NERVOUS SYSTEM. 
nerves. This power is that of special sensibility. 
The nerves, which convey these impressions of 
special sensation to the mind, are incapable of 
responding in any other way, even to a me- 
chanical stimulus. When the retina is stimu- 
lated with the point of a needle, the sensation 
of a flash of light is produced: when the audi- 
tory nerve is excited by a mechanical impulse 
upon the tympanum, sound is heard. 
The mechanism (so to speak) of sensations 
of whatever kind is exactly in the reverse di- 
rection to that of voluntary motions. In the 
latter the change begins in the mind and ends 
in the body; in the former the impression is 
made first on the body and is conveyed to the 
mind. In both cases mental change, whether 
active or passive, is necessarily associated with 
the nervous act. 
There are other actions in which the mind is 
concerned, although the wil/ does not take any 
Share inthem. These are, such as may be 
produced under the influence of those sudden, 
momentary, and involuntary mental changes, 
which are called emotions, and which may be 
excited, either by an impression conveyed to 
_ the mind from some external cause, through 
the senses, or by some change in the mind itself, 
arising in the train of its thoughts. Who has 
not feit the thrill which pervades every part of 
the frame, in listening to some harrowing tale 
of woe? or, when the imagination, in its mu- 
Sings, conjures up before the mental vision 
some fearful scene of lamentation and wretched- 
ness? How keenly do the emotions of joy 
and sorrow, anger and pity, cause themselves 
_ to be felt at every point of the system. The 
blush of shame, the pale curling lip of anger, 
_ “theeye in a fine frenzy rolling,” are al! examples 
of the emotions of the mind influencing bodily 
actions. The changes which the countenance 
undergoes in accordance with varying states of 
_ the mind result from the same cause. The will 
May acquire the power of controlling to a great 
extent this influence of emotion upon the ex- 
wession of the features; but to attain this 
aculty in great perfection requires great strength 
of will and frequency of exercise. Few men 
ever acquired such controul over the play of 
the features, and such power of resisting the 
influence of emotion as well as of imitating that 
influence, as Garrick and Talleyrand. 
The power of the emotions over the nervous 
system is shewn not merely in those actions 
which the will may controul, but also in others, 
which, as being in no degree voluntary, and 
therefore in a great measure disconnected from 
the animal functions, have been called organic. 
em none of these does emotion exert more 
influence than upon the circulation. In blush- 
ing, in the deadly paleness of the blood-de- 
serted cheek, in the cold sweat of fear, in the 
depression of the heart from syncope, we find 
unequivocal instances of actions of an involun- 
tary kind, produced by this influence. Many 
of the sensations which are felt in grief, fear, 
anxiety, or in the paroxysm of hysteria, are in 
a great measure due to local changes in the 
capillary circulation caused by the power of 
emotion over it. It would seem that there is 
589 
no part of the nervous system which mental 
emotion may not reach; and no fact\: of more 
general application than this, to the explana- 
tion of the multifarious forms of morbid sen- 
sation. 
Of the physical nervous actions.—When the 
eyelid is raised so as to admit light suddenly to 
the bottom of the eye, the pupil instantly con- 
tracts, nor is the individual conscious of the 
change which is taking place in it, and he is 
equally unable to controul or prevent it. The 
degree of contraction appears to be propor- 
tionate to the intensity of the stimulus. 
When a morsel of food is applied to the 
isthmus faucium, an action of deglutition is 
instantly induced. The palato-pharyngei mus- 
cles and the constrictors of the pharynx are 
immediately brought into play. This action is 
entirely involuntary, and cannot be controlled 
by the power of the will; it may be brought 
on even in the state of coma, when the indi- 
vidual is insensible to any external impression, 
and the fact is one of practical application, 
as shewing how persons in that state may be 
made to swallow by bringing the morsel 
into contact with the mucous membrane 
of the pharynx. Moreover it is an action 
which the will cannot imitate unless there be 
something to be swallowed. If the fauces be 
completely clear of any solid or fluid, this 
action cannot be performed; to call it into play, 
we instinctively bring mucus or saliva to the 
region of the fauces, and that stimulus brings 
about the required action. We have here, 
then, an instance of an action, which may take 
place despite of the will, which the will cannot 
imitate, which may be produced when. con- 
sciousness and will are in abeyance: it is, there- 
fore, an action independent of the mind’s in- 
fluence, and which may fairly be ascribed to a 
cause purely physical. 
The sudden application of cold to the surface 
of the body or to any part of it, more especially 
to the face, causes an immediate and involun- 
tary excitement of the muscles of respiration, 
and may be quoted as an instance of an action 
of similar kind to those mentioned in pre- 
ceding paragraphs. The will may control this 
action to a limited extent, but never entirely. 
A large number of movements in the living 
body, and especially of those which are com- 
monly called organic, might be referred to as 
examples of physical nervous actions, in which 
the stimulus acts independently of the mind. 
These actions may be produced by a physical 
change taking place in the nervous centre and 
propagated directly to the muscular texture of 
the part (the moving power), as in the case of 
convulsive movements produced by irritant dis- 
eases of a nervous centre; but the ordinary 
manner in which they are effected is by the 
application of a stimulus toa surface. Through 
afferent nerves this stimulus affects the nervous 
centre, and produces a change there, which 
excites certain other nerves proceeding from 
that centre to the organ in which the move- 
ment occurs. To take, as an example, the 
act of deglutition above referred to. The 
morsel of food stimulates certain nerves, the 
(Nervous Actions.) 
