PHYSIOLOGY OF THE NERVOUS SYSTEM. 
all continued up into the brain, and the (so- 
called) excito-motor fibres are implanted in the 
cord, that segment of the cord should be the 
largest in which the greatest number of these 
fibres is to be found. Now the great extent of 
excitor surface in the lower extremities, the 
magnitude of their muscles, the importance of 
their movements, and, at the same time, the 
great developement of reflex actions in them, 
would lead most reasonably to the expectation 
that the lumbar segment of the cord to which 
these nerves belong should exceed considerably 
in size the cervical segment which gives nerves 
to the upper extremities, where the excitor sur- 
face is of less extent, where the muscles are less 
powerful,and the reflex actions considerably less 
conspicuous. Moreover,the lumbar region of the 
cord would be, if Dr. Hall’s views were correct, 
the centre of those excito-motor acts connected 
with defzcation, micturition, parturition, &c., of 
which he speaks so much, and on this account 
might fairly claim a greater amount of substance. 
But the fact is, that the lumbar swelling of the 
cord is smaller than the cervical ; and that while 
it contains, and owes its bulk mainly to, a large 
quantity of vesicular matter, but a small pro- 
portion of fibrous matter is found in it. More- 
over, it is impossible to understand the great 
superiority of size of the lumbar portion over 
the dorsal segment of the cord, if we are to 
-admit that this latter segment contains in addi- 
tion to its own fibres (sensori-volitional and 
excito-motory) the sensori-volitional fibres of 
the lumbar, swelling also, which ought to be 
very numerous. 
It is very generally admitted that the only 
channel by which the will can influence the 
spinal cord is through the fibres of the anterior 
pyramids of the medulla oblongata, the greater 
number of which decussate each other along 
the median line. But it is in the highest de- 
gtee improbable that these fibres, occupying so 
small a space as they do, should form the ag- 
gregate of the volitional fibres (still less of the 
sensori-volitional fibres) of the trunk and extre- 
mities. The whole of these fibres (of both 
sides) collected together would scarcely equal 
in bulk the anterior portion of one of the antero- 
lateral columns of the spinal cord. 
It has been affirmed that much support is 
given to the excito-motory hypothesis by Dr. 
Carpenter’s and Mr. Newport’s supposed de- 
monstration of the two sets of fibres in the Arti- 
culata. But these observations are far from 
deserving the name of demonstration. The 
inferences from them are derived from the 
apparent direction of certain fibres, and not 
from any actual tracing of them by dissection 
or by microscopic inspection. The observa- 
tions, too, have been made with low powers, 
which are very insufficient for determining the 
precise disposition of the fibres and their rela- 
tion to the vesicular matter of the ganglia. 
These writers affirm that the longitudinal 
fibres of the ganglionic chain of Articulata 
pass up to the brain and constitute the sensori- 
volitional fibres, whilst other fibres pass in a 
transverse direction and are implanted in the 
ganglia. Were this the case, it might reason- 
722 
ably be expected that the brain would be the 
largest of the ganglia as containing the sum of 
the sensori-volitional fibres of the whole body. 
But let any one compare the size of the cere- 
bral ganglia of the scorpion (as figured by 
Mr. Newport*) with the size of the animal 
and that of its cord, and it will be evident to 
him how disproportionately small such a centre 
is to the number of sensori-volitional fibres 
which must be distributed over so large a sur- 
face and to so many muscles. Anatomy, how- 
ever, offers no objection to the hypothesis that 
the roots of the nerves are implanted in the 
ganglia, and that the longitudinal fibres act 
as commissures between different segments 
(adjacent and remote) of the cord. 
Neither do Mr. Newport's experiments on the 
myriapods and other Articulata throw any new 
light on the question of the existence of two 
orders of fibres; nor do they add anything to 
our knowledge beyond the important fact that 
actions take place in certain Invertebrata after 
decapitation, which are of the same nature with 
those which occur in Vertebrata after a similar 
mutilation. The mechanism of these actions 
has not been at all elucidated by these expe- 
riments. : 
The excito-motory hypothesis is sufficient for 
the explanation of the movements of decapi- 
tated animals, of parts in connection with small 
segments of the spinal cord, of limbs paralysed 
to sensation and voluntary motion from dis- 
eased brain or spinal cord. But there are two 
phenomena familiar to those who observe dis- 
ease with care, which cannot be explained by 
it; these are the movements which may be ex- 
cited by mental emotion in limbs paralysed to 
the influence of the will, and the total paralysis 
of the sphincter ani, which frequently accom- 
panies diseased brain, whilst at the same time 
the limbs are only affected to a partial extent or 
not at all. aes 
Cases occur sometimes in which hemiplegia. 
arises from an apoplectic clot, or other destruc- 
tive lesion in one hemisphere of the brain. The 
arm and leg, or either of them, are completely 
removed from the influence of the will; yet 
occasionally, as the effect of some sudden emo- 
tion, fear, joy, surprise, the paralysed limb is 
raised involuntarily. Even so slight a cause as 
yawning (an act of emotional kind) will excite 
the palsied limb. Every time the patient yawns 
the arm will be raised involuntarily. 
Such phenomena as these receive no ade- 
quate explanation from the excito-motory hypo- 
thesis. Mental emotions probably affect some 
part of the brain; if the only communica- 
tion between the brain and the limbs be by 
fibres of sensation and volition, it is impossible 
to understand how the emotional influence 
could be conveyed to them through a channel 
which has long been interrupted. If we are to 
adopt the excito-motory hypothesis, it will be 
necessary to suppose with Dr. Carpenter the 
existence of certain emotional fibres to explain 
the phenomena of this particular case. But it 
is difficult to admit the existence of three orders 
* Phil, Trans. 1844. 
