ELIX TROTOXIC CIRHEXTS 2:n 



but if a zinc wire is used as core and the cotton wool soaked in 

 zinc sulphate to prevent polarisation, neither anelectrotonic or 

 catelectrotonic currents are produced. 



Emphasis must be placed on the fact that these electrotonic 

 currents are absolutely distinct from the nerve impulse as well 

 as from the wave of negativity or current of action and the 

 current of injury, (i.) The former have a much greater velocity 

 than the nerve impulse, as indicated by the wave of negativity. 

 (ii.) Their E.M.F. may attain a value twenty-five times that of the 

 current of injury, (iii.) The direction in which electrotonic currents 

 flow depends entirely on the direction in which the primary 

 current is flowing, reversion of the latter leading to reversion of the 

 former. Action and injury currents always maintain a flow in the 

 nerve from a stimulated or injured part to a resting or uninjured 

 portion of the nerve. 



{b) Positive polarisation. A special type of injury current, 

 unfortunately named the positive polarisation current, may be 

 obtained immediately after 



the passage of a very strong /^l~^ Polarising 



current along a nerve (Fig. 60). ^ /^' 



When the " polarising " cur- 



rent is broken and the anode a '^'^y^^ Pos.tive polarisation 



connected through a galvano- W' 



r,^f^^-p>T fr> fVi<^ noflinrlfi I- ( niv FiG. 60.— Diagram to sliow direction of the positive 



lllCLCl to LUC CclLllOUe th \L-U- polarisation current, due to a break excitation at the 



cuit 2), a current will be """'*'' 



apparent flowing in the same direction as the " polarising " current, 

 i.e. from a to k. This current is not due to polarisation at all. 

 but to injury at the anode by the strong " polarising " current used, 

 A very significant series of experiments due to Lillie may be 

 referred to here. If an iron wire is immersed in dilute nitric acid, 

 in time it will be covered with a film of iron oxide. It is now in a 

 passive state, and as long as the environment does not alter, no 

 further changes will take place in or on the wire. But when the 

 film of oxide is broken, say, by a mere scratch near one end of the 

 wire, the break in the oxide is transmitted without decrement 

 along the whole extent of the wire regardless of its length. This is 

 accompanied by electrical changes and by the e\'olution of gas, and 

 is followed by a restoration of the film of oxide. The wire is now 

 passive and ready for further scratching. The model illustrates 

 several principles of nerve action dealt with above and emphasises 

 the point of view that the nerve impulse is associated with changes 

 on the surface of the axon. It might repay the student to turn 

 back to Chaps. VI. and IX., where consideration is given to the 

 properties of monomolecular films on surfaces. 



s 



