NOTES ON MEDIAN AND PAIRED FINS OF FISH. o49 



speakiiio'j even in this case the iiei'ves in all probability 

 remain faithful to the muscle substance of their own segment, 

 for it has been proved that each motor root supplies its own 

 special muscle-fibres^ which are merely bound together in the 

 same muscle (Sherrington 34). 



It seems to me very doubtful whether such compound 

 muscles are ever produced in the fins of fish, and I shall show 

 later (pp. 359 and 369) that there is good reason for believing 

 that the adult radial muscles are both unisegmental and 

 haploneurous. However, compound polyneurous muscles may 

 perhaps be found in fish, as they are in higher vertebrates. 

 Thus segmental nerves, involved in a limb-plexus, may 

 apparently, but only apparently, become connected Avith 

 muscles belonging to other segments than their own. 



The Shifting of Limbs Explained. 



Briefly we may repeat, the muscle and nerve-supply is 

 drawn in the embryo from the segments of the region 

 occupied by the limbs in the adult; in cases where the 

 development is unknown, the nerve-supply indicates to which 

 segments the limbs belong. The size of the nerves compos- 

 ing the plexus may be considered as proportional to the im- 

 portance of the share the several segments take in the 

 formation of the muscles. The muscle-buds and adult muscles 

 in fins are usually better developed in the central regions of 

 the fins than at their two ends. So the nerve components of 

 a limb-plexus are usually stouter in the middle than in front 

 and behind. Just as the muscular elements dwindle or 

 increase in size, owing to the backward or forward extension 

 of the base of a limb, just so far may the nerves increase or 

 diminish in thickness. 



The position of a limb-plexus may shift backwards or 

 forwards in all Gnathostomes ; no one would suppose that the 

 nerves actually pass up or down through the vertebra?, etc. 

 Furbringer has clearly shown how the shifting may take place 

 in his important and beautiful works on the anatomy of birds 



VOL. 50, PART 2. NEW SERIES. 25 



