CENTRAL NERVOUS SYSTEM IN VERTEBRATE ANIMALS. 
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to the internal surface of the tectum. Between them, and beneath the more superficial 
bundles, are scattered cells which resemble the larger cells in the cerebrum in size and 
shape. 
In the Basse ( Lcibrax Lupus) the smaller cells on the surface are not placed in a row,, 
but in groups, leaving spaces between them bare of cells, or in which only a few 
isolated ones occur; they are also found in groups in the interior, but not in connected 
rows. As if to make up for this deficiency a few large cells are found in the interior, 
which slightly exceed in size the larger ones in the cerebrum ; they are pyriform in 
shape, and have a nucleus and a distinct nucleolus, resembling, on a smaller scale, the 
cells in the spinal cord. In a section taken through the whole length of the torus 
about six of these cells were found in one plane. The parenchyma of the tori semi- 
circulares is composed of coarse granular neuroglia, in which fibres, mostly longitudinal, 
are indistinctly visible, chiefly towards the more superficial part. As in Mugil, the 
deeper part of the torus is occupied by the bundles of fibres of the crura lobi optici. 
Beneath the more superficial bundles of these, there occurs a thin layer of grey sub- 
stance in which other cells are found, which are slightly larger than those just men- 
tioned; they have a nucleus, nucleolus, and two or three processes; each cell occupies a 
space in the neuroglia ; they are much less numerous than in the Mugil, and occur in 
groups of two or three. Two groups and a single cell occur in a longitudinal section 
through the whole torus in one plane. 
The Hypoaria. 
The parenchyma of the hypoaria is composed of finely granular neuroglia, in which 
the ramifications of extremely small fibres form a network of inextricable tenuity. In 
this neuroglia bundles of nerve-fibres radiate from the posterior and upper side in 
distinct and well-formed cords. The nerve cells (fig. 21, a.) occur throughout the 
neuroglia scattered singly, but increasing in number towards the free margin of the 
hypoarium. These cells are pyriform in shape ; the thicker end is occupied by a nucleus, 
in the centre of which, in most cases, a spot-like nucleolus is seen ; occasionally, how- 
ever, this is represented by a group of granules. Each cell partly occupies a distinct 
chamber in the neuroglia, into which only the nucleus projects, the pointed end being 
closely surrounded by neuroglia ; occasionally a process, given off from the thicker 
end, converts them into bipolar cells. Their average length is 0'008 millim., and their 
width O'OOo millim. ; the nucleus usually has a diameter of 0'004 millim. 
The external surface of the hypoaria is not covered by an epithelial layer. Each 
lobe presents a ventricle which commences close to the inferior surface, where it 
extends from near the anterior to near the posterior end ; behind, this ventricle is 
narrow and slightly curved inwards ; anteriorly, it expands and becomes broader ; each 
ventricle gives off a passage from its anterior end, which passes with a curve upwards 
and inwards until the passage of one side meets that of the other in the infundibulum. 
These ventricles are lined by a distinct layer of epithelium, which is a continuation 
