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Proceedings of the Royal Society of Edinburgh. [Sess. 
behind, the free posterior horn. But it is evident that to describe in 
detail the position of the middle cells, some other plan must be adopted ; 
and the terms which I have used in the description are best explained by 
reference to the accompanying diagram (fig. 2). 
At most levels these different areas or regions can be recognised, but they 
are quite arbitrary, and only employed for topographical convenience, and 
it seems futile to attempt to define any exact boundaries for them. The 
middle cells are certainly not in the least regardful of boundary-line 
between these regions. In the cervical and lumbo-sacral enlargements 
there is, of course, no apical region ; Clarke’s column, though not always 
represented by cells of that column, still has its “ area ” there ; in addition, 
an external central area might perhaps be added. I shall now go on to 
the description of the middle cells as found in the various segments of the 
cord. 
C 1. — See fig. 3. — Vary considerably in number, on the whole fairly 
abundant, either scattered diffusely or in loose groups. In the base of the 
anterior horn always some scattered cells. The commonest group is a central 
one, tending to extend into the basal area, and especially into the para- 
central area ; may be 30 or more cells, forming a not very compact group. 
Frequently a small group in the position corresponding to that of the lateral 
horn. Merely a few scattered cells in the reticular formation, post-central 
area, and region of Clarke’s column. The cells vary in size in all areas ; 
those in anterior horn on the whole larger than the others, and sometimes 
with difficulty distinguished from motor cells, but they are never in such 
compact groups. They are all of polygonal or rounded polygonal outline, 
and their mode of occurrence is typical, i.e. irregular and casual — any group 
that may be found can never be traced through more than three or four 
sections at most; there is never any sign of regular segmentation such as 
is seen in the intermedio-lateral tract of the dorsal region. 
C 2. — Cells fewer in number than in C 1 . A central group of from 
12-15 cells is a very common feature of the segment; in fact, for long 
stretches it seems to be almost constant, particularly on the right side. 
Sometimes it is more para-central than central. A small group at or just 
in front of the apex — such as it is — of the lateral horn is also common. 
In the other areas their appearances are much as in C 1. 
C 3. — See fig. 11, Plate I. — Cells not very abundant, characteristically 
scattered, sometimes thickly, more often thinly. Small aggregations may 
be found in the area of Clarke’s column, the post-central region, and in the 
base of the anterior horn, either near the outer edge or nearer the centre, 
but none of them are very compact or very large. A central or central- 
