7200 
narcotic substances ; or, on the other hand, we 
may unduly excite them by applying a strong 
solution of strychnia. The contact of a solid 
body with a nerve may irritate and keep up a 
continual state of excitement, if it do not destroy 
its properties. A spiculum of bone, in contact 
with nervous fibres, is often the cause of the 
severest forms of neuralgia; inflammation may 
produce like effects. Various physical agents 
may produce similar consequences. The benumb- 
ing influence of cold is explained inthis way. Ex- 
sure to a continuous draught of cold air is a 
requent cause of facial paralysis. The giving 
way of a carious tooth will immediately occa- 
sion toothache by exposing the nerves of its 
pulp to the irritating influence of the air, or of 
the fluids of the mouth. And undue heat is 
likewise injurious to the physical constitution, 
and, therefore, to the action of nerves. These 
facts are of great interest in reference to the 
pathology of nervous diseases, and suggest that 
the attraction of a morbid material in the blood 
to a nerve or set of nerves, or to that part of the 
nervous centre in which such nerves may be 
implanted, may afford satisfactory explanation 
of many obscure phenomena of nerves of sen- 
sation. 
The organic change, whatever be its intrinsic 
nature, which stimuli, whether mental or phy- 
sical, produce in a nerve, developes that won- 
derful power long known to physiologists by 
the name vis nervosa, the nervous force. This 
force is more or less engaged in all the func- 
tions of the body, whether organic or animal. 
In the former its office is to regulate, control, 
and harmonize; in the latter it is the main- 
spring of action without which none of the 
phenomena can take place. Itis the natural ex- 
citant of muscular motion, and the display of that 
wondrous power depends uponits energy ; with- 
out vigour in the developement and application 
of the nervous force, a well-formed muscular 
system would be of little use, for it would 
quickly suffer in its nutrition if deprived of that 
exercise which is essential to it. 
In the various combinations of thought 
which take place in the exercise of the intellect, 
there can be no doubt that the nervous force is 
called into play in the hemispheres of the 
brain. Here the stimulus is mental ; the inde- 
pendent operations of the mind excite the ac- 
tion of the appropriate fibres of the brain, and 
the developement of the nervous force in the 
brain immediately succeeds the intellectual 
workings. It is thus that we explain the 
bodily exhaustion which mental labour in- 
duces; and thus, too, we can understand the 
giving way of the brain—the inducement of 
cerebral disease—under the incessant wear and 
tear to which men of great intellectual powers 
expose it. On the other hand, physical changes 
in the brain, of a kind different from those 
which are normal to it, the circulation of too 
much, or too little, or of a morbid blood, may 
excite mental phenomena in an irregular way 
and give rise to delirium or mania. 
Of the conditionsnecessary for the maintenance 
of the power of developing the nervous force.— 
From what has been already stated, it is mani- 
PHYSIOLOGY OF THE NERVOUS SYSTEM. 
fest that a healthy physical state of the nervous 
matter, whether in the nerves or in the nervous 
centres, constitutes the main condition neces- 
sary for preserving in them the power of deve- 
loping the nervous force. And as nerves will 
not maintain their healthy nutrition unless they 
be in union with the nervous centres, this union 
becomes an important condition for the main- 
tenance of this power in nerves. In the ner- 
vous centres the nerves form a connexion with 
the vesicular matter. We therefore infer that 
this connexion of the fibrous and vesicular matter 
is necessary for the exercise of the pa power — 
of nerves, because we know of no instance, 
either in the human economy or in that of the — 
inferior creatures, in which the nervous power is — 
developed without this union. ; 
It is true that if a motor nerve be separated — 
from the nervous centre, its peripheral segment — 
will evince a susceptibility to stimuli, or, in 
other words, it will retain the power of gene- 
rating the nervous force for some time after the 
separation. This is, however, only for a short — 
period, as the experiments of Longet disti 
show. Longet cut out a portion of the sciatie 
nerve in dogs, and irritated the lower segment 
of the nerve on each succeeding day by me 
of galvanism from a pile of twenty couples, 
by mechanical irritation. The nerve ceased to” 
be excitable on and after the fourth day, (* des” 
le quatrieme jour.”)* These results, although: 
they appear to differ from those obtained by 
Miller and Sticker, and Steinruch, are not really 
inconsistent withthem. Theseobservers, inste 
of examining and irritating the lower segn 
of the nerve each succeeding day after the 
tion, allowed it to remain for an arbitrary pe 
untouched, and then reopened the wound to ty 
the effect of stimulating the nerve. Thus Malle 
and Sticker waited eleven weeks in one it 
five weeks in a second, and two months and ; 
half in a dog, and in all the cases found 
nerve inexcitable ; and Steinruch waited 
weeks, at which time he found that the powel 
of the nerve had disappeared. It is obvio 
that there was nothing in any of these expe 
ments to cast a doubt on the possibility ¢ 
nerve having lost its excitability at a m 
earlier period after the section, and that 1 
selection of five or eight or eleven weeks, 
period when to inquire whether the ne 
tained its excitability or not, was entirely ar 
trary on the part of the experimenters. 
The pidag with which a nerve lo 
power after it has been separated from 
nervous centres clearly denotes that connec 
with the centre is a necessary condition for 
nutritive activity of nerves, and is, therefor 
necessary condition for their functional acti¥ 
or, in other words, for the full developem 
of the nervous force under its appropriate! 
muli. There are, however, ay facts wh 
inasmuch as they enhance the importan 
the vesicular i in the mani station 
nervous phenomena, give great wei 
proposition under consideration. 
os 
ng 
* Longet, Recherches Experimentales sur!’Irr 
bilité Musculaire: PExaminateur Med, Dec. | 
