Brain and Cranial Nerves of Acanthias vulgaris. 187 
when viewed from the side, like a truncated wedge, the truncated apex being 
ventral. This ventral portion is very thick and directly continuous, without 
any line of demarcation, with that of the Rhombencephalon, but the dorsal 
wall is much thinner and expanded into a pair of lobes, the Optic Lobes, 
which are larger than in Scylliwm in correlation with the relatively larger eye, 
and so project more in front of the Cerebellum. 
Two nerves leave the Mesencephalon. The Patheticus, IV, which 
emerges behind the Optic Lobes from under the Cerebellum, is, owing to 
the more forward position of the Obliquus Superior muscle, rather longer 
than in Scylliwm, but of just about the same diameter: it runs forward in 
contact with the Optic Lobe, and then, after passing through the cartilage 
of the cranium runs for a short distance with the Superficial Ophthalmic 
nerve before turning and entering the muscle. The Oculo-motor, III, arises 
from the ventral surface of the Mesencephalon near its anterior end, and 
enters the orbit in front of the anterior trunk of the VIIth and Vth complex. 
There it divides into three rami, two serving the Recti Superior and 
Internus muscles, and a third, running across the floor of the orbit and 
innervating the Rectus and Obliquus inferiores muscles (Fig. 1). 
The Fore-brain. The posterior portion of the Fore-brain, the Dien- 
cephalon or.Thalamencephalon, is, like the Mesencephalon, shaped like a 
truncated wedge, but in the reverse position. Its dorsal wall is quite thin, 
relatively very short, and at its posterior end gives rise to the Pineal Organ 
(Fig. 2). The stalk of this is a fine tube of considerable length : it is attached 
to the wall as far as the anterior end of the Diencephalon whence it projects 
freely forwards and ends in the roof of the cranium on a level with the anterior 
end of the Hemisphere region. 
The thin dorsal wall of the Fore-brain has been removed from the brains 
which were drawn, except the half-brain used for Fig. 5, where the well- 
developed Velum Transversum can be seen hanging down from it into the 
cavity of the IIIrd ventricle. The Infundibulum is well developed too and 
its parts very well marked (Figs. 3, 4, and 5)—the paired Inferior Lobes in 
front, the Saccus Vasculosus behind, and the paired Corpora Mammillaria 
above. These latter rather obscure the roots of the II[Ird nerves. Attached 
to the ventral surface is the Pituitary Body: it is almost the same length 
as the Infundibulum, but much narrower, particularly in front where it lies 
between the Inferior Lobes. It has a median longitudinal groove, and 
decreases in thickness from before backwards. 
The Optic Tracts, II, join the rest of the brain just in front of the 
Infundibulum, but neither the Chiagma nor the Thalami are at all 
conspicuous. 
