274 SOME CHAPTERS ON THE 



in Fig. 6, that is, if the central end of the internal popliteal was 

 simultaneously sutured to both external and internal popliteal 

 together, then when regeneration has occurred three points are 

 noted. (Langley and Anderson observed a similar phenomenon 

 when the central end of the crural nerve is sutured to the in- 

 ternal saphenous and also connects with its own muscular branch.) 



(1) Co-ordination is nearly as good as in the normal animal. 



(2) There are more new fibres in the two branches than in the 

 central trunk. 



(3) Excitation of, e.g., the peripheral end of the regenerated 

 external popliteal, gives a contraction of the muscles supplied by 

 the internal popliteal ; even when all connection with the spinal 

 cord is removed by section some distance above the line of suture. 

 This contraction is termed the axon reflex, following Langley's 

 terminology. 



Some of the new fibres in the trunk must therefore have a 

 double peripheral ending in the branches. The inference is that 

 either the peripheral fibres have joined together, which is a 

 process unknown elsewhere, or else that one central fibre has 

 branched into two at the line of suture. The latter explanation 

 is by far the most probable. The inference is that in this case 

 antagonistic muscles must be to some extent at least innervated 

 from the same anterior nerve cells. It is surprising, not that 

 co-ordination fails in the finer details, but that it exists at all 

 under these conditions. 



V. THEORIES OF NERVE ACTIVITY 



It is not possible to review, in the compass of a work of this 

 kind, one-tenth of the literature that exists on this subject. All 

 that can be done is to select such portions as seem worthy of con- 

 sideration, not so much for their intrinsic worth as for the indica- 

 tion they give as to the direction in which further researches will 

 advance. 



From the physico-chemical standpoint all the phenomena of 

 the electric currents in nerve (and in all tissues) must be due, as 

 far as our present knowledge goes, to the movement of ions carry- 

 ing charges of electricity. Current of injury, negative variation 

 and electrotonic currents, must all ultimately have this explana- 

 tion ; and as no other hypothesis seems at present at all probable, 



