138 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the cross section. Points on the cross section equidistant from the centre, or 

 on the longitudinal surface equidistant from the equator, have the same poten- 

 tial and give no current, while points placed unsymmetrically give a current. 

 Splitting the cylinder by separation of the parallel fibres gives pieces of mus- 

 cle which show the same electrical peculiarities, and without doubt the same 

 would be true of separate muscle-fibres or pieces of fibres. 



Theories as to Cause of Currents of Rest. Du Bois-Reymond, impressed by 

 the facts which he had ascertained as to the direction of action of the electro- 

 motive forces exhibited by the muscle, tried to explain the difference in elec- 

 trical tension of the surface and cross section on the supposition that the 

 muscle was composed of electro-motive molecules which presented differences 

 in electric tension similar to those shown by the smallest particles of muscle 

 which it is possible to study experimentally. Further, he considered these dif- 

 ferences in tension, and the consequent electric currents, to exist within the 

 normal muscle the longitudinal surface and normal cross section, i. e. the 

 point where the muscle-fibre joins the tendon, having the same sort of differ- 

 ence in electric potential as the normal longitudinal surface and the artificial 

 cross section. When the muscle is injured the balance of the electro-motive 

 forces within is lost, and they are revealed. It is difficult to refute such a 

 theory by experiment, because our instruments only record differences in tension 

 at points on the surface of the muscle to which we can apply the electrodes. 

 We cannot say that there is an absence of electric tension or lack of electric 

 currents within the normal resting muscle ; we can only say that there is no 

 direct experimental evidence of the existence of such currents. 



Another theory of the electrical phenomena observed in muscle, and one 

 which has found many adherents, was advanced by Hermann. 1 According to 

 Hermann's view there are no differences in electric potential and no electric 

 currents within the normal muscle ; the " current of rest " is a " current of 

 injury," a "demarcation current," i. e. it is due to chemical changes occurring 

 in the dying muscle-tissue at the border line between the injured and living 

 muscle-tissue. 



Although the greatest differences in potential are observed when many muscle- 

 fibres are injured, as when a cut is made completely through a muscle, injury 

 to any part causes that part to become negative as compared with the rest. 

 Even an injury to a tendon causes a difference in potential. It is exceedingly 

 difficult, therefore, to expose a muscle without injuring it; but this can be done 

 in the case of the heart ventricle, and Engelmann showed that this gives no cur- 

 rent when at rest, although a current is found as soon as any part is hurt, the 

 part becoming immediately negative in relation to other uninjured parts. In 

 experiments on isolated, long, parallel-fibred muscles, the current which is 

 caused by the injury of one extremity is found to fade away only very gradu- 

 ally (it may last forty-eight hours or more), and this current can be strength- 

 ened but little by new injuries. In the case of the heart-muscle the current 

 caused by cutting off a piece of the ventricle soon disappears, but another cur- 

 1 Handbuch der Physiologie, 1879, Bd. i. p. 226. 



