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 the second root of the vagus> 

 extend down to the central canal of the cord. This region is that o r " 

 the commissura cerebri infima of Haller, where the posterior columns 

 of the medulla are divaricated from each other so as to leave a wide 

 Y-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 the 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 commissura 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 tuber civereum with the 

 hypophysis connected with it, but also the large lobi inferior es (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, PI. 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 (oculomotorius) 

 leaves the base of the midbrain just in front of the posterior tip of 



