CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
765 
the floor of the widest part of the fourth ventricle. The cells forming this ganglion 
are of large size, like those found in the cord. This ganglion bears about the same 
relation to the floor of the anterior part of the fourth ventricle that the larger cells of 
the ganglion of origin of the vagus do to the floor of the posterior part of the same 
ventricle, being, if anything, a little nearer the floor and farther from the mid-line. The 
fine fibres which form the ventral side of this root are derived from a bundle, which 
comes forward from the external part of the lateral columns of the spinal cord, and 
crosses the posterior root of the vagus close to its emergence from the medulla 
oblongata ; it passes ventrad of the anterior root of the latter, then between the origin 
of the acusticus and the posterior part of the root of this nerve, and then joins the 
ventral part of the root under discussion. This bundle is very distinct and well- 
formed between the vagus and the point where it emerges from the medulla, but 
behind the former it gradually disappears, and is probably derived from the ventral 
horn of grey matter in the cord. In passing over the vagus it presents a delusive 
appearance of joining that nerve, all the fibres turning towards the external edge at 
that point ; but a careful inspection of a series of horizontal sections proves that such 
is not the case, but that the fibres pass on backwards as described. The posterior root 
of the trifacial arises as a bundle of fibres, which come forward from the central portion 
of the vagal tuberosity, and are collected into a cord which passes forward along the 
side of the narrow part of the fourth ventricle, until it arrives at a point close behind 
the expanded portion of the same, and opposite the decussation of the fibres of 
Mauthner, where it turns outward and slightly downward to form part of the 
posterior roots of the trifacial. 
The abducens (figs. 8 and 9, n. G) arises by two small roots from two little ganglia 
placed in the ventral grey matter of the medulla oblongata, beneath the narrow part 
of the fourth ventricle. A small nervous cord emerges from each of these ganglia, 
and the two unite together to form the trunk of the nerve at its exit from the ventral 
side of the medulla. 
The acusticus (fig. 9, n. 8) is derived from the lateral part of the medulla oblongata 
in the region of the anterior end of the ganglion from which the posterior root of the 
vagus arises. Fibres from this source pass upwards and inwards, and forming a distinct 
bundle become applied to the upper and outer side of the central longitudinal column 
in the medulla oblongata. This bundle follows that column, keeping entirely distinct 
therefrom, as far forward as the posterior end of the anterior part of the fourth 
ventricle ; here it turns outwards and downwards with a curve, and emerges from the 
medulla as a nerve trunk placed at a lower level than the last-described root of the 
trifacial. 
The vagus (figs. 2, 9, 10, and 11,??. 10) arises by two roots. The anterior is derived 
from two distinct sources — viz., from the cerebellum, and from the grey matter covering 
the narrow part of the fourth ventricle. The former appears as two bundles of nerve- 
fibres which pass back from that part of the cerebellum immediately covering the 
