746 MR. A. SANDERS ON THE ANATOMY OF THE 
The crura cerebri are derived from both edges of the groups situated in the inner 
wall, the two origins being more equal than is the case with the anterior commissure. 
The nucleus in each half of the cerebrum gives out a separate column, which goes to 
form the crus of its own side, and has no communication with that of the opposite 
side. 
The anterior commissure is confined to the ventral region of the cerebrum, and is 
not found above the level of the nuclei above described. A third bundle of fibres is 
mentioned by Rohon * as descending from the dorsal and middle cerebral regions 
obliquely towards the crura cerebri in Raja ; these he considered to represent the 
“fasciculi longitudinales tegmenti posteriores.” These bundles were not found in 
any of my specimens. 
In the Rays there is no continuous belt of neuroglia existing round the circum¬ 
ference ; in some places, the cells extend close to the external surface, in others the 
neuroglia is deficient in cells. In the interior of the cerebrum the cells are arranged 
still more decidedly in groups than in Scyllium ; there are a larger number of cells 
collected together, generally from nine to twenty-one, but the individual cells are 
smaller, not exceeding 4 /x or 5 g., and most of them are spherical. These clusters 
are separated from each other by small masses of neuroglia, and in some parts they 
are in contact forming an irregular network. At the base of the cerebrum there is a 
continuous stratum of cells which follows the curve of the posterior margin. These 
correspond to the groups described in Acantluas, and, as in that species, the crura 
cerebri are in relation to them, and pass through the posterior end of the group. 
Hypoaria . 
In Rhina the parenchyma of the hypoaria consists of an irregular network of 
minute fibrillse, resembling that found in the corresponding parts in the Teleostei. In 
this parenchyma numerous cells are scattered irregularly throughout the mass ; they 
are of various shapes, spherical, pyriform, oval, or fusiform ; they give off one, or more, 
generally several processes, which join the above mentioned network ; they differ in 
size from 13p, by 7/x to 6/x in diameter. They are not so regularly pear-shaped as 
in the Teleostei, neither do they present a distinct chamber or space surrounding 
them ; they also give off a greater number of processes. 
The absence of a chamber surrounding the cells here, and also the presence of a 
greater number of processes, may possibly have been conditioned by a difference in 
the mode of treatment. Chromic acid was used in preparing the brains of the 
Teleostei, this probably caused a greater contraction of the parenchyma, and so may 
have broken off the processes, while at the same tune it formed the space round each 
cell, and as the parenchyma is softer in the hypoaria, so the effect would be greater. 
The process followed in preparing the brains of the Plagiostomata, which was the 
'* Op. cit., p. 71. 
