646 
Fig. 370. 
pp 
i 
bat 
Dura mater of part of the spinal cord laid to 
git as teosasstoas delice. iii 
dddd, dentated processes. On the right the 
roots of the nerves and the ganglia of the pusterior 
roots are retained, 
all the characters of white fibrous tissue, of 
which it is chiefly composed. In its dentated 
processes, however, a considerable quantity of 
yellow fibrous tissue may be found. The simi- 
larity of its constitution with that of the pia 
mater evidently justifies its being regarded as a 
NERVOUS SYSTEM. (Nervous Centres. Grey Nervous Marrer.) 
, fibres of the nerves, and serve to connt 
process of that membrane, and not, as some 
anatomists thought, of the dura mater, } 
which it has a much less intimate and extensive 
connexion. Its anterior and posterior surfaces 
are uncovered by any membrane; are 
smooth, and have the glistening silvery appear- 
ance of white fibrous membrane. Itis evident 
that during life these surfaces must be bathed — 
by the subarachnoid fluid. wall 
The office of this peer 8 structure see 
evidently to be mechanical; to preserve t 
spinal sel in a state of equilibrium; and to 
prevent lateral movement of it, whilst at the 
same time it forms a partition between th 
roots of the nerves. a 
General remarks on the structure of the 
nervous centres.—It has already been shewn 
a former part of this article that the nerves 
properly so called are composed 
of one kind of nervous substance,—namely 
the fibrous nervous matter, which is disposed in 
bundles of peculiar fibres. It is only in the 
nervous centres or in continuations of t 
that we find an union of the white and the grey 
nervous matter; and, indeed, it may be ( 
in general, that the peculiar and distinctiv 
anatomical character of a nervous centre con 
sists in this combination of the two kinds « 
nervous matter. 
In the nervous centres the white matter 
bits, for the most part, the same essential ch 
racters of structure as in the nerves; that is 
say, it is disposed in tubes containing a cer 
pulpy matterin them, Ithas been found, 
ever, that these tubes are much more pron 
become varicose under the influence of press 
or of any other disturbing cause. They : 
not, as in the nerves, bound together by areo 
tissue, but are disposed in bundles and 
different planes, with their nutrient bloodvess 
ramifying among them, and in some situat 
the elements of the grey matter are interpos 
between them. Certain parts of the ner 
centres are composed exclusively of ¥ 
matter, as a portion of the hemis ol 
brain, and of the cerebellum, and upel 
parts of the spinal cord. r. 
The white fibres which are found in the 
vous centres may be distinguished acco 
to their physiological office into four di 
kinds. Two of these are continuations ¢ 
re 
aa 
PXCIU 
To 
a. 
nervous centres with other organs or te} 
either by conveying the influence of the: 
to them, or by propagating impression 
them to the centres. The er are 
efferent, the latter afferent fibres. In 
to these, we find a third and large series 0 
which serve to establish a connection b 
different centres, or between different port 
the same centre. These are called comm 
fibres ; they form a large portion of the n 
the brain and spinal cord. And Henle sug) 
that the brain contains a fourth series of 
associated with the operations of though 
We remark in the nervous centres, €s 
in the brain and spinal cord, a greater di 
as regards size between the different nerve 
than may be observed elsewhere, and it: 
to be a constant character that they dit 
