TRANSMISSION AND CONDUCTION 227 



death changes (Child, 1914^). Moreover, Adrian's 

 data on the frog's sciatic indicate that more peripheral 

 levels of the nerve are slightly less susceptible to the 

 action of anesthetics (Lucas, 1917, chap, iv), though the 

 author attributes the difference to defects in the appara- 

 tus. The frog's sciatic is unfavorable material for the 

 detection of a gradient because the length of nerve 

 available is not great. If a gradient is present in the 

 axon it certainly cannot be steep, and in many cases it 

 may be appreciable only in long nerves. As the matter 

 stands at present, the experimental evidence indicates 

 that gradients do exist, at least in some axons, and I 

 have shown that the "all-or-none" type of reaction is not 

 in the least incompatible with the existence of these 

 gradients. 



Even if we admit that physiological gradients are 

 present in axons after the growth period is over, the 

 limit of conduction is certainly not reached nor even 

 approached in the higher animals within the lengths 

 of axons attained in the body. Possibly in some of the 

 elongated lower vertebrates and invertebrates the axon 

 length may approach the limit, but on this point we 

 have no data. If it exists then in the higher forms, the 

 axon gradient is important, not as determining functional 

 relations, but rather as a record, an effect of the physio- 

 logical gradients of the body. 



When the vertebrate nerve is cooled or partially 

 anesthetized, a relatively steep decrement in conduction 

 appears, as many investigators have shown. Appar- 

 ently under these conditions the more susceptible parts 

 of the excitatory mechanism have been put out of action, 

 so that the excitation which occurs is only partial and 



