306 MESSRS. F. GOTCH AND V. HORSLEY 



usually a fall in the amount. Sometimes it has happened that a rapid rise for the 

 first few minutes has been observed which did not admit of explanation as being 

 caused by either of the two agents just indicated ; and the presence of such a rise 

 to a very marked degree in the case of the spinal cord as well as its presence in the 

 isolated nerves and muscles of the Frog, renders it probable that in the Mammalian 

 nerve, when present, its origin is physiological. 



The fall is undoubtedly connected with loss of vitality, and may vary between the 

 slow diminution which follows arrest of the circulation, and the rapid fall produced by 

 injury of the nerve in immediate proximity to the surface contact. 



The alterations which especially call for remark here are those which are entailed by 

 the death of the animal on the one hand, and by the severance of the nerve from the 

 spinal cord on the other. The first is probably due to several factors whilst the 

 second is due both to arrest of the local circulation and the destruction of the con 

 tinuity of the nerve fibres with their trophic centres in the cord. 



In the case of the sciatic nerve, the alteration seemed to us to be the same in 

 extent whichever event occurred ; that is to say, the resting difference for the first 

 30 minutes slowly diminished in amount at the average rate of ToWo Daniell per 

 minute, this rate of diminution becoming less afterwards. 



It is, however, questionable to what extent in the suspended and isolated nerve, the 

 physiological conditions which are dependent upon continuity with the cord are 

 maintained by any facilities which the preservation of vessels affords for keeping up a 

 circulation in the nerve, since this must be, undoubtedly, greatly impaired by the 

 exposure. The conditions of vitality would rather seem to be linked with the 

 maintenance of physiological connection with other nerve structures, around which 

 an active circulation is still being carried on, and which, therefore, retain their normal 

 vital characteristics. That this is probable is shown by the fact that when, as some 

 times occurs, the nerve difference, owing to rapid drying, shows a steady rise in 

 amount, which rise continues even when no evidence of circulation in the exposed 

 nerve is obvious, the rise is immediately counterbalanced and converted into a fall on 

 systemic death of the animal, this fall occurring pan passu with the loss of excita 

 bility in the central nerve structures. 



(c.) Physiological Changes Connected ivith Excitation. The alterations due to 

 excitation were after effects, that is, they followed the development of the transient 

 excitatory change and consisted in a permanent slight diminution in the previously 

 existing difference, such as has been observed to follow nerve excitation in the case of 

 the Frog. 



The slow after effect can be readily distinguished from the transient rapid electrical 

 changes due to the actual presence of excitatory processes evoked by the application 

 of the stimulus, since these are synchronous with the excitation, whereas the after 

 effect is very variable in its duration, but always follows cessation of the stimulus, and 

 persists for many seconds. 



