GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 137 



move from the direction of greater to that of lesser tension, descending deeper into the 

 tube if the pressure be raised on the mercury side, or lowered on the acid side, and vice 

 versa. As seen through the microscope the picture is reversed (d), and the movements 

 of the mercury appear to be in the opposite direction to that stated. The extent of the 

 movements of the mercury column can be estimated by a scale in the eyepiece. More- 

 over, the movement of the mercury can be recorded photographically, by placing a strong 

 light behind the column of mercury, and letting its shadow fall through a slit in the wall 

 of a dark chamber, upon a sheet of sensitized paper stretched over the surface of a revolv- 

 ing drum or a sensitized plate moved by clockwork or other suitable mechanism. This 

 instrument, of which there are a number of different forms besides that originally devised 

 by Lippmarm, is very delicate, recording exceedingly slight differences in electrical poten- 

 tial. 



2. Currents of Rest. A normal resting nerve or muscle presents no dif- 

 ferences in electric tension and gives no evidence of electric currents, wherefore 

 we say it is iso-electric. If any part of the structure be injured, its electrical 

 condition is forthwith changed, and if the injured portion and some normal 

 part be connected with a galvanometer, an electric current is observed to flow 

 from the normal region to the point of injury. These muscle -currents were 

 discovered at about the same time by Matteucci and Du Bois-Reymond, and 

 the latter wrote a now celebrated treatise upon the electrical phenomena to be 

 observed in the nerve and muscle under varying conditions. 1 



Directions of Currents of Rest. If a striated muscle, with long parallel 

 fibres, such as the sartorius or the semimembranosus of a frog, be prepared 

 with care not to injure the surface, and then be given a cylindrical shape by 

 cutting off the two ends at right an- 

 gles to the long axis, the piece will 

 present two cross sections of injured 

 tissue and a normal longitudinal sur- 

 face (see Fig. 60). If non-polarizable 

 electrodes, connected with the coils of 

 wire of a galvanometer, be applied to 

 various parts of such a piece of mus- 

 cle, it will be found that all points on 

 the longitudinal surfaces are positive 

 in relation to all points on the cross 

 sections, but that the differences of 

 tension will differ according to the 

 points which are compared. Suppose 

 that the cylinder be divided into 



equal halves by a plane parallel to the galvanometer. The broken lines connect points 

 _. : xi v i A of equal potential from which no current would be 



cut ends. Points on the line bound- obtained 

 ing this plane, the equator, show the 



greatest positive tension, and the farther other points on the longitudinal sur- 

 face are from the equator the less their tension. Points on the cross section 

 show a negative tension, and this lessens from the centre to the periphery of 

 1 Untersuchungen uber thierische Elektricitdt, Berlin, 1849. 



FIG. 60. Schema to show the direction of cur- 

 rents to be obtained from muscle. The schema 

 represents a cylindrical piece of muscle with nor- 

 mal longitudinal surface (a, c and b, d), and two 

 artificial cross sections (a, b and c, d). The position 

 of the equator is shown by line e. The unbroken 

 lines connect points of different potential, and the 

 arrows show the direction which the .currents 

 would take were these points connected with a 



