76 PHYSIOLOGY CHAP. 



or swollen with water, which are totally inexcitable and apparently 

 dead do, on the contrary, manifest differences of electrical 

 potential. 



Another important fact discovered by Hermann and confirmed 

 by Biedermann and others is that wholly uninjured muscles 

 are isoelectric, i.e. manifest no difference in potential at their 

 surface in the resting state. When the hind -leg of a frog is 

 very carefully skinned, precaution being taken to avoid contact 

 between the cutaneous secretion and the exposed surface of the 

 muscle, no current can be led off from the latter to the galvano- 

 meter. When, on the contrary, an exposed muscle is injured at 

 any point of its surface by cauterisation, chemical burns, partial 

 poisoning with potassium salts, mechanical crushing, etc., the 

 injured spot invariably becomes negative to the intact parts of the 

 surface. So that injured points react like transverse or oblique 

 surfaces produced by section. Hermann, therefore, formulated the 

 general law which is applicable to all cases, that " In every injured 

 muscle fibre the surface of demarcation between the living and 

 dead portions of the fibre is the seat of an electromotive force 

 directed towards the living part." He gave the name of demarca- 

 tion current to the so-called " current of rest," because it does not 

 pre-exist in the normal muscle, but first appears when any part of 

 the latter suffers alteration. [Current of injury : Hering.] 



Another phenomenon brought out by Hermann is that a 

 general rise of temperature increases the strength of the demarca- 

 tion current up to a certain limit, beyond which it decreases 

 again, till it disappears with the onset of heat rigor. A drop in 

 the temperature, on the contrary, lowers the e.m.f. Again, in 

 intact muscle heated points are electro-positive to cooler parts 

 (Hermann and Worm-Mliller). Finally, fatigue from protracted 

 muscular activity weakens the demarcation current (Eb'ber), and 

 abolishes it if pushed as far as rigor. 



Another fact in favour of Hermann's views is that in muscle 

 prisms or cylinders freshly cut with a razor and connected with 

 the galvanometer the demarcation current is absent, or almost 

 absent, during the first moments, but increases rapidly to its 

 maximum. This phenomenon can only be explained by assuming 

 that the surface of the section alters with exposure to air, and 

 that its negative potential increases in proportion with this 

 change. The alteration shown in the acidification of- the muscle 

 gradually extends over the whole, till it becomes perfectly rigid. 

 The demarcation current as shown on the galvanometer suffers 

 a parallel slow diminution, till it eventually disappears. 



We must next study the phenomena of active muscle. If the 



nerve of a muscle-nerve preparation that is showing a demarcation 



current on the galvanometer is tetanised, the current is diminished 



the galvanometer needle swings back towards the zero of the 



