724 A MANUAL OF PHYSIOLOGY 



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 demarca- 

 tion current being accurately compensated), because the circuit will 

 be opened before the positive 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 positive 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 excitatory electrical response 

 is a normal physiological phenomenon. In human skeletal 

 muscles the current of action has been demonstrated by con- 

 necting a galvanometer with ring electrodes passing round the 

 forearm, and throwing the muscles into contraction. 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, in glands during secretion, in the central nervous system 

 during the passage of impulses along its conducting paths. 

 Some of these will be further considered a little later on. 



As to the interpretation of the facts we have been describing, and 

 which are summed up in the three propositions on p. 718, two chief 

 doctrines long divided the physiological world : (i) the theory of 

 du Bois-Reymond, the pioneer of electro-physiology, and (2) the 

 theory of Hermann. It was believed by du Bois-Reymond that 

 the current of rest seen in injured tissues is of deep physiological 

 import, and that the electrical difference which gives rise to it is not 

 developed by the lesion as such, but only unmasked when the 

 electrical balance is upset by injury. He looked upon the muscle 

 or nerve as built up of electromotive particles, with definite positive 

 and negative surfaces arranged in a regular manner in a sort of 

 ground-substance which is electrically indifferent. The ' negative 

 variation ' he supposed to depend on an actual diminution of 

 previously existing electromotive forces ; and from this conception 

 arose its historic name. Hermann and his school assumed that 

 the uninjured muscle or nerve is ' streamless,' not because equal and 

 opposite electromotive forces exactly balance each other in the 

 substance of the tissue, but because electromotive forces are absent 

 until they are called into existence (by chemical changes) at the 



