638 A MANUAL OF PHYSIOLOGY 



objection to these theories is that in nerve, so far as we know, no 

 sensible mechanical change whatever takes place during excitation, 

 and that differences of potential exist or may be developed in tissues 

 of the most diverse structure. 



Although the electromotive changes caused by excitation are much 

 more transient than those caused by injury, everything suggests that 

 there must be some deep analogy between the two conditions. 

 Some have supposed that what may be called a subdued and more 

 or less permanent excitation exists in the neighbourhood of the 

 injured tissue, and that this explains the similarity of electrical con- 

 dition in activity and injury. 



Polarization of Muscle and Nerve.* We have already spoken 

 of electrical excitation and of the changes of excitability 

 caused by the passage of a constant current (p. 595). We 

 are now to see that these physiological effects are accom- 

 panied by, and indeed very closely related to, more physical 

 changes which the galvanometer or electrometer reveals to 

 us. When a current is passed by means of unpolarizable 

 electrodes (Fig. 156, p. 542), through a muscle or nerve for 

 several seconds, and the tissue thrown on to the galvano- 

 meter immediately after this polarizing current is opened, a 

 deflection is seen indicating a current (negative polarization 

 current) in the opposite direction. 



This negative polarization, like the polarization of the electrodes 

 seen after passage of a current through any ordinary electrolytic 

 conductor, dilute sulphuric acid, e.g., depends on the liberation of 

 ions (p. 362) at the kathode and anode. It is seen not only in 

 muscle, nerve, and other animal tissues, but also in vegetable struc- 

 tures, and indeed, to a certain extent, in unorganized porous bodies 

 soaked with electrolytes. In muscle and nerve, however, it is par- 

 ticularly well marked ; and although it is not bound up with the life 

 of the tissue, and may be obtained when this has become quite 

 inexcitable, it is nevertheless dependent on the preservation of the 

 normal structure, for a boiled muscle shows but little negative 

 polarization. 



When the polarizing current is strong, and its time of closure 

 short, we obtain, on connecting the tissue with the galvanometer 

 after opening the current, not a negative, but a positive deflection, 

 indicating a so-called positive polarization cut rent in the same direc- 

 tion as that of the polarizing stream. This is really an action stream, 

 due to the opening excitation set up at the anode (p. 553). It is 

 only obtained when the tissue is living, and is far more strongly 

 marked in the anodic than in the kathodic region. 



* The portions in small type on pp. 638 to 643 may be omitted, except 

 by students interested in the subject or reading for a special purpose. 



