721A 
continue their hold for as long as a quarter of 
an hour or twenty minutes after the removal of 
the head and the posterior segment of the 
body. But let the portion of the cord which 
is connected with the anterior extremities be 
destroyed, and all such power of movement 
becomes completely annihilated. 
In birds and mammalia phenomena of this 
kind are less conspicuous than in the cold- 
blooded animals, because in them the nervous 
power becomes extinct so speedily after any 
mutilation of the body. The power itself is 
no doubt more energetic, as the muscular 
power is, but it is less lasting. 
In the articulate classes movements of pre- 
cisely the same nature may be observed. The 
common earthworm may be divided into seve- 
ral pieces, and each piece will continue to 
writhe so long as the irritation produced by the 
subdivision remains, and after that has ceased, 
movements may be excited in any seginent by 
stimulating its surface: the same phenomena 
are observable in leeches and various insects. 
These actions are exactly analogous to those in 
the segments of the divided body of a verte- 
brate animal. 
creature has in its proper ganglion the analogue 
of the piece of the spinal cord remaining with 
the segment of the vertebrate animal. These 
phenomena of function, conjoined with certain 
anatomical resemblances, make it quite certain 
that the abdominal ganglionic chain of the 
articulata is analogous, not, as formerly sup- 
posed, to the sympathetic system, but to the 
cerebro-spinal centres of Vertebrata. In both 
the Vertebrata and the Invertebrate Articulata 
each segment of the body is provided with its 
proper ganglionic centre, which is to a certain 
extent independent of the rest. In the latter, 
the centres of the segments remain distinct, 
although connected by fibres which pass from 
one to the other; but in the former they are as 
it were fused together at their extremities, and 
from that fusion results the single cylindrical 
nervous centre which we call the spinal cord. 
An experiment, to which attention has been 
directed by Flourens, illustrates very well the 
difference in the character of the actions of two 
ortions of the spinal cord, according as the 
rain is connected with or dissociated from it. 
The spinal cord of an animal is divided about 
its middle; when the anterior segment (that 
which still retains its connection with the brain) 
is irritated, not only are movements of the 
anterior extremities produced, but the animal 
evinces unequivocal signs of pain; when, how- 
ever, the posterior extremity is irritated, the 
animal seems not only insensibie to pain, but 
unconscious even of the movements that have 
been excited in the posterior extremities. If a 
frog be divided in the back into two segments, 
the anterior portion crawls about, exhibiting all 
the indications of sensation and volition; the 
png segment remains quite motionless un- 
ess some stimulus be applied to it, when 
movements more or less active may be ex- 
cited. 
Nothing can be more conclusive than such 
an experiment, in illustration of the fact that 
connection with the encephalon is necessary to 
Each portion of the articulate. 
.Jiritation of its substance, or by the infly 
PHYSIOLOGY OF THE NERVOUS SYSTEM. 
sensation; and that movements, not only with- 
out volition, but also without consciousness, 
may be excited by stimulating the segments 
separated from it. But there is nothing in this 
experiment to justify the conclusion that during 
the entire and unmutilated state of the cerebro- 
spinal axis the mind has no connection with 
the spinal cord. The experiment only shows 
that when a portion of that great centre has 
been removed, the mind retains its connection 
with the higher or encephalic portion, deserting 
that which is merely spinal. . 
Direct irritation of the spinal cord is capable 
of exciting these movements as much as when 
the stimulus is applied to the skin. 
All these motions cease when the spinal cord 
is removed ; no movement of any kind, volun- 
tary or involuntary, can then be excited, except 
by directly stimulating the muscles, or the nerves 
which supply them, and such movements want 
the combined and harmonious character which 
belongs to those which are excited through the 
nervous centre. 
Division of all the roots of the nerves at 
their emergence from the spinal cord annihi- 
lates these movements as completely as the 
removal of the cord itself. Under sueh circum- 
stances no motion can be excited by stimulation 
of the surface of the body, nor by irritating the 
cord itself; and this fact may be regarded as 
an unequivocal proof that the nerves, in ordi- 
nary actions, are propagators of the change 
produced by impressions to or from the centres ; 
and that in the physical nervous actions -the 
stimulus acts, not from one nerve to another 
directly, but through the afferent nerve upon — 
the centre, which in its turn excites the motor — 
nerve. i 
All these facts in the physiological history 
of the spinal cord lead unequivocally to the 
following conclusions respecting its office :— 
1. that the spinal cord (that term being used in 
its simple anatomical sense, the intra-spinal 
mass ) in union with the brain is the instrument 
of sensation and voluntary motion to the trunk 
and extremities; 2. that the spinal cord may — 
be the medium for the excitation of moveme 
independently of volition or sensation in p 
supplied by spinal nerves, either by di 
1 
™ 
of a stimulus conveyed to it from some surface 
of the trunk or extremities by its nerves distri- 
buted upon that surface. 
Of the physical nervous actions of the co 
We must pause here to make a more exte 
reference to those actions of the De gp 
which are capable of being excited by per 
pheral stimulation, and which are independent 
of mental change. There is no point in the 
physiology of the nervous system of more inte 
rest or importance than this, inasmuch as these 
actions are not limited to the cord, but 
place in other portions of the cerebra 
centre, in which nerves are implanted, and 
even in ganglions from which nerves take their 
rise. 
The existence of a class of actions like th 
has long been known to physicians and physio- 
logists. By the name of sympathetic act 
they excited great interest as to the mode 
