68 'Journal of Comparative Neurology and Psychology. 



Chapter IV. 



THE HIND-BRAIN, METENCEPHALON. 

 I. The Huid-hraui of the Teleosts. 



The medulla oblongata in the teleosts forms the transition 

 between the posterior part of the mid-brain and the medulla 

 spinalis. The region where the mid-brain and hind-brain join is 

 covered dorsally by a highly developed wall through which several 

 groups of fibers continue their course. This roof of the aqueduct 

 is the velum anticum cerebelli. Under and behind the velum 

 the aqueduct continues into the ventriculus quartus, or rhom- 

 boidalis, which is larger and deeper in immature Lophius and 

 whose upper part, which is roofed by the widely extended cere- 

 bellar arms (Fig. Ixxiv, Plate V), is in the full grown fishes nar- 

 rowed to a fissure by means of a mass, the nucleus lateralis cere- 

 belli (Figs. Ixxxvii to Ixxxix, Plate VI), situated at the union of 

 these cerebellar-arms and the oblongata, protruding them outward 

 and especially inward. This nucleus has been described by 

 others as "Rindenknoten," or secondary vagus nucleus. 



Behind the cerebellum the fourth ventricle is for a short dis- 

 tance covered by the tela choroidea, from which the plexus 

 choroideus grows into the ventricle. In Gadus the part of the 

 ventricle which is covered by tela is still less than in this young 

 specimen of Lophius, though in the latter a commissure appears a 

 little farther caudad in this part of the roof between the caudal 

 ends of the tubercula acustica (Fig. Ixxviii, Plate V). In the full- 

 grown codfish there is not only a commissure between the poste- 

 rior parts of the acustica (Fig. xcvii, Plate VI), but the tubercula 

 acustica themselves are fused (Figs, xcv, xcvi). Accordingly, 

 the whole roof of the sinus rhomboidalis is of nervous character 

 and this greatly reduces both the membranous covering and the 

 cavity of the ventricle itself. Behind the acoustic region the roof 

 is again for a space ependymal, but soon again becomes massive 

 by the union of the lobi vagi, behind which both in Gadus and in 

 Lophius another small commissure connects the two sides, the 

 commissura infima Halleri (Fig. Ixxx). 



At the plane of this commissure in Lophius a new and more 

 important enlargement of the sensory field begins to appear, in 



