800 



are practically continuous ^), and their fronto-caiidal extension is 

 about the same. 



Some smaller cells continue the column in a backward direction equally as in 

 Petromyzon, they seem not to be connected with rootfibres though. 



As in Petromyzon also in Myxine the caudal shifting of the facialis 

 nucleus to the level of the glossopharyngeus root — otherwise 

 present in all fishes — does not take place. This is not strange 

 though and can even be expected — since the caudal tastecentrum, 

 which causes the shifting in other animals is hardly or not developed 

 in this animal : The sensory IX^'' being absent while the sensory 

 VII''' is very small. 



Moreover — most fibres of the sensory VII'*' in Myxine are no 

 taste-fdires, but general sensory fibres as can be deduced from the 

 fact that they join the descending V''' and consequently are — 

 according to our knowledge about other animals — of the same 

 character as these (compare for the different components of the 

 sensory VII''' in Cyclostomes the important contributions of Johnston : 

 Morphologisches Jahrbuch Bd. 34, 1905 and Anatom. Anzeiger Bnd. 

 37, 1910). 



Just the fact that even the sensory VIP'' is chiefly of the same 

 character of the sensory V''' and unites with it explains why the 

 corresponding motor nuclei are attracted to the region of the descend- 

 ing V and VIF'' fibres, which constitute their chief, if not only 

 reflectory centre, their periferal endings forming the sensory invest- 

 ment of the muscular apparatus of the V and VII (the lips and the 

 so-called tongue or sucking ai^paratus). This determination of the V 

 and VII nuclei through reflectory or neurobiotactic influences is the 

 more striking, because the facial nucleus does not only lack the 

 caudal displacement, but even sliifts in frontal direction as appears 

 from the fact that its root-fibres show a frontal course in the medulla 

 (see fig. 2). The nucleus of the Vll thus approaches still more the 

 entrance of the sensorv V''' that dominates its functions. 



1) Where tlie somewhat smaller cells of the Vlflh nucleus touch the Vth nucleus, 

 the column is less compact, has an indication of a gap. It is known that Tretjakoff 

 (Archiv. f. microskopische Anatomie Bnd 74, 1909, p. 713} supposes the VI 

 nucleus of Ammocoetes (the young Petromyzon) to lie at the end of the Vth cells 

 in the V — VIll column, because the root-fibres of the abducens. accompany the 

 Vth root to the oblongata and cannot be separated h-om them in the bulb. I do 

 not know whether this opinion is right, but I only want to call attention to the 

 fact that in Myxine, where certainly no VI nucleus is present, there is indeed a 

 sort of a gap at the place where Tretjakoff supposes the VI nucleus to be in 

 Ammocoetes. 



