CENTRAL RELATIONS OF COMPONENTS. 65 



buds of the trunk, too, we have reason to believe, have 

 grown back into it from the head, as the organs of the 

 lateral line are known to do. We have no evidence that 

 the terminal bud system was primitively present in a 

 metameric way in the trunk, but, as in the lateral line 

 system, all the evidence at hand points to its cephalic 

 origin. The theoretical problems connected with the 

 communis system are further discussed in Section 12. 



The secondary connections have not been fully worked 

 oiit for any of the cranial nerves. In the case of the 

 vagus, as with the acustico-lateral nerves, these connec- 

 tions are of two types, crossed and uncrossed. The un- 

 crossed fibres, or secondary vagus bundle, in the sense of 

 Mayser, gather mesally and ventrally of the spinal V 

 tract and maintain this relation up to the exit of the latter 

 from the brain, when they pass directly up into the cere- 

 bellum (Figs. 17, 18, 19, Sec. X). The crossed fibres, 

 after reaching the opposite side through the commissura 

 accessoria, enter the tractus bulbo-tectalis {tr. b. /.) and a 

 large part, if not all of them, reach the optic tectum. 



IV. — The Motor Components. 



For the description of the motor nuclei the reader is 

 referred to the accounts of the motor nerves in the follow- 

 ing sections. Some points of a more general morpho- 

 logical interest regarding these components have been 

 suggested in the preceding pages of this section. It need 

 only be added here that I confirm, in general, the division 

 of the motor cranial nerves in two series, somatic and 

 visceral. They all belong to the latter type in the 

 fishes save the eye-muscle nerves. The nuclei of the 

 branchio-motor type, viz., nucleus ambiguus, motor VII 

 and motor V, I consider to be cranial differentiations of a 



