NERVOUS SYSTEM. (NERVOUS ACTIONS.) 



589 



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 tirst 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 ivili does not take any 

 share in them. 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 felt 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 tine frenzy rolling," are all 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- 

 pression of the features ; but to attain this 

 faculty 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. 

 Upon 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 



no part of the nervous system which mental 

 emotion may not reach ; and no fact is 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 arid 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 to a 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 



