610 Proceedings of the Koyal Society of Edinburgh. [Sess. 
well as its more usual limits. At the same time the cells become progressively 
fewer in number and also smaller in size. More scattered than forming de- 
finite groups ; may be in any part, but most abundant in the anterior horn, 
and here also, on the whole, larger than in central and post-central regions. 
In the reticular area are cells indistinguishable in every way from other 
middle cells, but mingled irregularly with others which are quite different, 
sometimes the one set, sometimes the other predominating. [Those cells, 
probably representing the intermedio-lateral tract, are much smaller, pale, 
and almost circular in outline. They seem to consist almost entirely of 
nucleus. They appear first in the extreme lower end of S 3.] In character 
and irregularity of occurrence the middle cells are as in other segments. 
S 5. — Cells gradually diminishing in number. At the lower end only 
occasional in their occurrence — one or two cells only. May be found in 
any part of the grey matter, but chiefly in two situations : firstly, the 
extreme anterior part of the anterior horn ; secondly, near the reticular 
angle, along with cells of the intermedio-lateral tract, as in S 4. In the 
first-named area they are usually larger in size. 
Coccygeal Segment . — At its upper end there are some straggling middle 
cells both anteriorly and near the reticular formation ; they gradually 
become rarer and rarer. Some of the intermedio-lateral tract cells seem 
also to be present. 
Summary. 
The middle cells are present throughout the whole length of the spinal 
cord. They are situated in the middle region of the grey matter, between 
the free anterior and free posterior cornua; but they sometimes also 
extend into the regions usually occupied by the anterior cornual cells, by 
the intermedio-lateral tract or by Clarke’s column. The small cells in the 
base of the anterior horn (scattered cells) cannot be sharply separated from 
the middle cells, nor can most of the small cells about the base of the 
free posterior horn. 
Although some of the middle cells may be found in all this wide area 
of distribution at practically any level of the cord, there are certain 
arrangements of cells which may be looked upon as typical of each segment, 
or at least of each region of the cord. In the upper cervical region they 
are not on the whole very abundant (this differs from Waldeyer’s account), 
but are best developed in the central and para-central fields. In the 
cervical enlargement they are much more numerous, particularly within a 
broad band extending from the formatio reticularis to about the anterior 
grey commissure. Throughout the dorsal region they are again com- 
