Phenomena oí ncurobiotaxis iii thc optic svátcni 271 



In sharks, the situation however appearecl to be quite clilVerent. Here 

 the same nucleus lies on an extremely dorsal level, as is clearly shown in 

 figure 3. 



This observation led me to define precisely the positions occupied by 

 the nucleus abducens and by the other motor nuclei of the oblongata in 

 the whole series of vertebrates. 



These investigations produced very surprising results. 



Concerning the )iuclens abducens the following was tound: 



In its most primitive position — with Lampreys — , the VI, nucleus accor- 

 ding to Tretjakojf is inclosed in a frontal cell column, lying in the an- 

 terior part of the oblongata, in which column (more laterally), also the nu- 

 cleus trigeminus and the nucleus facialis are to be found. This nucleus 

 holds a dorsal position in front of the VIL nucleus. Its root, together with 

 the trigeminal one, leaves the bulb cióse to the facialis root (Tretjakoffi. 



With higher fishes there appears another state of things, inasmuch the 

 nucleus as well with sharks as with bony-fishes, is lying beinnd the en- 

 trance of the VII root, being thus moved backward into the región of 

 the oblongata, were the nervus vestibularis makes its entrance and, part- 

 ly at least, ends. 



With sharks the nucleus has a more dorsal and moreover a more cau- 

 dal position, whilst bony-fishes some times have it partly lying on the 

 level of the facialis root entrance and ahvays nearer the base of the bulb. 



This difTerence between sharks and bony-fishes, on closer examina- 

 tion, throws a clear light on the factors, causing the shifting of the nu- 

 cleus abducens. 



When comparing the tracts which stimulate, in these animáis, the 

 nucleus abducens, we find, two capital systems, acting on the nuclei of 

 the eyemuscles: the vestibular and the optic reflectory system, the for- 

 mer being strongest in sharks, who are possessed of an enormously de- 

 veloped fase, longitud, posterior (f. 1. p.) 



Several shorter and longer reñextracts, starting from the vestibu- 

 lar termini in the oblongata, end in or next to the f. 1. p., on the late- 

 ral side of which the cells of the nucleus abducens in these animáis are 

 spread (fig. 3). 



Also the optic reflextracts are well developed in sharks and spread 

 over the área of the fasciculus longitud, post. and the base of the oblon- 

 gata. The last system, the socalled ventral tecto-bulbar tracts (^specially 



