78 THE INVOL UNTAR Y NER VO US S YSTEM 



monstrated by the methylene blue method. The axons of 

 many of them are seen to divide while still in the central nervous 

 system, thus forming two nerve fibres one of which is usually 

 larger than the other : the one fibre passes out into one nerve 

 while the other passes into another nerve. The one fibre passes 

 out as the large motor fibre to a group of muscles, the other 

 passes into a nerve which supplies with motor fibres the opposing 

 musculature if we may judge from Hardy's researches on the 

 motor n-erves to the flexor and extensor muscles and Celesia's 

 work on the two nerves which supply respectively the opening 

 and closing muscles of the claw with motor fibres. Such a nerve 

 fibre can hardly be a motor fibre but may possibly be an inhibitory 

 nerve fibre to the opposing muscle. 



The only evidence which I know in the vertebrate kingdom 

 of the axon of a presumably motor neuron dividing into two 

 fibres, which travel to different destinations, is given by Dogiel 

 in the case of some of the cells of Auerbach's plexus. The cells 

 of Auerbach's plexus are arranged in groups along the intestine 

 and, as already mentioned, nerve fibres coming from elsewhere 

 (vagus in my opinion, sympathetic according to Dogiel), make 

 connexion with many of these ganglia by means of collaterals ; 

 further these groups of nerve cells are generally considered to 

 consist of motor neurons so that their axons are motor axons. 

 The connector fibres to these cells and their motor fibres form a 

 mesh-work of nerve fibres which is so marked a feature of Auer- 

 bach's plexus. If in this mesh- work two sets of motor fibres 

 cross each other on their way to the muscles of the intestine, then 

 it is clear that the one set of motor fibres will in all probability 

 innervate a different part of the musculature from that of the 

 other set. Now Dogiel points out, that among these nerve cells 

 of Auerbach's plexus, certain cells exist, each of which has an 

 axon which passes undivided along a strand of fibres until it 

 reaches the junction point with another strand ; it there splits into 

 two fibres which pass to their destination, one along one strand 

 and the other along the other. If these are, as seems probable, 

 motor axons, then we have here a most suggestive resemblance 

 to a common occurrence among invertebrates, which must be 

 taken into account in the consideration of reflex action in the 

 isolated intestine. The further consideration of this question will 

 be taken in a later chapter. 



