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blood vessels-and from each ganglion proceed two sets of nerves, one terminating in an organ of sense, the other in some organs of motion. A circle is formed of these parts in the following order, beginning at the origin of the sensation: a sensitive surface; a nerve arising in it and proceeding inwards to a certain ganglion; the ganglion itself; and lastly, a nerve issuing from the ganglion towards a moving organ, and terminating in its muscles. If, for example, we take the sense of touch: the parts of the circle are the integument of the fingers; the nerves proceeding from this integument to the ganglionic centre appropriated to touch; the ganglion itself; the efferent or outgoing nerves of the ganglion, of which a certain number at least proceed to the fingers themselves, or to the muscles that move them. Such is the fundamental structure of the nervous anatomy;
and next, as to its mode of working.
When a stimulus is applied to any sensitive surface-to the tips of the fingers, for example-this stimulus immediately tells upon the fibres of the nerve embedded in that surface. The nerve-fibres have for their special function the communication or transmission of any influence brought to bear upon their extremities-they are what is termed conductors. The pressure exerted upon the nerves of touch when the fingers are squeezed is rapidly conveyed in some shape or other to the ganglion of the sense of touch; it is not swallowed up or stifled there, but as a part of its nature it acts upon the vascular globules or vesicles of the ganglion, influencing the circulation of the blood of those vesicles, and developing a motive-force which issues along the outgoing nerves, and is transmitted to the muscles of the fingers, or the parts affected by the sensation. This, and nothing less than this, constitutes a complete act or operation of sense. The original stimulus in the sentient surface always tends to produce a reflex stimulus of the organ that carries that surface. This movement will be either a movement of closer contact with the thing or object of sensation, or à movement of repulsion and retraction of the member, in case of the stimulus being painful or disagreeable. Put a ball in a child's open hand, and the effect of the touch will be, through the steps above described, to clench the hand and grasp the ball. If it is hot, or cold, or prickly, or in any way uncongenial to the organs, a different set of muscles will be communicated with and complete the round, and the hand will be rapidly withdrawn from the unwelcome touch. Until one or other of these two effects have been produced, the sensation cannot be said to have accomplished its natural course. If a stimulus or impression from without stops short at the ganglionic centre, the fact shews either that the impression is feeble, that the ganglion or outgoing nerve is paralysed, or that some other stimulus of a more powerful kind and of a contrary nature has found its way to the same ganglion-a thing that often takes place in the complex organisation and multiplied communications of the nervous system.
In our search into the causes of the motive-power of the animal body, this view of the nature of sensation is all-important. It reveals to us at once a direct and unfailing connection between sense and activity— the two being only different portions of the same mechanism. A sensation is never complete till it brings forth an action. The permanency of the sensation as a recollected or revivable impression depends on its having had full scope and effect upon the moving organ concerned in the case.
The circulation of a nervous current, or propagation of a nervous vibration, whose nature is unknown to us, constitutes the entire fact of the sensation taken by itself: when this current or vibration has ceased there would appear to be no feeling present-no manifestation of sense any more than of movement. When we retrace a past sensation, we apparently do nothing beyond reviving the current of excitement between the sensitive surface and the moving part of the sensational circle.
Keeping in view, therefore, this relation between sensation and action, as between parts of the same whole, we will now pass in review the different classes of animal sensibilities, adverting in each instance to the special movements generated by the inherent activity of the circles of sense. We shall thus ascertain what amount of active power nature has associated with the very fact of sensibility, and shall thereby provide an adequate explanation of a certain fraction of the phenomena now under consideration.
1. Sensations of Organic Life.—It is necessary for us to recognise a class of feelings arising from the general well or ill being of the system at large as something distinct from the feelings of the special senses. Accordingly, physiologists have singled out those feelings under the name of general sensibility. The various processes at work in the waste and renovation of the tissues of the system give forth an influence upon the consciousness, and make part and parcel of the happiness or misery of the individual existence.
The circulation of the blood, the respiratory action in the lungs, the secretions and excretions, the formation of new cells, and the absorption of decayed matter, seem all to give a certain amount of indication of their working, without in general drawing any special attention towards themselves. It is reckoned a criterion of good health to be utterly unconscious of any one of these processes; and the maxim is so far true, for it is only in case of some disorder that the consciousness is strongly acted on by the organic processes of the system. But yet the vigorous action of the nutritive functions of the frame, and the purgation from every kind of waste matter, tell powerfully upon the whole state of feeling of the individual, by enhancing the pleasure of existence, and rendering more vivid all the special senses and susceptibilities of the being. On the other hand, disease, laceration, insufficient nutriment, loss of repose, exhaustion, or any cause tending to interrupt the work of renewal and waste-the stream of vitality-make themselves felt by the same class of nerves, and produce a painful and irritated consciousness, whose influence overshadows all the other regions of conscious existence.
The obscurity that hangs over the nervous mechanism of organic sensibility must necessarily extend itself to the returning and motive portion of the nerve circles. The clearness and certainty of our knowledge of the complete round of the nervous current in the special senses do not belong to this more vague and diffused portion of our sensibility. This much we know, that when any part of the body becomes keenly conscious-in consequence of a painful disease, for example-there is a constant tendency kept up to move the part hither and thither, in the vain endeavour to withdraw it from the gnawing influence. In this tendency we can recognise the general fact of the reflex influence of the senses; for as a primary
law, it is seen that the returning nerves enter the muscles of the part affected, and the rebound of the sensation is shewn in either keeping up the sensitive part to the exciting object, or in retracting it, as the case may be. If the foot happen to be disordered and in pain, the muscles of the limb are kept in a constant state of solicitation to move the member about, and the utter uselessness of the attempt only adds to the irritation. In the case of the breathing, which is one vital part of the organic system, every stimulus on the surface of the lungs reacts immediately on the muscles of respiration: the connection of action and sensation is here quite apparent. Pure air increases the rapidity of the breathing, impure air relaxes the energy of the breathing muscles; and there is the same opposition between the effects of cold and warm air. As in the other senses, the reflex current of the respiratory sensations goes to the muscles controlling the sensitive organ—that is, the muscles of the chest.
There is one remarkable fact that goes to confirm the assumption now made as to the existence of sentient circles in all their completeness over every part of the body, for the purpose of making conscious the organic vitality of the system. This fact is no other than our sense of the direc tion or precise locality of any local irritation. In the human system the only means of indicating direction is by the movement of some member towards the place that may be in question. The movement of the eye tells direction in sight, and the movement of the hand tells the place of an object of touch. A point within the body which sends an impression towards the brain has its locality discovered by the stimulus given to the muscles adjoining the part, and by the movement thus set agoing. An uneasiness in the forearm is identified as to its place by the reflex action that it causes in the muscles of the forearm, and the movements consequent on this stimulus. In the interior of the abdomen, at a distance from any muscular actions, there is a great ambiguity and indistinctness as to the seat of a disordered feeling; and but for this tendency to reflect a stimulus upon the muscles nearest to the part, or the muscles carrying the part, we could not guess where the evil lay. There are artificial means of identifying the place of an acute organic sensation, by probing about till the sore gives evidence of being touched in the aggravated feeling of a new irritation; but nature's own method of indicating locality within the body is through the completion of each circle of sense by a muscular movement.
Inasmuch as the organic states of the body are affected by the atmospheric and other circumstances and conditions that surround it, the organic sensibility is acted on by all these causes, and a certain cognisance of external nature is the consequence. Changes of temperature alter the entire adjustment of the animal system: the rate of the breathing and of the circulation of the blood is changed, and many other alterations follow in train. So changes in the degree of moisture of the air affect the circulation, the action of the skin, and we know not how much besides; and some indication is given of this in the feelings of organic life. On the eve of a rain-storm, when the barometer falls, shewing a diminished pressure of the atmosphere, and when the air is getting charged with vapour to the point of saturation, the disturbance of the animal system is great and palpable. The human subject is generally conscious of an altered state of
things; and very many of the animal species are powerfully affected, and exhibit some marked peculiarity of manner which enables them to be cited as a prognostication of the coming storm. A general and pervading uneasiness of the system would, under the reflex influence that we are now considering, cause a general agitation and flutter of the system, ending in no specific movement; but the other endowments of the animal are usually brought into play in the circumstances, and it acts according to the best of its judgment in endeavouring to get away from the evil influence. In this it may or may not succeed; but the efforts that are made are a proof of the presence of a stimulus to action whenever the feelings of organic life assume an uncongenial cast. Quadrupeds, birds, reptiles; all seem to feel the influence of atmospheric changes, and in all of them some action or other follows up the sensation-those actions being usually something more than the mere reflex influence upon the corresponding muscular parts, which is a necessary part of every sensation.
We can thus see an innumerable variety of causes under this one sense tending to stimulate the movements of animals through the mechanism that joins sense and motion into one whole.
2. Sensations of the Alimentary Canal.-The peculiar process of the digestion of the food, and its absorption along the surface of the intestine, appears to yield a state of sensation or feeling over and above the feelings of organic life. There is an approach to specialty in this department of the organism: a set of nerves would seem to be expressly designed for conveying to the general consciousness certain impressions derived from the changes going on over the mucous membrane of the alimentary canal, through the presence or absence of nutritive material.
The contact of the food with the coat of the stomach causes a secretion of gastric juice, and the dissolved food begins to be absorbed into the blood at the very commencement of the process of digestion; and all along the intestines a double transudation appears to take place, certain matters being constantly given out of the mucous membrane, and others taken in. There is a sensitiveness developed by this action, and two distinct results are produced by it. By a reflex action from the ganglionic system (nerves which do not enter the conscious brain), the vermicular motion of the bowels is sustained; this being an exact parallel to the case of an ordinary sensation, in the cardinal peculiarity of the union of force with stimulus. In the second place, impressions are conveyed to the conscious system of a cast corresponding with the nature of the processes at the time, sometimes pleasant and luxurious, sometimes indifferent, and at other times painful or oppressive.
If we inquire to what muscles the stimulus of the alimentary states is reflected in the first instance, in order to complete the circle of sensation, the probable answer is the diaphragm and the muscles of the abdomen. These form the set immediately enclosing and controlling the digestive viscera, and all analogy would point to them as the recipients of the reflected influence. There is a considerable likelihood that the healthy and vigorous action of the digestive processes communicates a vigour of tone to the abdominal and respiratory muscles, and that a perverted action of the secretions paralyses the corresponding muscular parts.
The mass or amount of sensation derived from the alimentary canal is
very great, and it forms a considerable fraction of the total happiness or misery of all animal tribes. Nature has evidently associated the circles and ganglia of digestion in intimate and powerful bonds of union with the general centre of the nervous system; so that the impressions of the digestive states, besides their proper reflex actions on the digestive muscles, rouse up extensive operations over the active organs at large. This secondary and wide-ranging influence (whose explanation belongs to the higher functions of instinct) may be seen in many forms and ways. The tendency to repose after a full meal, the fretfulness of a hungry man, the setting of the wits to work upon the pursuit of food as a primary object of life, are instances of the powerful alliance between the digestive circles and the other regions of activity. Of the pains and agonies of existence, none are more horrible or more unhinging to the general system than some of the perverted states of the stomach.
3. Sensations of Taste.-The organ of Taste is situated at the entrance of the alimentary canal, and serves as a means of discriminating the substances proper to be taken as food. In so doing it contributes a certain quota to the pleasures of existence.
The structure of the organ is a mucous membrane spread over the tongue and palate, and secreting a liquid to combine with a portion of the food. The act of combination between the liquid and the food affects the nerves of taste embedded in the membrane.
The circle of sensation is completed by the action of the muscles of the mouth, tongue, and lower jaw; an agreeable sensation stimulates the processes of mastication and swallowing; a repulsive sensation causes an opposite action, such as to expel the obnoxious mouthful.
There is evidently some harmony between agreeableness to the taste and agreeableness to the stomach and to the organic system; hence what passes the ordeal of the palate is usually suitable to the real wants of the individual. This arrangement gives to animals the power of instinctive discrimination of food. In the inferior orders of creatures the sense of taste seems very powerful and predominant, and the instinctive capacity arising from it is proportionally great. The human subject, less frequently repelled by the tastes of substances found in nature, can indulge in a greater variety of articles of food, and in consequence may sin more largely against the wellbeing of the system than creatures of more narrow and exclusive likings can do.
We thus find in the mechanism of the sense of taste the origin of a class of practical judgments of the greatest importance in the guidance of animal life. This is a true case of instinct, and in it we can see a very large and comprehensive end effected by great simplicity in the means.
4. Sensations of Smell.-The sense of Smell is placed at the portal of the lungs, to test the quality of the inspired air, and give timely warning of a noxious atmosphere. The organ consists of a membraneous expansion covering the convoluted cavities of the nose, and connected by nervous fibres with a central ganglion of a conspicuous character. The circle is completed by nerves returning to the muscles of the nostrils in the first instance, and, in the second place, to the muscles moving the chest. An agreeable flavour stimulates one set of movements, tending to increase the inhalation; a flavour of an unsuitable kind has the contrary effect of checking the