720 
change rapidly propagated along the nerve, 
from molecule to molecule, from the point of 
ao of the stimulus. The change is 
obviously analogous to that which takes place 
in the particles of a piece of soft iron, in virtue 
of which the iron acquires the properties of a 
magnet, so long as it is maintained in a certain 
relation to a galvanic current; the magnetic 
power being instantly communicated when the 
Circuit is completed, and as rapidly removed 
when it is interrupted. 
The action of the stimulus, then, excites a 
state of polarity of the particles of the nerve 
stimulated ; and this polar state may be in- 
duced in other icles, whether muscular or 
nervous, with which the nerve stimulated may 
be in organic connexion. Just as the polar 
state of the electrical apparatus is capable of 
being communicated to the piece of soft iron, 
which thereby acquires the well-known mag- 
netic properties during the continuance of the 
excited polarity. 
Thus, then, we learn that such is the nature 
of the nerve fibre, that under the application 
of a stimulus, mechanical, chemical, or galvanic, 
it is capable of generating a polar force analo- 
gous in many particulars to that of muscle; 
this force we call the nervous force, vis nervosa, 
or nervous polarity.* 
And if we examine the ordinary mode of the 
development of the nervous force, in the usual 
actions of the frame, we find that under the 
influence of a mental stimulus, the will, it is 
propagated from the nervous centre along the 
netves to muscles, or under the influence of a 
physical stimulus it is propagated along the 
nerves to the centres, where it is capable of 
exciting either a sensation or muscular motion 
in a secondary manner, or both. 
But the application of a physical stimulus 
to a nervous centre may cause the development 
of nervous force, which may be conducted 
away from it by nerves which are implanted in 
it. And thus we learn that the same polar 
condition which may be produced in nerves is 
equally capable of being excited in nervous 
centres. The polar condition of the nerve fibre 
may be propagated to the nervous centre, or that 
of the nervous centre to the nerve fibre. 
In some of the nervous centres, however, no 
visible change of any kind takes place upon the 
_ irritation of the nervous matter, nor does the ani- 
mal seem to suffer pain. Such is the case when 
the hemispheres of the brain are the subject of 
experiment. We are not to infer from this that 
the nervous force is not developed in these 
centres, but that they have no direct connexion 
with the muscular system, nor have they that 
peculiar organization which would enable them 
when irritated to excite painful sensations. 
There are certain nerves which when stimu- 
lated excite neither muscular motion nor com- 
* I have been in the habit of taking this view of 
the nervous force in my lectures for the last four or 
five years, and of using the term, nervous polarity, 
as expressive of the nature of the nervous force. 
This term has likewise been adopted by Mr. Bow- 
man and myself in our work on the Physiological 
Anatomy and Physiology of Man, vol. i. p. 56, and 
in the chtiptiies on the Nervous System, passim. 
PHYSIOLOGY OF THE NERVOUS SYSTEM. 
mon sensation or pain, but a sensation peculiar 
to themselves. Thus if the optic nerve be sti- — 
mulated by a mechanical or galvanic stimulus, — 
a sensation of light is produced ; if the auditory 
nerve be stimulated in like manner, a sensa 
of sound is produced. 
These facts prove not only that a peculia 
force is generated by the nervous matter, but 
they also show that the nerve fibres in the cen 
tres, as well as in the nerves, specia 
endowments depending, in all probability, upoi 
their central as well as upon their peripher 
connexions. Thus nerve-fibres conne wit! 
muscles are capable of exciting muscular con 
traction, and are therefore called motor or mus. 
cular nerves. Nerve-fibres, which are di: 
buted to a sentient surface, as the skin or mi 
cous membrane, and have a certain relatio 
with that part of the nervous centre which ¢ 
stitutes the centre of sensation, (vide p. 711 
are when stimulated capable of exciting a fet 
ing which may be agreeable or painful, ace 
ing to the degree of stimulation. These ¢ 
called sensitive nerves, or nerves of comm 
sensation. To the class of sensitive ner 
belong those which, owing no doubt to a pe¢ 
liarity in their connexion with the centre, 
well as to their relation to a special apparat 
at their periphery, develope peculiar sensation 
as the nerves of sight, hearing, taste, &c., a 
they have been distinguished as nerves of spec 
sensation. & 
Very many sentient nerves are implanted 
the nervous centre near to certain motor ner 
so that a stimulus applied to the former 
capable of reacting upon the latter, and of 
citing motion through their connexion with t 
muscles. Dr. M. Hall, however, ingeniou 
supposes that this power resides only in a] 
ticular class of nerve-fibres (and not it 
ordinary sentient nerves through their close 
of relation with the ordinary motor nerves), 
nerve of this kind would constitute an 
consisting of an incident and a reflex portic 
which are united at the nervous centre. — 
stimulus is conveyed to the centre by the’ 
dent portion, and is then reflected into 
reflex or motor portion. Such nerves, Dr. 
designates ercito-motor. We shall exa 
further on the grounds of this hypothesis. 
It is an important fact, which Sir C. 
was the first clearly to prove, that 
of different endowments may be bound t 
in one sheath, forming, in anatomical lang 
one nerve. Thus a nerve may contain set 
and motor fibres, as the median nerve” 
arm, or if we admitted Dr. Hall’s 
it might contain sentient, motor, and € 
motor fibres. And most nerves in the 
rent regions of the body are of this desert 
i. €. compound nerves, made up of sentiet 
motor fibres bound together in the sa 
in very different proportions. In ma 
nerves, as in the spinal nerves, and 
pair, the separation of the fibres of mot 
those of sensation exists at the implan' 
the centre, and there the fibres of each 
ment are collected into a separate | 
which possesses the endowment proper t 
constituent fibres, These are the roots of 
vo 
i? 
10; 
Te 
