This is a subject far from being exhausted in our philosophical works; but at present we may look on these vocal cords, not as connected with voice, but in another of their offices, as forming the commissure which opens and shuts in breathing to protect the lungs from the intrusion of extraneous bodies. And here it is pertinent to remark, that in the structure of an animal body, one organ is frequently made subservient to several functions, and that without interference with the performance of any of them. This is especially true of the larynx. It is to one of its uses only that we have at present to attend. The chink of the glottis formed between the cordæ vocales opens at every inspiration, and closes at every expiration, expanding and contracting as we see the nostrils do in breathing. But the admirable thing is the acute sensibility given to this part, and the immediate influence of that sensibility upon the muscles connected with it. As soon as the lightest husk, or seed, or smallest fly, drawn in with the breath, touches the margin of the chink, there is a rapid action of the muscles which move the vocal cords, the aperture is closed, and the object is arrested. This provision is an effectual means of preventing the entrance of foreign matters into the delicate cells of the lungs. But how is the object carried thus far expelled? The same sensibility of the aperture of the glottis animates another, and that a very extensive class of muscles, viz., all those which, seated on the chest, compress it, and force out the air, in coughing; these combining in one powerful and simultaneous effort, whilst the glottis is closed, overcome that constriction, and propel the breath through the contracted pipe with an explosive force, which brushes off the offending body. There is one thing more necessary to this most important though familiar action. The lungs are never empty of air: in breathing, we do not fully expel it. If we did, there would be a period of danger occurring seventeen times in a minute; for in the first part of each inspiration, something might be drawn into the windpipe which would suffocate: but by this reserve of air in the lungs, the act of coughing can take place at any instant and the object be expelled. The sensibility seated in one spot of the throat, so beneficently, does not extend into the windpipe or lungs; for we cannot more admire the perfect adaptation of this property to its object, than the fact of its never being bestowed in a superfluous degree, or given where it is not absolutely required. Just as we have seen that the sensibility of the skin suffices to protect the parts situated beneath, so the sensibility of the top of the windpipe protects all the interior of the tube and the lungs themselves, without extending through the whole continuous surface. The simple act of sneezing affords a curious instance of the mutual relation between the muscular activity and the governing sensibility. The sensation which gives rise to this convulsive act, is seated in the membrane of the interior of the nostrils; we are not surprised, therefore, at its differing from the sensibility in the throat which excites coughing, the seat of both being different. But as regards the muscular action succeeding the irritation in the nostrils, is it not curious, that in the powerful expiration which constitutes sneezing, some twenty muscles or more, which had been excited in the similar act of coughing, are thrown out of action; while a different set, about equal in number, which had not acted in coughing, are called into action; the difference in the combination of the muscles being for the obvious purpose of directing the strong current of air, past the mouth, along the tubes of the nostrils? By no act of the will could the air be propelled so successfully through the nose, to the effect of brushing off the offensive and irritating particles from the membrane, as by this co-operation of the muscles, excited by the peculiar sensibility. It is surely admirable to find in the Mouth so many faculties combining and consenting in action, each with its appropriate organisation, and each most curiously connected with other structures. Thus we have the power of mastication, of deglutition, of modulation of the voice, the senses of taste and of touch, concentrated in one apparently simple organ. Not to speak of other relations, can there be any better proof of design, than the effects of the excited sensibility of the tongue? No sooner have the gustatory points of the nerves been excited, than there is poured out into the mouth most abundantly, by four distinct tubes, the saliva; that fluid which facilitates mastication, and directly prepares the food for the action of the stomach. We presume that this fluid is chiefly useful in mastication, as the glands are large, and the fluid most abundant, in animals which chew the cud. In all, these glands are so disposed as to receive gentle pressure from the motion of the jaw; so that, whilst their vascular apparatus is excited by the sensibilities of the tongue, the fluid is urged from the ducts by the pressure of the jaw, and the action of the muscles which move it. And however well we might imagine such a supply of fluid to assist deglutition, this is not all that is here done in preparation; for whilst the morsel is moved by tongue, and lips, and jaws, an appropriate fluid is collecting in what appear to be mere irregularities in the back part of the throat, but which are, in truth, so many receptacles, that, pointing towards the stomach, give out their contents as the morsel passes. There is one curious circumstance which we may notice before quitting this subject. Eating seems always to be an act of the will, and attended with gratification. Yet it is well known that the operation of mastication, or what is very nearly the same, may go on within the stomach, without any outward sign at least of pleasure. The gizzard (with which we are most familiar in fowls, though found in the vegetable feeders of different classes of animals) is correctly enough described as an organ of mastication, in which an incessant and alternate action of opponent muscles takes place, like the motions of the jaws. In the stomach of the lobster, these are not merely the muscles of chewing, but the teeth also: so that it appears the function may be performed altogether internally and without the volition, and probably without the sensations that accompany the offices of the mouth. We mention this, as drawing the reader to comprehend that many organs may be in operation in the internal economy, without our consciousness. Let us advert to the mode of swallowing by the crocodile, as an instance of the changes in the organisation which adapt an animal to new conditions. In terrestrial animals, the act of swallowing must be accommodated to the atmosphere; but if the creature live in water, and still breathes the air, the structure of the parts must be changed. The crocodile seizes its prey, and descends into the water with it. Its power of descending does not result, as in the fish, from compressing the air-bladder, but is owing, as we have shown, to a provision in its ribs and lungs. Unless the crocodile could expel the air from its lungs in a greater degree than the mammalia are capable of doing, it could not crawl upon the bottom, nor retain its place there without continual exertion. There is in the mouth, as well as in the thorax and lungs, an adaptation to this mode of destroying its prey by carrying it under water. The crocodile has no lips; it lies on the shore basking with its mouth open and teeth exposed, so that flies light upon and crawl into its mouth. Against these the air tubes are protected, not by lips, or a sensibility of the mouth, but by an apparatus which separates the mouth from the throat and windpipe. This partition between the cavities is necessary when the animal seizes its prey: for as it plunges under the water with open mouth, the air tube must be protected against the ingress of the water. For that purpose, there is a transverse ridge, arising from the body of the bone of the tongue, which raises a duplicature of the membrane, so as to form a septum across the back part of the mouth below,, whilst the curtain of the soft palate, hanging from above, meets the margin of the lower septum, and they form together a complete partition between the anterior and posterior cavities. Thus the animal is enabled to hold its prey in the open mouth, without admitting the water to the air passages. HUNGER AND THIRST.-Hunger and thirst are in truth senses, although the seat or organ is not easily ascertained. The wants, and desires, and pains accompanying them resemble no other sensations. Like the senses, they are given as monitors and safeguards, at the same time that, like them, they are sources of gratification. Hunger is defined to be a peculiar sensation experienced in the stomach from a deficiency of food. Such a definition does not greatly differ from the notions of those who referred the sense of hunger to the mechanical action of the surfaces of the stomach upon each other, or to a threatening of chemical action of the gastric juice on the stomach itself. But an empty stomach does not cause hunger. On the contrary, the time when the meal has passed the stomach is the best suited for exercise, and when there is the greatest alacrity of spirits. The beast of prey feeds at long intervals; the snake and other coldblooded animals take food after intervals of days or weeks. A horse, on the contrary, is always feeding. His stomach, at most, contains about four gallons, yet throw before him a truss of tares or lucerne, and he will eat continually. The emptying of the stomach cannot therefore be the cause of hunger. The natural appetite is a sensation related to the general condition of the system, and not simply referable to the state of the stomach; neither to its action, nor its emptiness, nor the acidity of its contents; nor in a starved creature will a full stomach satisfy the desire of food. Under the same impulse which makes us swallow, the ruminating animal draws the morsel from its own stomach. Hunger is well illustrated by thirst. Suppose we take the definition of thirst-that it is a sense of dryness and constriction in the back part of the mouth and fauces-the moistening of these parts will not allay thirst after much fatigue or during fever. In making a long speech, if a man's mouth be parched, and the dryness merely from speaking, it will be relieved by moistening; but if it come from the feverish anxiety and excitement attending a public exhibition, his thirst will not be so assuaged. The question, as it regards thirst, was brought to a demonstration by the following circumstance. A man having a wound low down in his throat, was tortured with thirst; but no quantity of fluid passing through his mouth and gullet, and escaping by the wound, was found in any degree to quench his thirst. Thirst, then, like hunger, has relation to the general condition of the animal system-to the necessity for fluid in the circulation. For this reason, a man dying from loss of blood suffers under intolerable thirst. In both thirst and hunger, the supply is obtained through the gratification of an appetite; and as to these appetites, it will be acknowledged that the pleasures resulting from them far exceed the pains. They gently solicit for the wants of the body: they are the perpetual motive and spring to action. Breathing, as we have seen, is even more directly necessary to life than food; but to this we are differently admonished. An appetite implies intervals of satiety and indifference. The uninterrupted action of breathing could not be supported by a perpetual desire: we cannot imagine such an uniformity of sensation. The action of breathing has been made instinctive, while pain and the alarm of death are brought as the only adequate agents to control the irregularities of a function so necessary to life. Pain does here what desire and the solicitation of pleasure could not accomplish. |