eS. —-~S 
2 
PHYSIOLOGY OF THE NERVOUS SYSTEM. 
portant to ascertain what is the mechanism by 
which these various actions take place. 
The most convenient way to discuss this 
point will be to examine into the value of cer- 
tain hypotheses which have been framed to 
explain it. We shall find it necessary in this 
discussion to keep before us two propositions 
in favour of which sufficient evidence has al- 
ready been adduced. These are, 1. That the 
brain or some part of it is essential to the 
production of mental nervous actions ; in other 
words, that acts of volition and sensation cannot 
take place without the brain: and, 2. That the 
vesicular is the truly dynamic nervous matter, 
that which is essential to and the source of the 
developement of all nervous power. 
The first hypothesis which we shall notice is 
one of so much ingenuity that one is tempted 
thereby to adopt it, and would gladly do so if 
it were found sufficient to explain the pheno- 
mena, and if it were consistent with that sim- 
plicity which characterises the mechanism of 
the body. It originated with Dr. Marshall 
Hall, and has been advocated by him with great 
zeal and ability; it may be distinguished as 
the hypothesis of an excito-motury system of 
nerves, and of a true spinal cord, the centre of 
all quvsical nervous actions. 
Is hypothesis may be stated as follows.* 
The various muscles and sentient surfaces of 
the body are connected with the brain by nerve 
fibres which pass from the one to the other. 
Those fibres destined for. or proceeding from 
the trunk to the brain pass along the spinal 
cord, so that that organ is in great part no 
more than a bundle of nerve fibres going to 
and from the brain.- These fibres are specially 
for sensation and volition—sensori-volitional. 
* I am very desirous that this hypothesis should 
be stated correctly, as I consider that both physio- 
logy and practical medicine are greatly indebted to 
Dr. Marshall Hall for the attention his labours 
have awakened to the inherent powers of the ner- 
vous system. Nevertheless I have shown in the 
text that great advances in our knowledge of these 
powers had been made by certain physiologists of 
the last century, whose views and researches had 
been completely or almost forgotten. 
I have collected the statement of Dr. Hall’s 
hypothesis in the text chiefly from his later wri- 
timgs. The history of Dr. Hall’s labours on this 
part of physiology, as I gather it from his writings, 
ae eT to be as follows : — 
n 1832 a paper was presented by him to the 
Zoological Society, of which, so far as I can ascer- 
tain, no other record has been kept than that which 
is to be found in the printed summary of the Proceed- 
ings of that Society. 1 have not had any opportunity 
of consulting these proceedings, but find an extract 
from them printed in Dr. Hall’s work entitled Me- 
moirs of the Nervous System, published in 1837, 
This paper was entitled, “<A brief Account of a par- 
ticular Function of the Nervous System,” and its 
object was to point out the existence of a source of 
muscular action distinct from all those hitherto no- 
ticed by physiologists. The peculiarity of this mo- 
hon is stated to consist in its being excited by irrita- 
tion of the extreme portion of the sentient nerves, 
whence the impression is conveyed through the 
corresponding portion of the brain and spinal marrow 
as a centre, to the extremities of the motor nerves. 
Experiments upon salamanders, frogs, and turtles 
were detailed, from which Dr. Hall drew the follow- 
721u 
But, in addition to these, there is, according 
to Dr. Hall, another class of fibres proper to 
ing conclusions: 1. that the nerves of sensibility 
are impressible in portions of an animal separated 
from the rest; in the head, in the upper part of 
the trunk, in the lower part of the trunk; 2. that 
motions similar to voluntary motions follow these 
impressions made upon the sentient nerves; and, 
3. that the presence of the spinal marrow is essen- 
tial as the central and cementing link between 
the sentient and motor nerves. 
Other experiments were detailed in this paper 
upon frogs rendered tetanic by a solution of opium ; 
these showed that in this state the cutaneous nerves 
became ‘‘ extremely susceptive, and the motor 
nerves extremely excitative.” Decapitation of a 
tetanized frog did not destroy the tetanic condition 
of the trunk and extremities. ‘‘'The exalted con- 
dition of the function of the sentient and motor 
nerves continued in each part.” ‘“ All was changed 
in removing the brain and the respective portions 
of the spinal marrow.” 
«« These experiments,” Dr. Hall continued, ‘‘ ap- 
pear to me to establish a property or function of 
the nervous system, of the sentient and motor 
nerves, distinct from sensation and voluntary or in- 
stinctive motion,’ 
Dr. Hall’s next publication appears to have 
been a paper read before the Royal Society on 
the 20th of June, 1833. This paper is entitled 
“© On the reflex function of the medulla oblongata 
and medulla spinalis.”” Having noticed the con- 
clusion arrived at by Le Gallois, and confirmed by 
the reporters of the Institute, that section of the 
spinal marrow in the neck arrests only the respira- 
tory movements, leaving sensation and voluntary 
motion to remain in the whole body, he points out 
that the causes of muscular motion may be centric 
or eccentric in the nervous system. When the 
cause is eccentric, that is, distant from the nervous 
centres, Dr. Hall states that the phenomena are 
due to a peculiar function, which he considers 
had not previously been understood. Its charac- 
teristic is that it is ‘* excited in its action and reflex 
in its course ; in every instance in which it is ex- 
cited, an impression made upon the extremities of 
certain nerves is conveyed to the medulla oblon- 
gata or medulla spinalis, and is reflected along 
other nerves to parts adjacent to, or remote from, 
that which has received the impression.” 
“« It is by this reflex character,” he adds, ‘ that 
the function, to which I have alluded, is to be 
distinguished from every other.” Yet, curious to 
say, he assigns to it powers which certainly cannot 
be excited in the reflex manner. He says, ‘ the 
reflex function exists as a continuous muscular 
action, as a power presiding over organs not ac- 
tually in a state of motion, preserving in some, as 
the glottis, an open, in others, as the sphincters, a 
closed form, and in the limbs a due degree of equi- 
librium or balanced muscular action, a function 
not, I think, recognised by physiologists.” 
Dr. Hall points out the distinctness of this func- 
tion from sensation and voluntary motion, and re- 
lates experiments on decapitated animals, (snakes, 
turtles, vipers, toads, frogs, and efts,) to show 
that the motions which occur in them are not spon- 
taneous but only excited, and that these ‘* excited 
motions in decapitated animals are dependent upon 
a principle different from sensation and volition.” 
He then shows the difference between the reflex 
movements and those arising from irritability, by 
comparing the motion of the heart, when touched, 
with that of the glottis of an animal when similarly 
stimulated. Both movements take place equally 
after the removal of the brain; but if the medulla 
oblongata be removed, the contractions of the 
larynx cease, while those of the heart continue. 
«« The difference consists, then, in the presence of 
the medulla oblongata, which is essential to the 
Q 7 Qeese* 
