THE MUSCLES AND NERVES. 289 



bifurcation, distinct from and larger than the nuclei of the 

 sheath. These he named ' axis cylinder nuclei.' I have fre- 

 quently been able to demonstrate the fact that these are really 

 the nuclei of fusiform bipolar ganglion cells. On the proximal 

 side of each cell the process is an axis cylinder, but on the 

 distal side it is a tubular nerve, which has some resemblance 

 to a' medullated nerve fibre (PI. XVII., Figs. 9 and 10). Such 

 large fibres are seen in transverse sections of the branches of 

 nerves surrounded by fine axis cylinders. 



Although the larger tubular fibres are blackened by osmic 

 acid, they have far less fatty matter in their composition 

 than the medullated nerves of Vertebrates. It appears to 

 me that the proximal fibres should be regarded as nerve-root 

 fibres, belonging properly to the central ganglia, whilst the 

 larger distal fibres are true nerve fibres. At each bifurcation 

 of the nerve one or more ganglion cells occur, so that the 

 number of larger fibres increases. I regard the cells which 

 appear at the angles of bifurcation as trophic nerve elements, 

 similar to those of the ganglia on the posterior spinal nerve- 

 roots of Vertebrates. 



Whether all the nerve fibres, or only the sensory ones, pass 

 through these ganglion cells is a matter on which I am still 

 doubtful ; but those which do, terminate in peripheral ganglion 

 cells, the branching processes of which again assume the form 

 of simple axis cylinders. I have not observed any such 

 terminal ganglion corpuscles on the motor nerve fibrils which 

 end in the muscle fibres. Hence I think it probable that all 

 such large tubular nerve fibres are sensory in function. 



Although I have compared the large nerve tubules with the 

 medullated fibres of Vertebrates, I have never been able 

 to find any indications of ' nodes of Ranvier,' or the rodded 

 structure characteristic of the latter. The nerve tubules 

 described by Weismann and others correspond with the large 

 nerve fibres. 



Motor End Organs.— I have sought in vain for well-marked 

 motorial end plates both in the larva and the imago of the 

 Blow-fly. The nearest approach to such organs are small, more 



