NERVOUS SYSTEM. (Comparative Anatomy.) 
Jon when they are observed lying out- 
sori between ie deposited substance and 
the cell-membrane, as in the muscles (c). The 
remaining cavity appears to be filled by a pretty 
consistent substance, the band of Remak, and 
discovered by him. In the adult a nerve, 
consequently, consists, 1st, of an outer pale 
thin cell-membrane—the membrane of the ori- 
ginal constituent cells, which becomes visible, 
when the white substance is destroyed by de- 
grees (d); 2d, of a white fatty substance de- 
ited on the inner aspect of the cell-mem- 
e, and of greater or less thickness; 3d, of 
a substance, which is frequently firm or con- 
sistent, included within the cells, the bund of 
Remak.” re , 
From the ‘description given in the foregoing 
we have seen that the prevailing anato- 
mical element of nerves is a tube composed of 
homogeneous membrane containing a soft pulpy 
matter, the true nervous substance, divisible 
jnto the white substance of Schwann and the 
‘band of Remak, and that through the medium 
of these nerve-tubes or fibres the grey matter 
‘of the central masses is brought into connection 
with the peripheral textures and organs. 
Whether he fibres which Henle has desig- 
nated gelatinous fibres, which resemble very 
‘much the central band of the nerve-tubes de- 
prived of tubular membrane and white sub- 
i , perform a sithilar office, or whether they 
" serve to establish a connection between the grey 
matter of the several nervous centres, are ques- 
tions which we must leave for future considera- 
_ The sagacity of Galen long ago pointed out 
that every part, which is capable of motion, 
and at the same time possesses sensibility, 
‘must receive two classes of nerves, motor and 
; sensitive. And it was reserved for the genius 
of Bell in our own times to demonstrate that 
__ the office of a nerve depends upon the powers 
or endowments of its component fibres or 
_ tubules, and that a nervous trunk may be 
_ made up of fibres of different endowments 
Tying in juxta-position with each other. 
" It is at the roots of the nerves that tubules 
of distinct endowments are isolated from each 
other. Thus Bell’s experiments, which have 
been confirmed by subsequent observations, 
shewed that the anterior roots of spinal nerves 
were motor, and the posterior sensitive; and 
the determination of this important fact is the 
foundation of all our knowledge of the phy- 
siology of nerves. 
The difference in the powers or endowments 
of the nerve-tubes does not appear to depend 
upon any variety in their structure, or other 
physical characters, (size perhaps excepted,) for 
repeated examination has failed to detect any 
such, but rather upon their peripheral and cen- 
tral connexions. A sensitive nerve, while it is 
organized at its periphery in such a manner as to 
adapt it to the reception of impressions, must 
be connected with that part of the brain whose 
office it is to perceive the changes which such 
impressions can produce. And a motor nerve 
must be on the one hand connected with mus- 
cular fibres, and on the other associated with 
. 
601 
such a part of the brain or other nervow centre 
as is capable of exciting in it that change which 
when communicated to a muscle will stimulate 
it to contract. 
The precise mechanism of those nervous acts, 
which I would distinguish as purely physical, 
by reason of their independence of the mind, 
is as yet unknown. It is still undetermined 
whether a distinct series of fibres (excitomotory) 
is necessary for them, or whether they may 
not be performed by the same fibres which are 
the channels of the mandates of the will, and 
of the impressions of those stimuli which are 
capable of producing sensation. 
(R. B. Todd.) 
NERVOUS SYSTEM, Comparative Ana- 
tomy of.*—In the following article it is in- 
tended to describe the anatomy of the nervous 
system in the different classes of animals as 
they rise upwards in the scale. 
Acrita.—tThe class acrita consists of ani- 
mals whose very characteristic is, that in them 
the nervous system is molecular, consisting 
of globules diffused through the cellular 
tissue of their bodies. Amongst them we 
distinguish, first, the Polygastrica, which are 
minute microscopic animals, furnished with 
numerous digestive cavities, in whom no ner- 
vous filaments have as yet been traced; still 
they, many of them, possess eye-specks, they 
show some indications of the sense of taste, 
and perform their various motions in the diffe- 
rent fluids as if under the well-directed gui- 
dance of nervous power. These animals appear 
as a punctiform homogeneous mass, in which 
a nervous system does not as yet exist in a 
distinct form: the nervous matter may be, 
perhaps, every where diffused through the cel- 
lular tissue of their body. These latter re- 
marks will equally apply to the next class— 
the Porifera; of which the spongia officinalis 
may be cited as an example. Its texture is 
soft and gelatinous, and is probably made up 
of nervous and muscular globules. 
Potypirera.—No nervous filaments have 
been discovered, or described, in any of the 
various forms and sizes of polypiferous ani- 
mals, excepting in the genus actinia, respecting 
which a doubt, almost amounting to a denial 
of the statement, exists. The actinia may be 
considered as an isolated polypus; it has no 
calcareous skeleton, and fixes itself to the rocks 
by its fleshy base. Spix, a German anatomist 
of high repute, gave plates of its nervous sys- 
tem thirty years ago, and described it as con- 
sisting of filaments with minute ganglia, sur- 
rounding the fleshy base just mentioned, from 
which were given off nerves to the different 
parts. [ Professor Rymer Jones believes he has 
detected a delicate nervous thread, passing 
round the roots of the tentacles, embedded in a 
strong circular band of muscle, which sur- 
rounds the orifice of the stomach.] Mr. Bell, 
in dissecting several of these actinia, has not 
been able to detect any nervous filaments ; 
* The Editor is responsible for the passages in- 
cluded between brackets. 
