GENERAL CONCLUSIONS. 265 



The communis system is also probably very ancient in 

 its simple visceral form. It has, however, no pre-facial 

 representative, and in the nerves in which it persists it 

 has been highly modified in connection with the taste bud 

 and terminal bud apparatus. 



The acustico-lateral system seems to have been differen- 

 tiated rather late and in connection primarily with the 

 facialis segment. 



The criteria of these components are primarily the 

 central and peripheral relations of the nerves. It happens, 

 however, that each component has certain characteristic 

 and quite constant differences in the character of its nerve 

 fibres which make it possible to separate them, as a rule, 

 throughout their peripheral courses, even when several 

 components are bound up together in a common trunk. 

 Thus, the somatic motor fibres are always large with wide 

 medullary sheaths and large axis cylinders, the visceral 

 motor fibres are usually very small with very feeble 

 myelination, the communis fibres are very minute and 

 with still more delicate medullary sheaths, the general 

 cutaneous fibres are of small or medium size with 

 occasional larger fibres scattered among them, their 

 sheaths being somewhat heavier than those of the com- 

 munis fibres, and the lateralis fibres are the largest of all, 

 having very large axis cylinders and wide sheaths which 

 usually stain more densely than those of the somatic 

 motor fibres. 



These fibre characters, however, are not absolutely 

 constant, but vary with the degree of development of the 

 organs innervated. The clearest illustration of this is in 

 the motor components. The branchial muscles, known 

 to be of visceral origin, have acquired in the fishes 

 the striation and large size of the fibres characteristic 



