CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
745 
8/x to 6/x broad. Towards the internal surface the smaller cells again predominate in 
number and are more scattered than in the central portions occupied by the larger 
cells, but are yet closer together and more numerous than in the outer layer. The 
ventricles are lined by an endothelium composed of a single layer of columnar cells. 
In Scyllium the cells of the inner layer appear to average a smaller size than in 
Rhina, being from 5 /x to 6 /a in diameter, the smaller ones predominating ; they are 
arranged in elongated groups parallel to the endothelium. The fibrillse have princi¬ 
pally a tendency to a radial direction. On the outer side the cells are more sparingly 
distributed, they are generally about 7/x in diameter; in the central - portions some 
cells have a tendency to be arranged in groups of four or five ; sometimes so many as 
nine occur in one cluster, but these are of the smaller kind. Large cells resembling 
those of Rhina are not so numerous as in that species ; they usually occur singly, and 
measure about LO/x or 11/x; some of them are elongated or pear-shaped. The neuroglia 
is of the same nature as in Rhina, 
In Acanthias the parenchyma of the cerebrum resembles that of Rhina. On the 
outer side there is the usual space of granular neuroglia and network of fibrillse, in 
which very small cells are scattered. Then comes the layer of cells of greater size, 
which are generally more elongated than in Rhina ; them dimensions vary from 14/x 
to 7/x in length and from 10/x to 6/x in width. Towards the inner surface smaller cells 
predominate ; as a rule the cells occur singly, but small groups, consisting of two or 
three cells, are sometimes to be found. This brain, therefore, forms a transition 
between Scyllium and Rhina. 
At the base of the cerebrum (fig. 19), even with or slightly above the floor of the 
ventricles, there are four elongated groups of the larger cells agglomerated together 
with little if any neuroglia between ; they are placed one in each outer wall and one 
in each inner wall, all running longitudinally ; they are situated at one-third of the 
thickness from the outer and inner wall respectively. The remainder of the section is 
composed of neuroglia, with cells scattered through it resembling the arrangement at 
the dorsal part of the brain. In the space between the outer margin and the groups 
above described the neuroglia is more compact and presents a smaller number of cells, 
which, however, are more abundant on the side nearer the ventricles, where the 
neuroglia is also of a looser texture. The anterior commissure is principally derived 
from the outer edge of the group situated.in the external wall, but some few fibres 
are derived from the inner edge of the same ; they cross over to the opposite side of 
the brain and bear the same relation to the group on the other side. 
This commissure, which I think must homologize with the anterior commissure of 
mammalia, appears to correspond to what Bellonci* terms the olfactory commissure 
in Anguilla vulgaris; but in these fishes there are no visible bundles connecting it 
with the olfactory lobe. 
* “Intorno alia struttura e alle connessioni dei Lobi Olfattorii, &c. ‘Roma, R. Accad. Lincei Mem.,’ 
vol. 13, 1882. 
MDCCCLX XXVI. 5 0 
