Kingsbury, Brain of Necturus. i8i 



cephalad of the ninth nerve in Acipenser (Goronowitsch), the 

 Cyptinid(E (Mayser), sharks and Rays {L(2margiis, Lamna, Raid) 

 (Ewart), and Galeocerdo (Strong, '95), and Wright was incHnedto 

 regard its origin similar in Amiiinis. In the Cyprinidce the fibers 

 were derived from the tiibcnidiun acusticuni. In Acipenser Go- 

 ronowitsch found the lateral nerve formed of ascending and de- 

 scending fibers from the " dorso-lateral" tract, of which the 

 descending bundle was much the larger and was thought by him 

 to come from the cerebellum. In Necinnis the origin of these 

 fibers was restricted to the dorsal region of the oblongata in the 

 region caudad of the eighth nerve, which might be regarded as 

 representing the tubercidiini aciisticiun of the teleosts, although 

 no tuberculum acusticum proper exists. Certainly no fibers can 

 be traced cephalad to the cerebellum. 



In Necturus, as compared with the other urodeles exam- 

 ined, this nerve was greatly developed, a condition to be ex- 

 pected, both from its larger size and the greater number of 

 sense organs in Necturus. The ratio of IX^+^ to IX^+* in this 

 form is approximately 2:1. In Aniblystoma, both roots, IX^ 

 and IX^ are distinct but their ratio to IX^+^ is about 1:1. 



Acustic Nerve. — As far caudad as the exit of the X^ large 

 fibers begin to appear in the region dorsad to the ascending V 

 and between it and tract b. These increase in number cephal- 

 ad, and at last, about i millimeter cephalad of the entrance of 

 IX^ they enter the eighth nerve. This forms a considerable and 

 indeed the largest source of the fibers of the eighth nerve. The 

 fibers are large with thin myelinic sheaths and are very easily 

 traced. A far smaller portion of the acustic fibers, which are 

 also of finer caliber, turn cephalad. Osborn ('88) regarded 

 such in CryptobrancJius as decussating through the cerebellum 

 with the corresponding fibers of the eighth nerve on the oppo- 

 site side. A direct relation of acustic fibers to the cerebel- 

 lum in the lower forms appears probable from the results of 

 Mayser, Goronowitsch, and Koppen, ('88) on the species inves- 

 tigated by them. Ahlborn however considered that the fibers 

 entering the cerebellum in Petroniyson were commissural be- 

 tween the acustic nidi of the two sides. In higher forms the 



