THEORY OF NERVE COMPONENTS. 13 



sense organs of like nature all over the skin of the 

 head and trunk, the so-called terminal buds. Fishes 

 generally have an elaborate system of taste buds 

 in the mouth, all of which are related to this cen- 

 tre; but they differ widely in the number of ter- 

 minal buds on the outer skin, and in all known cases 

 the size of the lobi vagi is increased where these latter 

 organs are numerous. 



The cat-fishes and some others present an interesting 

 modification of this case. Here the terminal buds which 

 are supplied by the vagus nerve, are reduced in number, 

 but on the head and especially on the barblets, these 

 organs are exceedingly numerous. Accordingly, the 

 branches of the facial nerve which supply these regions 

 are enlarged and a terminal centre in front of the lobus 

 vagi is developed for them, the lobus facialis (the so- 

 called lobus trigemini of the older authors). This lobus 

 is only a pre-auditory derivative of the lobus vagi and all 

 nerves related to these two homodynamous centres can be 

 treated as a morphological unit, the " communis system." 

 (Text-figure 3). 



These lobes constitute visceral or special sensory cen- 

 tres and can in no respect be compared with the dorsal 

 horns of the spinal cord. They are new structures de- 

 veloped in the head in correlation with distinctively 

 cranial sense organs. If represented in the trunk at all, 

 it could only be in the feebly developed visceral sensory 

 centres of the spinal cord. 



The cat-fish illustrates another one of the medullary 

 centres, whose size is exceedingly variable among the 

 vertebrates. This is the tuberculum acusticum, in man 

 related only to the auditor}^ nerve, but in the fishes 

 serving as the terminal nucleus of the entire lateral line 



