726 A MANUAL OF PHYSIOLOGY 



oxygen (p. 401) ; (2) Oxidation of the contractile substance by this 

 oxygen ; (3) The assimilation of oxygen by the contractile substance 

 i.e., its passage into the molecules of this substance (p. 264). 

 According to him, it is the first of these stages which is associated 

 with the abrupt development of the difference of potential between 

 the excited and unexcited portions of the muscle. which we call the 

 negative variation. This first stage he assumes to be completed 

 before the visible contraction begins ; and he originally asserted 

 that the same was true of the negative variation. It is now known 

 that the latter, although it begins before the contraction, and very 

 rapidly reaches its maximum, declines more gradually, so that it 

 overlaps the mechanical change of form. This is particularly well 

 seen in veratrinized muscles (p. 654), in which the electrical variation, 

 like the contraction, is greatly prolonged (Garten). Nevertheless, 

 Bernstein's theory, even with this limitation, agrees with what is 

 known as to the influence exerted on the action current by the 

 mechanical conditions of the contraction. For the electrical change 

 is little, if at all, affected by the tension of the muscle or the load 

 it has to lift ; and this is 'what we should expect it if depends on a 

 process which is mainly completed before the contraction begins. 



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. 683). We are now to 

 see that these physiological effects are accompanied by, and 

 indeed very closely related to, more physical changes which the 

 galvanometer or electrometer reveals to us. Since these throw 

 light on the physical, and therefore ultimately on the physio- 

 logical structure of the tissues, they have been deeply studied, 

 especially in nerve. There is no question that they depend 

 upon the presence in the tissues of membranes presenting a 

 relatively great resistance to the passage of ions. When a 

 current is passed by means of unpolarizable electrodes (Fig. 213, 

 p. 625) through a muscle or nerve for several seconds, and the 

 tissue thrown on to the galvanometer 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 ol 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. 401) at the kathode and anode. It is seen not only in 

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

 structures, and indeed, to a certain extent, in unorganized porous 

 bodies soaked with electrolytes. In muscle and nerve, however, it 

 is particularly 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, 



