48 THE CELL 



face be connected, no differences of tension are shown (Fig. 29, dotted lines). 

 From these facts it follows that the entire longitudinal surface of the mus- 

 cle possesses a positive tension, every cross section a negative tension. 



It has been shown by Hermann (1867) that in an entirely uninjured, rest- 

 ing muscle there are no such differences of tension. If the skin be removed 

 very carefully from the gastrocnemius of a frog and great pains be taken that 

 it does not come in contact with the secretion of the skin, no current at all, or 

 only the very weakest one, is obtained when the muscle is connected with the 

 galvanometer. If on the other hand the muscle be injured in any way in the 

 neighborhood of one electrode, a strong current appears. The uninjured, resting 

 heart also gives no current (Engelmann). 



These and other discoveries among which should be mentioned the fact 

 that the current of injury does not appear in its full strength immediately but 

 develops gradually induced Hermann to propound the following theory in 

 explanation of the current of rest, which at this time is the one most acceptable 

 to physiologists. The cause of the difference of electrical tension in the resting 

 muscle lies in the injury which it receives. In a partially injured muscle every 

 point of the injured portion is negative to every point of the uninjured part. 

 The facts may be expressed in the following general proposition: in every 

 injured muscle fiber the demarcation surface between the living and the dead 

 contents of the fiber is the seat of an electromotive force directed toward the 

 living part. On this account Hermann designated the current of rest as the 

 demarcation current. 



Exactly the same electrical phenomena as in the resting muscle appear in 

 resting nerve. 



Electrical currents appear also when a muscle or a nerve is active, and 

 these currents are intimately connected with the functional condition of the 

 tissue. The general law of these currents (action currents) may be com- 

 prehended in the following statement: Every active portion of muscle or of 

 nerve maintains a negative relation toward the resting part (Bernstein, 

 1867). We can therefore condense the law for the rest current and the 

 action current into the following simple formulation : In muscle and in nerve 

 every active or injured part maintains a negative electrical relation toward 

 every other part which at that time is at rest or is uninjured. 



Further study of the action current will be postponed to the discussion 

 of the general physiology of muscles and nerves (Chapter XV). 



We meet with^ analogous electrical phenomena in many other tissues. The 

 currents present in glands are of special interest. Du Bois-Keymond proved that 

 the skin of the Amphibia is the seat of a current directed from without inward 

 which he ascribed to the secretion of the skin peculiar to these animals. Later 

 investigations have made us acquainted with similar currents in many other 

 objects (mucous membrane of the frog's tongue, of the pharynx and cloaca of 

 the frog, skin of the Amphibia and fishes, and of the leech, etc.), and have 

 shown that they are generated by the mucus-forming epithelia composed of 

 single-celled glands, as well as by other epithelia not glandular (Reid). Both 

 the strength and the direction of the current of action may be modified by such 

 agencies as pilocarpine, which stimulates secretion, by excitation of the mucosa 

 or of the glandular nerves, by changes of temperature, of the blood supply and 

 of the water content. 



