CH. XII. ] FATIGUE 133 



most important factor in fatigue, and that the fatigue products pro- 

 duced in the muscles during work cause part of their injurious 

 effects by acting on the central nervous system and diminishing its 

 power of sending out impulses. 



One of the most striking of Mosso's experiments illustrates in a 

 very forcible manner the fact that the central nervous system is more 

 easily fatigued than the nerve-endings in muscle. A person goes on 

 lifting the weight until, under the influence of the will, he is unable 

 to raise it any more. If then without waiting for fatigue to pass off, 

 the nerves going to the finger muscles are stimulated artificially by 

 induction shocks, they once more enter into vigorous contraction. 



Mosso has also shown that the introduction of the blood of a 

 fatigued animal into the circulation of a normal one will give rise in 

 the latter to all the symptoms of fatigue. The blood of the fatigued 

 animal contains the products of activity of its muscles, but still 

 remains alkaline ; the poisonous substance cannot therefore be free 

 lactic acid. Lactic acid is doubtless one only of the products of 

 muscular activity; we have at present no accurate knowledge of 

 the chemical nature of the others. 



It should, however, be mentioned that some physiologists (Lee, 

 loteyko, etc.), still regard the effect on the end plates as the most 

 important factor in fatigue. 



The statement that nerves are not fatiguable, does not mean that the nerve- 

 fibres undergo no metabolic changes when transmitting a nerve impulse, but that 

 the change is so slight, and the possibilities of repair so great, that fatigue in the 

 usual acceptation of the term cannot be demonstrated. Waller made the interesting 

 but tentative suggestion that the medullary sheath is a great factor in repair, or, in 

 his own words, " the active grey axis both lays down and uses up its own fatty 

 sheath, and it is inexhaustible not because there is little or no expenditure, but 

 because there is an ample re-supply." 



A year or two after these words were written, Miss Sowton, at Dr Waller's 

 suggestion, undertook a piece of work in order to test the truth of this hypothesis. 

 If the absence of fatigue is due to the presence of the fatty sheath, fatigue ought 

 to be demonstrable in nerve-fibres in which the fatty sheath is absent. She 

 selected the olfactory nerve of the pike as the non-medullated nerve with which to 

 try the experiment, and her results confirmed Dr Waller's expectation ; the galvano- 

 metric replies of the nerve became somewhat feebler after repeated stimulation. 



It appeared to me advisable to test the question in another way. The splenic 

 nerves seemed to be the most convenient large bundles of non-medullated fibres 

 for the purpose. Dr T. G. Brodie was associated with me in carrying out the in- 

 vestigation. A dog is anaesthetised, the abdomen opened, the spleen exposed, and 

 the splenic nerves which lie by the side of the main splenic artery are laid bare. 

 It is quite easy to dissect out a length of nerve sufficient for the experiment (1^ to 2 

 inches). The nerve is then cut as far from the spleen as possible, and the spleen 

 is enclosed in an air oncometer connected to the bellows volume recorder invented 

 by Dr Brodie. On stimulating the nerve with a weak faradic current the organ 

 contracts, and the recording lever falls. The diminution of the size of the spleen 

 is quite visible to the naked eye, however, without the use of any apparatus. The 

 next thing to do is to put a block on the course of the nerve, which will prevent 

 the nerve impulses from reaching the spleen. Here we met with some difficulty. 

 Curare and atropine are both ineffective: the constant current has a great dis- 

 advantage ; non-medullated nerves are so much affected that very feeble constant 



