THE INFLUENCE OF PREVIOUS EXCITATION. 523 



to half the amount present in the freshly excised nerve, and in twelve 

 hours to a comparatively small amount. This is undoubtedly due to 

 the passing away of the specific change in the neighbourhood of the 

 cross section, since it may be restored to nearly its full amount by a new 

 cross section. 1 The gradual decline is more rapid in summer than in 

 winter frogs ; though here also, after the difference has subsided, it can 

 be renewed by a fresh cross section. 



The decline and the susceptibility to renewal by fresh cross section 

 are phenomena common to medullated nerves, the spinal cord, and non- 

 medullated nerves. There is thus little ground for the belief either that 

 the decline is due to the gradual physiological failure of the nerve 

 fibres from the point of cross section as far as the nearest node, 

 or that the renewal with a fresh section is due to parts being 

 involved above such nodes. Both the decline and the renewal are 

 rather to be regarded as indicating that the sudden installation of the 

 difference is a functional change, localised to the neighbourhood of the 

 cross section ; the duration of this change is determined by the func- 

 tional capacity of the uninjured nerve and the extent of the alterations 

 produced by the injury. 



It often happens that the difference in a freshly excised nerve 

 increases in amount for a short time after its preparation ; this might be 

 due to loss of liquid from the nerve through evaporation, thus increasing 

 the resistance, and so altering the distribution of the derivation surface 

 currents, since moistening the nerve always causes the reverse effect. 



It is probable, however, that in most instances both the functional 

 capacity of the uninjured tissue and the extent of the reaction of the 

 altered tissue increases for a certain time, since such augmentation is a 

 familiar phase of excitation phenomena in the case of both nerve and 

 other excitable tissues. 



An interesting fact connected with the circulation and the maintenance of 

 resting electromotive phenomena is demonstrable on mammalian nerve. The 

 sciatic nerve, or the exposed spinal cord, of the cat or monkey is ligatured and 

 divided, and the end having been freed from the neighbouring structures, is 

 connected with a galvanometer. In this condition a cross sectional difference 

 is obtained which declines very slowly, provided that the few centimetres of 

 exposed nerve are in structural and physiological connection with the deeper 

 undisturbed portions of the same tissue. On the death of the animal, or the 

 complete excision of the tissue, the difference commences to decline with 

 considerable rapidity. It thus appears that the maintenance of a connection 

 with functionally active portions of the tissue, prolongs the duration of the 

 cross sectional difference in the exposed nerve of the mammal. 2 



Finally, although the resting difference of an excised nerve declines, even 

 in the case of the frog's sciatic, to little or nothing in twenty-four hours, it may 

 be reproduced by fresh cross section several days after excision, provided that 

 the nerve is kept from drying, by placing it in 0'6 per cent. NaCl, containing 

 traces of Ca salts ; and the same is true of the excised mammalian nerve. 

 The amount of the resting differences in such stale mammalian nerves is 

 notably less than those of fresh ones, as might be expected from the impair- 

 ment which has taken place in their functional attributes. 



(e) The influence of previous excitation. The gradual decline in the 

 difference which occurs in an excised nerve is greatly modified by 



1 Engelmann, Arch. f. d. ge/t. PhysioL, Bonn, 1877, Bd. xv. S. 138. 



2 Gotch and Horsley, Phil. Trans., London, loc. cit. 



