CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 747 
same as that employed for the nervous system of Mormyridge, avoided this contraction 
of the tissues. 
The ventricle in Ehina is of great size, and occupies nearly the whole of the lobe, 
leaving only very narrow walls. This ventricle is lined by an endothelium which 
consists for the greater part of its extent of one layer of columnar cells, the internal 
end of which sends processes into the interior of tire parenchyma ; they are backed 
up by a layer one, two, or three deep of spherical cells of an endothelial nature, which 
sometimes project more or less into the substance of this lobe. Next to these cells is 
found a layer comparatively free from cells ; here the general direction of the fibrillse 
is longitudinal, but the direction of the processes from the endothelial cells is radial, 
crossing the former at right angles; the result being that this layer has a squared 
striation; the cells, which are occasionally present in this layer, are spherical in 
contour, resembling in this respect the inner cells of the endothelium. At the 
posterior portion, where the hypophysis cerebri is attached, the endothelium is com¬ 
posed of the rounded cells only, which are extended from four to seven deep ; they 
are continuous with the endothelial cells of the hypophysis cerebri. The processes 
from the endothelial cells resemble those from the “ Stiftzelle ” of the cerebellum ; they 
are present even when the endothelium consists only of the spherical cells, but they 
are not so well marked as when the columnar cells are found. In the layer of paren¬ 
chyma, which is next to the endothelial lining of the ventricles, there are more cells 
to be found in that part where the endothelium consists of spherical cells than where 
it is composed of columnar cells only; which seems to show some connexion between 
the two kinds of cells, that is to say, between the spherical cells which go to form the 
endothelium, and those which are dispersed through the ventricular part of the 
parenchyma, 
Scyllium and Acanthias resemble Ehina in the arrangement of the endothelium 
and parenchyma in this lobe. The cells in both these species are on an average 
smaller, and the smaller cells predominate. 
In the Eays (fig. 18) the parenchyma is more compact, the fibrillse form a closer 
network, and the cells are scattered with larger intervals between them. There are 
no ventricles in the hypoaria in the Eays, and consequently no endothelial layer; 
the shape of the cells is more generally fusiform than in the other species, and their 
average size is greater. 
In order to conclude the description of the region of the third ventricle (fig. 18), a 
tuberosity may be mentioned which exists in the dorsal part of the wall of this 
ventricle and in front of the anterior end of the optic lobe. In it are found numerous 
small cells embedded in a granular neuroglia, but no ganglion cells are to be seen. A 
well-defined bundle of fibres proceeds from it backwards and slightly downwards, but 
disappears at the ventral side of the longitudinal columns. This tuberosity corresponds 
to the tuberculum intermedium of Gottsche.'" AhleornI describes similar structures 
* Muller’s ‘ Arckiv,’ 1835. 
+ “ Enters, liber clas Geliirn d. Petromyzonten.” 1 Zeitschr. TVissensch, Zool.,’ vol. 39, 1883. 
5 C 2 
