350 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
by the commissura accessoria of Mauthner, and the extremely broad 
‘fibres of Mauthner’ are found in the upper compartments. At 
either side of the central canal is the nucleus of one of the motor 
roots of the second part of the vagus, and on either side of the ven- 
tral longitudinal fibres the nucleus of the first spinal nerve. 
Fig. 2 represents a section passing through the anterior part of the 
vagus lobes, and through the origin of the first part of the vagus. 
Those fibres which join the nerve from the tuberculum acusticum are 
cut transversely, and are seen above the eighth nerve in the next 
figure. The sensory vagus fibres arise chiefly from the periphery of 
the lobe, while fibres which originate near the wall of the fourth 
ventricle collect themselves into a strong bundle, reinforced by simi- 
larly originating fibres from the trigeminal lobe (Fig. 3) and are thence 
to be traced forwards into the cerebellum as the secondary vago- 
trigeminal tract of Mayser. (Sec. V. 7.) This strong fasciculus lies 
immediately below the ascending roots of the fifth nerve. The fourth 
ventricle is slit-like in section, except where it becomes somewhat 
wider above where its roof is formed only by ependyma and pia. 
The slit-like section is’retained except where encroached on by the 
trigeminal lobes, until it becomes opened out immediately in front of 
these (Fig. 4) to be closed again by the commissure of grey matter 
which joins the tubercula acustica (Fig. 5). These ganglia are 
further connected by fibres which decussate below the floor of the 
fourth ventricle. (Figs. 3 and 4). 
From the various parts of the tuberculum fibres converge to form 
the auditory nerve (WV. VI/7.), but it also receives a contingent from 
a nucleus lying below the secondary vago-trigeminal fasciculus. 
The whole of the trigeminal lobe serves to give origin to the sen- 
sory fibres of the fifth nerve which form the powerful ‘ dorsal genicu- 
lated root,’ trending outwards in Fig. 4. In the same plane the motor 
fibres of the facialis (V. VII.) escape, partly derived from a nucleus 
represented in the figure, but largely composed of a strand which 
stretches outwards, forwards and downwards from the floor of the 
fourth ventricle. It may be recognized in transverse section in 
Fig. 3, before it has begun to assume the cowrse above named. 
Fig. 5 illustrates a section passing through the trigeminal roots. 
The fibres derived from the tuberculum acusticum are most superficial, 
the ascending and transverse fibres most anterior and ventral ; the 
change of position which the latter undergo with regard to the dorsal 
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