158 ANATOMY OF VERTEBRATES. 



fourth ventricle, joining fasciculi from the cerebellar crus and 

 those of the anterior root. This ( consists of two portions, of 

 which the chief penetrates the medulla beneath the restiform 

 body and enters both parts of the acoustic nucleus : the other 

 portion runs backward along the upper border of the restiform 

 body, which it accompanies over the superior peduncle to the 

 inferior vermiform process of the cerebellum.' * The ( flocculus,' 

 fig. 64, n, with which the acoustic nucleus is connected, is large 

 in the Cat, the Aye-aye, the timid Rodents, and all the small 

 Mammals with acute hearing; it is relatively small in the Sheep 

 and most Ungulates. 



The acoustic nerve quits its origin in contact with the facial, 

 fig. 128, g, a small artery to the labyrinth runs between them: 

 it takes a short course to the ( meatus internus,' longer in Cetacea 

 than in other Mammals, receives a filament or two from the 

 intermediate part of the facial, figs. 132, 133, d, on entering the 

 meatus, and then divides. The part penetrating the fore half of 

 the cribriform plate supplies the cochlea ; its large size is a mam- 

 malian characteristic, and is most remarkable in the Cetacea : the 

 posterior division, answering to the main part of the acoustic in 

 lower Vertebrates, is spent upon the vestibule and semicircular 

 canals. 



The eighth cerebral nerve, in anthropotomical enumeration, 

 includes the three nerves called ' glosso-pharyngeal,' ' vagal,' 

 fig. 128, h, and { spinal accessory,' ib. /. The roots of the glosso- 

 pharyngeal are traceable to a nucleus of grey matter at ?z, fig, 57. 

 The vagal nuclei, ib. h, are forward (in Man upward) extensions 

 of the grey or vesicular myelonal columns from which the spinal 

 accessory rises : they lie on each side of the hypoglossal nuclei, 

 ib. g, on the floor of the fourth ventricle, but are united by the 

 commissure forming the roof of the central canal before this opens 

 into the ventricle : higher up the vagal roots penetrate the 

 ' caput cornu,' like the posterior or dorsal myelonal roots. There 

 is a partial decussation at the raphe. 



Both glosso-pharyngeal and vagal nerves emerge at the angle 

 between the olivary and restiform tracts of the macromyelon, 

 k, k, fig. 57, and are soon joined by the aggregate of the roots of 

 the ( spinal accessory : ' these, commencing at about the fifth cer- 

 vical, advance, between the dorsal roots of the cervical nerves 

 and the ligamentum denticulatum, gathering successive slender 

 accessions, all of which, originating as above defined, emerge at 

 the dorsal border of the restiform tract. 



The glosso-pharyngeal is relatively smaller in Mammals than 



