SENSE ORGANS OF AMIURUS. 355 
vagus lobes are never so large as the trigeminal ; the slit between 
them is always wider, and no encroachment beyond the middle line 
is observable. The slit becomes shallower posteriorly and does not 
in the posterior planes of the origin of tke second root of the vagus: 
extend down to the central canal of the cord. This region is that of 
the commisswra cerebri infima of Haller, where the posterior columns 
of the medulla are divaricated from each other so as to leave a wide 
V-shaped slit on section, which, however, does not extend to the 
central canal. The posterior boundary of this slit may be regarded 
as the point of passage of the medulla oblongata into the spinal cord, 
a point which is indicated by no marked constriction, for immediately 
behind the vagus lobes the brain tapers off quite gradually into the 
cord. 
From the ventral aspect various other parts of the brain may be 
seen. (Fig. 14.) The ventral surface of the cerebral hemispheres 
is marked by the formation of a lateral lobe which gives on trans- 
verse section tlie outline represented in Fig. 18, Pl. V. Immediately 
behind the cerebral hemispheres is the crossing of the optic nerves, 
which can be followed in the form of the optic tracts towards the 
optic lobes. Behind the optic chiasma is the commisswra transversa 
of Haller; the latter structure lies on the anterior part of the floor 
of the primary forebrain or thalamencephalon. We shall see after- 
wards that the roof of this part of the brain is extremely short from 
before backwards; its floor on the other hand is extraordinarily 
developed, for not only is there the large twher cinerewm with the 
hypophysis connected with it, but also the large obi inferiores (LI), 
and the saccus vasculosus enclosed between the posterior tips of 
these, all of which structures contain prolongations from the third 
ventricle. 
Owing to the small size of the optic lobes these are barely visible 
from the ventral aspect, and the floor of the midbrain being chiefly 
developed into the swellings, tori semicirculares, which nearly fill up 
the optic lobes, is practically excluded from the basal aspect of the 
brain. The ganglion interpedunculare (Fig. 7, Pl. V.) represents the 
boundary between the midbrain and oblongata. 
The points of origin of most of the cranial nerves can be studied 
from the ventral aspect. Those of the olfactory and optic tracts 
have already been referred to above. The third nerve (ocwlomotorius) 
Jeaves the base of the midbrain just in front of thé posterior tip of 
