THE CRANIAL GANGLIA IN AMEIURUS 311 



The realizacion of this fact and its use as a working basis in 

 the attempt to determine the relationships of the cerebral nerves 

 is the most striking characteristic of recent work on the ner- 

 vous system of the lower vertebrates. The recognition of func- 

 tional units rather than serial homologies as the keynote in this 

 work has tended to free the study of the cerebral nerves from 

 subordination to larger problems and to call attention to the 

 need as well as to the possibility of a more thorough knowledge 

 of the composition of the nerves before this knowledge can be 

 applied to such problems as the metamerism of the head or to 

 the relation of the head to the trunk. It has also served to 

 bridge the gap that has existed between the structural and func- 

 tional conceptions of the nerves by adopting a unit that is not 

 only structural in character but functional as well. 



Such an analysis of the cerebral nerves has been possible, 

 owing to the fact that, among other things, there are structural 

 differences between the various components that have made it 

 possible to follow them throughout their whole course in favor- 

 able types. The analysis has been materially strengthened of 

 course by a clearer understanding of the various types of end 

 organs to which the fibers are distributed peripherally as well as 

 of the central connections in the brain and cord. 



The net result of this work seems to have changed the basis 

 for homologizing the cerebral nerves from the nerve trunks to 

 the nerve components. These are anatomical and physiological 

 units characterized by similarity in function, peripheral distri- 

 bution and central connections; but they are not uniformly dis- 

 tributed throughout the various cerebral nerves even in closely 

 related types. Nerves with the same name and the same general 

 topographical relations may vary totally in composition. This 

 fact makes it evident that there could be no agreement as to 

 the homology of the cerebral nerves as long as their exact com- 

 position was unknown. 



A more thorough knowledge of the components of which the 

 cerebral nerves are made up and of the composition and posi- 

 tion of the ganglia from which they arise opens the question as 

 to the origin of the discrete elements of these ganglia. Are 



