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 dmdly 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 einotion may not reach; and no factkt 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 isthrnus faucium, an action of deglutition is instantly induced. The palato-pliaryngei 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 he completely clear of ally solid or fluid, this action cannot be perforrned; to call it into play, we instinctively bring mucus or saliva to the region of the fauces, and that stimulus brings ahout the required action. We have here, then, an instance of an action, which may take place despite of the will, which the will menot 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 iluence, 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 !United 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 org-an 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 glosso-pharyngeal, which are freely distributed upon the mucous surface of the pharynx. This stimulus causes a change in the medulla oh longata, in which those nerves are implanted, and that change is propagated thence by the pharyngeal branches qf the veva to the mus cles which contract the pharynx in the act of deglutition.
In such an action the change in the nerves by which the muscular contraction is excited must take place in a two-fold direction,—first, from the circumference to the centre, from the point of application of the stimulus to that at which the nerves become implanted in the centre; and, second/y, from centre to circum ference, from the point at which the stiinulus falls upon the centre to that where the nervous fibres mingle themselves with the muscles of the part to be moved. The stimulus, which is incident upon the nervous centre, is said to be reflected by it to the muscular textures, the immediate agents of the required movement. Hence Dr. Marshall Ilall has proposed to dis tinguish a special system of incident and rylex nerves, or of excito-motory nerves, the former being excitor, the latter motor. This distinc tion, considered anatoniically, is as yet quite hypothetical, for we have no unequivocal proof that the nerves of sensation and volition, which in their ordinary mode of action are otrerent and efterent as regards the brain, may not be competent by the relation which they must necessarily bear to the spinal cord, to perform the actions which are thus assigned to a dis tinct system. The determination of this point depends mure upon the solution of certain pro blems respecting the anatomical constitution of the nervous centres, than upon any purely physiological experiments. It will be made the subject of careful examination at a subse quent part of this article.