Phenomena of neurobiotaxis in the optic system 277 



mammals and in man, the nucleus has shifted ¡n a more dorso-lateral di- 

 rection, lying immediately adjacent to the dorsal principal vestibular 

 nucleus and Deiters nucleus, as is most apparent in the rabbit (fig. ~) in 

 the horse and man. By this last shifting the nucleus acquires a position 

 no longer underneath but laterally to the horizontal Vil root. 



Terni again found in Mus rattus the accessory VI nucleus, also des- 

 cribed by Lugaro and Valkenburg in a more ventrolateral position near 

 the descending trigeminus root in which it sends its dendrites. Terni 

 points out that this is the nucleus of the membrana nictitans and has 



nucí, trochlearis nerv. trochlearis 



Fig. 8. — Position of the trochlearis nucleus above the sulc. limitans in the velum 



cerebelli, in the Lamprey. 



demonstrated also the trigeminal reflexes which apparently determine its 

 position neurobiotactically (it fails in mammals having no membr. nic- 

 titans). 



Thus it is made obvious by a phylogenetic review oí the position, 

 occupied by the nucleus abducens and its accessory cells, that their to- 

 pography is determined by the strongest stimuli acting on it, these being 

 different in different animáis and different tbr the chief and accessory nu- 

 cleus. These differences make us acquainted with neurobiotaxis as the 

 leadingf factor in the structure of the nervous svstem. 



It is ¡nteresting to note that the above described phylogenetic frontal shifting 

 of the nucleus abducens is repeated in ontogénesis, as is demonstrated by Bok in 

 embryos of birds, hy Slrcclcr in those of men: an engrammatic reproduction. 



The shiftings of the nucíais trochlearis are equally evident. To begin 

 with the Lamprey, which in this investigation may serve as the most 

 primitive stage, see figure 8, we find the nucleus of the trochlearis in 



