PHYSIOLOGY OF THE NERVOUS SYSTEM. 
which each part of this great system takes in 
the production of nervous phenomena. This 
inquiry naturally divides itself into two branches, 
namely, first, the functions of nerves ; secondly, 
those of nervous centres. 
Of the functions of nerves——Nerves are in- 
ternuncial; they possess in themselves (sepa- 
rate from the nervous centre) only a very limited 
power of developing the nervous force, and that 
only in response to a physical stimulus, for 
connection with a centre is necessary for the 
exercise of a mental stimulus. 
In inquiring into the function of any parti- 
cular nerve, the problem is to determine whether 
it propagates the nervous force centrad or periphe- 
rad, and whether it be connected with the centre 
of sensation or with the centre of volition; whe- 
ther, in short, it be sensitive or motor, It must 
be always borne in mind that most nerves con- 
tain nerve-fibres of different endowments, and 
that the office of any given nerve will be deter- 
mined by the endowment of the greatest por- 
tion of its fibres. When we say, therefore, that 
a nerve is motor or sensitive, it is not to be un- 
derstood that all its fibres are exclusively of that 
function, and that it contains no others of a dif- 
ferent endowment. 
In enquiring into the function of a nerve, 
the first point to determine is its anatomy, where- 
by we learn whether it be distributed to mus- 
cular parts or to sentient surfaces; and then to 
ascertain whether its distribution in man corre- 
sponds with that in the inferior animals. Ana- 
tomy, human and comparative, affords by far 
the most certain grounds to enable us to decide 
upon the endowment of a nerve: if the nerve 
be distributed to muscular parts, it is evident 
that it cannot be a merely sentient nerve, 
although it may contain some sentient fibres. 
Experiment upon animals recently dead also 
affords considerable aid in reference to questions 
of this kind. . Mechanical or chemical or galvanic 
irritation of a nerve will cause muscular contrac- 
tions if it be a motor nerve, and will produce 
no perceptible change in either nerve or muscles 
if it be not muscular. Under certain circum- 
stances, however, simple irritation of a nerve, 
while it evinces no change in the nerve itself or 
in the parts with which it is connected, will affect 
the portion of the nervous centre in which it is 
implanted, and will through that excite certain 
motor nerves to stimulate their muscles. To 
affect motor nerves through sensitive ones, it is 
generally necessary to stimulate their peripheral 
fibres, the entire trank remaining uninterrupted 
in its course; and it would appear as if a cer- 
in peripheral organisation, as for instance a 
evelopement of papille on the tegumentary 
urface, were necessary for this purpose. Very 
ely irritation of the trunk of a sentient nerve 
roduces this effect; the least equivocal in- 
tance indeed in which, so far as I know, 
uscular action can be produced in this way, 
. €., by irritation of the central segment of the 
unk of a’nerve, is in the case of the glosso- 
oharyngeal nerve. Dr. John Reid has suc- 
peeded, after section of this nerve, in producing 
traction of the pharyngeal muscles by stimu- 
ing its central segment. 
MM. Longet and Matteucci affirm that a 
720T 
motor nerve may be distinguished from a com- 
pound one by the different effect produced on 
each by opening or closing a galvanic current, 
according to the direction in which it passes in 
the nerve. We have referred above to the 
results of experiments on compound nerves, the 
sciatic for instance, by means of the electric 
current. Compound nerves, as has been shown 
by these means, may at first be affected equally 
On opening as on closing the circuit, whether 
the current be direct or inverse; but after a 
time they are excitable, as shown by the con- 
traction of the muscles below the point stimu- 
lated, only on closing the direct current or 
opening the inverse. With a purely motor 
nerve, however, such as the anterior root of a 
spinal nerve, a different result is obtained after 
the first period has passed; inasmuch as the 
contractions of the muscles can only be excited 
on opening the direct current or closing the in- 
verse.* 
Experiment upon living animals likewise 
affords us some assistance in determining the 
functions of nerves. This mode of inquiry, how- 
ever, must be used with great circumspection, and 
great caution must be observed in the inter- 
pretation of the results which it elicits. Section 
of a nerve paralyses its function, and occasions 
loss of motor or of sensitive power, according 
to the nature of the parts to which the nerve is 
distributed. Experiment of this kind, however, 
frequently leads to very unsatisfactory results, 
because it is often a matter of extreme difficulty 
to reach the nerve in question; the operation 
for that purpose may involve other parts and 
nerves as well, and sometimes it may be impos- 
sible to divide one nerve without injuring ano- 
ther immediately adjacent toit. Moreover, the 
shock of a severe operation frequently produces 
so much disturbance in the entire system of the 
animal as to render it extremely difficult to 
form any accurate opinion as to the effects of 
the section of the nerve under examination. 
Clinical medicine gives very important aid 
to physiological enquiries of this nature. 
Disease or injury of certain nerves impairs or 
destroys or modifies certain functions. The 
various forms of partial paralysis, especially 
those affecting the face, may be referred to in 
illustration of this assertion. Thus a very dis- 
tinct series of signs accompany disease of the 
facial nerve or the portio dura of the seventh 
pair; and these signs mark it very distinctly as 
* Matteucci et Longet, sur la relation qui existe 
entre le sens du courant electrique, et les contrac- 
tion musculaires dues a ce courant. Paris, 1844. 
It is an extraordinary circumstance that the 
excitability of motor nerve-fibres should be mo- 
dified by their simple juxtaposition with sensitive 
fibres. 
I learn from a recent communication from Prof. 
Matteucci, (May, 1847,) that he finds that etheriza- 
tion in dogs modifies the excitability of the nerves, 
so that the mixed nerves, while connected with the 
nervous centre, react with the direct or inverse 
current as the motor nerves do, and excite contrac- 
tions on opening the direct current or closing the 
inverse ; but the moment their connection with the 
cord is destroyed they exhibit the phenomena 
of mixed nerves, causing contraction with the 
direct current on closing, and with the inverse on 
opening, 
