1 62 PHYSIOLOGICAL PHYSICS. [Chap. xv. 



an artificial stimulus and causes the paralysed muscles 

 to work. So that the rule remains that the irritability 

 is unaffected by the lesion. There is an exception, 

 however. It occasionally happens that the irritability 

 seems to be increased. This will occur when the 

 lesion in the brain or upper part of the spinal cord is 

 an irritative one, and irritates the ends of the fibres 

 which it has cut off from their centres. In the 

 absence of any ground for supposing an irritative 

 lesion, a physiological explanation would be that the 

 moderating influence of the higher centres had been 

 removed, and the response of the lower was, therefore, 

 more easily elicited. 



In the peripheral lesion communication has been 

 cut off with the centres in the cord. These centres 

 are not only reflex, but trophic ; the nerves, therefore, 

 cut off from their centre, degenerate, and the retro- 

 grade changes will in time also affect the muscles. 

 The rapid loss of irritability, then, is due to degenera- 

 tion. Here a curious circumstance arises which it is 

 difficult to explain. What has been said refers to 

 electricity used as induced currents, applied by 

 moistened electrodes. It is found then that in some 

 peripheral lesions, where, as is to be expected, response 

 to the induced or faradic current is entirely absent, 

 the muscles will respond to the galvanic current if it 

 be slowly interrupted, and the muscles of the paralysed 

 side will often respond vigorously to a galvanic current 

 so weak that it has no effect on the sound side. 

 Further, in such cases the nature of the response is 

 altered. As was seen when considering the law of 

 contraction, nominally the excitability is greater in 

 the neighbourhood of the cathode on closing, and in 

 the neighbourhood of the anode on opening the 

 circuit ; but in these cases it is contraction at the 

 cathode on opening and at the anode on closing that 

 is marked. It has been found difficult to explain 



