ELECTRO-PHYSIOLOGY 635 



every revolution a horizontal wire b on the fixed ring, thus making 

 and breaking the primary circuit P of an induction machine, and so 

 causing stimulation of a muscle or nerve M connected with the 

 secondary S ; and, <:, a double contact, either in the form of two 

 platinum wires, which dip into two mercury troughs, or o f two wire 

 brushes rubbing on copper blocks d, at a certain part of the revolu- 

 tion. The troughs or blocks are connected with a circuit containing 

 a galvanometer G, and a portion of the muscle or nerve arranged so 

 as to give a strong action current. This circuit is completed by the 

 wires or brushes, which are in metallic contact with each other ; and 

 the relative position of the fixed contact in the primary circuit and of 

 the troughs or copper blocks can be altered so as to alter at will the 

 interval between stimulation and closure of the galvanometer circuit. 

 The proportion of the whole revolution during which this circuit is 

 closed can be varied by changing the relative position of the two 

 copper blocks. Suppose the tissue is stimulated at one end while 

 the leading-off electrodes are at the other. When the contact a, b, is 

 made at the same time as c, d, no deflection will be shown by the 

 galvanometer if the rheotome is revolving rapidly (the demarcation 

 current being accurately compensated), because the circuit will be 

 opened before the negative change has time to travel to the leading- 

 off electrodes. But as the distance between b and d is increased, a 

 small deflection will appear, which, with further increase of the dis- 

 tance, will become larger, reach a maximum, and then begin to fall 

 off again. The first small deflection corresponds to the position in 

 which the negative change has just had time to reach the leading-off 

 electrodes before the galvanometer circuit is opened. The maximum 

 deflection corresponds to a period a little later than this, because the 

 electrical variation does not at once reach its maximum at any point. 



There is ample evidence that the negative variation is a 

 normal physiological phenomenon. In human skeletal 

 muscles the current of action has been demonstrated by 

 connecting a galvanometer with ring electrodes passing 

 round the forearm, and throwing the muscles into con- 

 traction. A diphasic variation is thus obtained; and the 

 electrical change travels with a velocity of as much as twelve 

 metres per second, which is greater than the velocity in frogs* 

 muscles. Electromotive changes are likewise associated with 

 the beat of the heart. Action currents have also been detected 

 in the phrenic nerves of living animals accompanying the 

 respiratory discharge (Reid and Macdonald), in the vagi 

 accompanying the movements of the lungs, in the oesophagus 

 during swallowing, in the cutaneous sensory nerves in 

 response to the 'adequate' stimulus of pressure (Steinach), 

 in the retina in response to the adequate stimulus of light. 



