92 NERVE COMPONENTS OF BONY FISHES. 



central nucleus at the level of the canalis centralis. The 

 former ceases cephalad of the first spinal nerve and the 

 latter is nearly or quite interrupted for a short distance 

 there also, its place being taken, however, by the motor 

 nucleus of the vagus, the nucleus ambiguus, which lies 

 close to the floor of the fourth ventricle on each side. 

 The rather large multipolar cells which compose this 

 nucleus are arranged in dense rosettes, the cell group ter- 

 minating very abruptly caudad. In none of the fishes 

 which I have examined does the nucleus ambiguus gradu- 

 ally merge into the ventral cornu, as described by Haller 

 ('96). Its relations are with the paracentral nucleus 

 rather than with the ventral horn. 



At the level of the exit of the most caudal fibres of the 

 vagus nerve (663) a small compact bundle of fibres leaves 

 this nucleus, probably deriving some fibres also from the 

 nucleus of the opposite side, and passes to the ventral 

 surface of the chief vagus root. The nucleus, continuing 

 cephalad, is somewhat reduced for a short distance, but 

 at about the level at which the most cephalic vagus fibres 

 leave the oblongata (640) it is again greatly enlarged and 

 at this point gives off a broad band of fibres, which is also 

 probably re-enforced from the opposite side and which 

 enters the vagus root and there joins the other motor root 

 just described. The nucleus continues cephalad to 625, 

 but crowded farther ventrad by the enlarged lobus vagi, 

 so as to lie laterally of the fasciculus longitudinalis dorsalis. 

 Several bands of fibres arise at intervals from the pre- 

 vagal portion of the nucleus ambiguus and curve back to 

 join the other motor fibres. This nucleus is only diagram- 

 matically indicated in Fig. 4. 



Quite separated from the nucleus ambiguus, but a very 

 short distance cephalad is another very small nucleus of 



