant from the axial points 

 (Fig. 48). 



