276 



C. U. Aricns Kappers 



In reptiles (Gongylus) and birds (Gallus) Terni found an accessory 

 \1 nucleus as described by Van Gehuchten in the chick and he stated 

 Ihat the number of its cells corresponda with the number of nervefibers 

 entering the m. retractar biilbi and ni. bursalis in Reptiles, in the m. m. py- 

 ramidalis and qnadratns in birds which form in these animáis the mus- 

 cles of the membrana nictitans. The position of this accessory nucleus 



Nucí. 

 Clenu Nervi jjrincipalis 

 \'1I vestibularis 



■'■% 



■v^ 



^J^X'^r-- ;>. 





' / i - ' 





\ .'Vi l r\ 



l-i". -. — J Jorso-lateral position ot the abducens nucleus in the rabbit, near the 

 Xu. principalis vestibularis and Xu. Deiters. 



is a more ventrolateral one, near the descending V root, whose stimuli 

 determine the reflexes of the membrana nictitans. 



The successive frontal shifting of the chief VI nucleus in these animáis 

 however is caused by the position occupied by the frontal part of Deiters 

 nucleus and by the dorsal vestibular nucleus which, in these animáis are 

 the most important reflectory centres for the chief VI nucleus (figs. 6, 7). 



That especially these centres in the higher classes of vertebrates de- 

 termine the position of its cells, is also proved by the dorso-lateral posi- 

 tion held by the nucleus abducens in most mammals. Only in Carnívo- 

 ra, Cetácea, and Insectivora, the position next to the fasciculus longitudi- 

 nalis posterior and under the horizontal VII root is kept. In all other 



