PHYSIOLOGY OF THE NERVOUS SYSTEM. 
nerves, of which one has been satisfactorily 
proved to be sentient, the other motor, the 
former being generally the larger, and having 
the peculiar feature of a ganglion being formed 
upon it. 
There is scarcely a nerve in the body, which, 
in strictness, ought not to be regarded as a com- 
pound one; the physiological character of each 
nerve must depend on the endowment of the ma- 
jority of its fibres, and the nerve will be called 
sensitive or motor, according to the predomi- 
nance of motor or sensitive fibres in it. For 
example, the facial nerve, or portio dura of the 
seventh pair, is called motor, because it is almost 
wholly composed of motor fibres; but it 
contains, besides, in very much _ smaller 
number, some sensitive filaments which it 
derives from anastomoses with neighbouring 
nerves. The third, fourth, and sixth nerves 
are of similar constitution to the facial. In the 
ramifications of the fifth nerve, on the other 
hand, the filaments of sensation are predomi- 
nant; those of motion being much fewer, and 
confined to the ramifications of its inferior 
maxillary division. 
There is no difference between a motor, and 
a sensitive nerve as regardsstructure. Ehrenberg, 
indeed, endeavoured to establish that the vari- 
cose character of the fibre belonged to nerves 
of special sense; but subsequent observation 
showed this to be incorrect. We can attribute 
the difference of endowment of the fibres to 
no other cause, but to the nature of their peri- 
pheral and central connections. The same 
nervous force is propagated by the fibres of 
each kind, but whether that force is to excite 
motion or sensation must depend on the connec- 
tion of the fibres with muscles in the one case, 
and with the centre of sensation in the other. 
The terms afferent and efferent have been 
used in expressing the function of different 
fibres, and they are convenient terms to a cer- 
tain extent. But the use of them tends to con- 
yey erroneous ideas respecting the change which 
takes place in a nerve when stimulated, as if 
that change took place only in one direction. 
It is true that, in a motor nerve, the stimulus 
ordinarily acts from the centre, and the nervous 
force is propagated peripherad; and on the 
other hand, in the sentient nerve, the stimulus 
is usually applied at the periphery, and the 
nervous force proceeds centrad. It is the place 
at which the stimulus is applied which usually 
determines the direction in which the nervous 
force travels. But there are no good grounds 
for supposing that the molecular change con- 
sequent upon the stimulation of a nerve is 
limited to that part of the nerve-fibre which 
is included between the point stimulated, and 
the centre or the muscles, where the effect 
of the stimulation appears; on the contrary, 
t is not improbable that, at whatever point 
the stimulus be applied, the whole length 
of the nerve-fibre participates in the change. 
his is not unlikely in the case of motor 
nerves. For a continued or violent irritation of 
motor nerve, in some part of its course, caus- 
ng spasm or convulsive movement of the 
nuscles it supplies, may be propagated along 
VOL. III. 
7201 
its whole length to the centre, and may there 
give rise to irritation of neighbouring fibres, 
whether motor or sensitive, exciting more con- 
vulsion and pain. The phenomena of many 
cases of epilepsy, in which the fit begins with 
irritation of a few muscles, may be referred to 
in illustration of this position.* And it is also 
very probable as regards sensitive nerves. If 
the ulnar nerve be irritated when it passes be- 
hind the internal condyle, a sensation of tingling 
is excited, which is referred to the sentient 
surface of the ring and little fingers; and if 
the irritation is kept up, the skin of those fingers 
becomes tender to the touch, its sensibility 
being very much exalted. This fact cannot be 
explained unless upon the supposition that 
the molecular change in the nerve-fibres, pro- 
duced by the irritation, extended to the peri- 
phery as well as to the centre, exalting the 
excitability of their distal extremities. 
It is a highly interesting physiological fact, 
which has an important practical bearing, that 
at whatever part of their course sentient nerve- 
fibres be irritated, the same sensation will he 
produced, whether the seat of the irritation be 
the centre, the periphery, or the middle of their 
course, provided only the same fibres are irri- 
tated in the same degree. Thus it frequently 
happens that sensations are referred to the ex- 
tremities of a nerve when the existing irritation 
is situated at its point of implantation in the 
centre. The sensation of tingling or formica- 
tion, in the hand or foot, arm or leg, is fre- 
quently an indication of cerebral or spinal 
disease ; but the practitioner should not forget 
that precisely the same sensation may be caused 
by an irritation taking place in the course of the 
nerve. I have frequent occasion to estimate 
the importance of this fact in the treatment of 
cases of Sciatica. This disease generally con- 
sists in an irritated state of the nerve in some 
part of its course by a gouty matter, and it may 
be treated with the best effects by blisters ap- 
plied over the nerve. As, however, the morbid 
impregnation may have taken place at any part 
of the course of the nerve, it is a very useful 
practice, when a single application fails, to apply 
the blisters over different parts in succession, 
instead of confining the vesication to one region. 
This law of action of sensitive nerves gives 
the clue to the explanation of the extraordinary 
but well-attested fact, that persons who have 
suffered amputation will continue to feel a con- 
sciousness of the presence of the amputated 
limb, immediately after, and often for a long 
time, or even always, after its removal. I have 
met with two cases, in one of which the arm, 
in the other the leg, was amputated so long 
before as forty years; yet each person declared 
that he-had the sensation of his fingers or toes 
as distinctly as before the operation. And not 
only does the consciousness above referred to 
exist, but likewise, when the principal nerve 
of the limb is irritated, the patient complains of 
pains or tingling, which he refers to the fingers 
* Tam aware that these phenomena admit of 
another explanation, but there is no reason why 
they might not likewise originate, in many cases, in 
irritation of a few motor fibres. 
Q2z *** 
