44 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



zinc plate, and inside the cup is the copper sulphate solution and the copper plate 

 The zinc plate is acted upon by the sulphuric acid, and, as a result of the chemical 

 change, a difference of electric potential is set up between the 

 metals, so that if the zinc and copper be connected by a piece of 

 metal, what we call an electric current flows from the zinc to the 

 copper inside the cell, and from the copper to the zinc outside the 

 cell. The zinc plate, being the seat of the chemical change, is 

 called the positive plate, and the copper the negative plate. 

 Several such cells may be connected together to form a battery, 

 each cell adding to the electro-motive force, and hence to the 

 strength of the current. As the current is always considered to 

 flow from + to , we call the end of the wire connected with the 

 copper (negative plate) the positive pole, or anode, and the end 

 of the wire connected with the zinc (positive plate) the negative 

 pole, or kathode. If one of these wires be touched to a nerve, 

 under ordinary circumstances no effect is produced ; but when the 

 other wire is likewise brought in contact with the nerve, the 

 moist tissues of the nerve form a conductor, complete the cir- 

 cuit, and an electric current at once flows through the nerve from 

 the anode to the kathode. The effect of the sudden flow of 

 electricity into the nerve is to give it a shock as we say, it 

 irritates the nerve and the muscle which the nerve controls is seen to contract. 



In the place of using ordinary wires for applying the electricity, we use electrodes. 

 These are practically the same thing, but have insulated handles, and have a form better 

 suited to stimulate nerves or other tissues. The two wires may be held in two different 



FIG. 5. Daniell cell. 



FIG. 6. a, Ordinary electrode for exciting exposed nerves and muscles, consisting of two wires 

 enclosed, except at their extremities, in a handle of non-conducting material ; 6, c, non-polarizable elec- 

 trodes. When metals come in contact with moist tissues a galvanic action is likely to occur and polariz- 

 ing currents to be formed. These extra currents would complicate or interfere with the results of many 

 forms of experiment, and they are avoided by the use of non-polarizable electrodes. A simple form con- 

 sists of a short glass tube, at one end of which is a plug of china clay mixed with a 0.6 per cent, solution 

 of sodium chloride, and at the other end a cork through which an amalgamated zinc rod is thrust. The 

 zinc rod dips into a saturated solution of zinc sulphate, which is in contact with the clay. The clay plugs 

 touch the tissue to be excited, and the current passes from the zinc rods through the zinc-sulphate and 

 sodium-chloride solutions in the clay to the tissues ; d-f, electrodes for exciting human nerves and mus- 

 cles through the skin (after Erb) : these may be of various forms and sizes, and are arranged to screw 

 into handles (g), to which the wires are attached ; they are usually made of brass and covered with 

 sponge or other absorbent material wet with salt-solution. The smaller electrodes are used when a dense, 

 well-localized stream is required, and the larger electrodes when little action is wished and it is of 

 advantage to have the stream diffuse. 



handles, in which case we speak of the positive and negative electrodes, or the anode 

 and the kathode, or they may be held in the same handle (Fig. 6). 



Keys. It is not as convenient to stimulate a nerve by touching it with the electrodes as 



