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topics of inquiry;—the cause of the first inspiration in the newborn infant? and of the regular alternation of inspiration and expiration during the remainder of existence?
The first of these questions will fall under consideration, when we investigate the physiology of infancy; the latter will claim some attention at present.
Haller attempted to account for the phenomenon by the passage of the blood through the lungs being impeded during expiration, a reflux of blood into the veins, and a degree of pressure upon the brain being thus induced. Hence, a painful sense of suffocation arises, in consequence of which the muscles of inspiration are called into action by the will, for the purpose of enlarging the chest, and, in this way, removing the impediment. The same uneasy feelings, however, ensue from inspiration, if too long protracted; the muscles, therefore, cease to act, and, by their relaxation, the opposite state of the chest is induced. Whytt conceived, that the passage of the blood through the pulmonary vessels is impeded by expiration, and that a sense of anxiety is thus produced. This unpleasant sensation acts as a stimulus upon the nerves of the lungs and the parts connected with them, which arouses the energy of the sentient principle; and this, by causing contraction of the diaphragm, enlarges the chest, and removes the painful feeling. The muscles then cease to act, in consequence of the stimulus no longer existing.
These, and all other methods of accounting for the phenomena are, however, too pathological. From the first moment of respiration the process appears to be accomplished without the slightest difficulty, and to be as much a part of the instinctive extra-uterine actions of the frame, as circulation, digestion, or absorption. It is obviously an internal sensation, after respiration has been once established; and, like all internal sensations, is inexplicable in our existing state of knowledge. The part, which developes the impression, is probably the lung; the pneumogastric nerves convey the impression to the brain; and the brain calls into action the muscles of inspiration. We say, that the action of impression arises in the lung from some internal cause, connected with the office it has to fill in the economy; but in so saying we sufficiently exhibit our total want of acquaintance with its nature.
Let us now inquire into the movements of inspiration and expiration, which, together, constitute the function of respiration. These acts are entirely accomplished by the dilatation and contraction of the thorax. The air enters the chest, when it is expanded, and is driven out, when the chest is restored to its ordinary dimensions; the thorax thus seeming to act like an ordinary pair of bellows with the valve stopped: when the sides are separated, the air enters at the nozzle, and is forced out, when they are brought together.
Inspiration.—The augmentation of the capacity of the thorax, which constitutes inspiration, may be effected to a greater or less
extent, according to the number of muscles which are thrown into action.
The chest may, for example, be dilated by the diaphragm alone. This muscle, as we have seen, in its ordinary relaxed condition, is convex towards the chest, as in the marginal figure, and in Fig. 107. When, however, it contracts, it becomes more horizontal; and assumes the position indicated by the dotted line d, in this manner augmenting the cavity of the chest in a vertical direction. The sides or lateral portions of the diaphragm, which are fleshy and correspond to the lungs, descend more, in this movement, than the central tendinous portion, which is moreover kept immovable by its attachment to the sternum and its union with the pericardium. In the gentlest of all breathing, the diaphragm appears to be the sole agent of inspiration; and, in cases of inflammation of the pleura costalis, or of fractured rib, our endeavours are directed to the prevention of any elevation of the ribs by which the diseased part can be put upon the stretch. Generally, however, as the diaphragm descends, the viscera of the The relaxed diaphragm. abdomen are compressed; the abdominal muscles assume the position of the double dotted line f, and the ribs and the breast bone are raised so that the latter is protruded as far as the dotted lines e.
a. The thorax.-b. The abdomen.-c
When both the diaphragm acts, and the ribs and sternum are raised, the cavity of the chest is still farther augmented.
The mechanism, by which the ribs are raised, has been productive of more controversy than the subject merits. Haller asserted, that the first rib is immovable, or at least admits of but trifling motion when compared with the others; and he denies, that the thorax makes any movement, as a whole, of either elevation or depression; affirming, that the ribs are raised successively towards the top of the cavity; and this to a greater extent as they are more inferior. Magendie, on the other hand, denies that they are elevated in this manner; and endeavours to show, that they are all raised at the same time; that the first rib instead of being the least movable is the most so; and that the disadvantage, which the lower ribs possess in the movement, by their admitting of less motion in their posterior articulations, is compensated by the greater length of these ribs. This compensation he considers to have its
advantages; for as the true ribs, with their cartilages and the sternum, usually move together, and the motion of one of these parts almost always induces that of the rest, it would follow, that if the lower ribs were more movable, they could not execute a more extensive movement than they do; whilst the solidity of the thorax would be diminished. They who are desirous of seeing this question canvassed will find it in the third volume of the Elementa Physiologiæ of Haller, and in the "Précis" of Magendie. By the elevation, then, of the ribs and the depression of the diaphragm, the chest is augmented, and a deeper inspiration effected than when the diaphragm acts singly. In this elevation of the ribs, we see the advantage of their obliquity as regards the spine. Had they been horizontal, or inclined obliquely upwards, any elevation would necessarily have contracted the thoracic cavity, and favoured expiration instead of inspiration.
The muscles, chiefly concerned in inspiration, are the intercostals, and those muscles, which arise, either directly or indirectly, from the spine, head, or upper extremities, and which can, in any manner, elevate the thorax. Amongst these muscles, are the scaleni antici and postici; the levatores costarum; the muscles of the neck, which are attached to the sternum, &c.
As no air exists in the cavity of the pleuræ, it necessarily happens, that, when the capacity of the chest is augmented, the residuary air, contained in the air-cells of the lungs after expiration, is rarefied; and, in consequence, the denser air without enters the larynx, by the mouth and nose, until the air within the lungs has attained the density, which the residuary air had prior to the inspiration, not that of the external air, as Sir Charles Bell has affirmed. At the time of inspiration, the glottis opens by the relaxation of the arytenoidei muscles, as Legallois proved by experiments, performed at the Ecole de Médecine of Paris. On exposing the glottis of a living animal the aperture is found to dilate very distinctly at each inspiration, and to contract at each expiration. If the eighth pair of nerves be divided low down in the neck, and the dilator muscles of the glottis, which receive their nerves from the recurrents—branches of the eighth pair—be thus paralyzed, the aperture is no longer enlarged during inspiration, whilst the constrictors the arytenoidei muscles—which receive their nerves from the superior laryngeal,—given off above the point of sectionpreserve their action, and close the glottis more or less completely.
When the air is inspired through the mouth, the velum is raised, so as to allow the air to pass freely to the glottis; and, in forced inspiration, it is so horizontal, as to completely expose the pharynx to view. The physician takes advantage of this, in examining morbid affections of those parts, and can often succeed much better in this way than by pressing down the tongue. On the other hand, when inspiration is effected entirely through the nose, the velum
palati is depressed, until it becomes vertical, and no obstacle exists to the free entrance of the air into the larynx. In such case, where difficulty of breathing exists, the small muscles of the alæ nasi are frequently thrown into violent action, alternately dilating and contracting the apertures of the nostrils; and hence this is a common symptom in pulmonary affections.
Mayow conceived, that the air enters the lungs in inspiration as it would a bladder put into a pair of bellows, and communicating with the external air by the pipe of the instrument. The lungs, however, are not probably so passive as this view would indicate. In cases of hernia of the lungs, the extruded portion has been observed to dilate and contract in inspiration and expiration. Reisseisen believed this to be owing to muscular fibres, which Meckel and himself conceived to perform the whole circuit of the bronchial ramifications. These are not, however, generally admitted by anatomists, and the phenomenon is usually ascribed to the bronchi having in their composition the highly elastic tissue, which is an important constituent of the arteries. Laennec affirms, that he has endeavoured, without success, to verify the observations of Reisseisen; but that the manifest existence of circular fibres on branches of a moderate size, and the phenomena, presented by many kinds of asthma, induce him to consider the temporary constriction and occlusion of the minute bronchial ramifications as a thing well established. In the trachea, an obvious muscular structure exists in its posterior third, where the cartilages are wanting. There it consists of a thin muscular plane, the fibres of which pass transversely between the interrupted extremities of the cartilaginous rings of the trachea and bronchi. The use of this muscular tissue, as suggested by Dr. Physick, and since him by Cruveilhier, is, to diminish the calibre of the air tubes in expectoration; so that the air having to pass through the contracted portion with greater velocity, its momentum may remove the secretions that are adherent to the mucous membrane. The explanation is ingenious and probably just.
Magendie asserts, that the lung has a constant tendency to return upon itself, and to occupy a smaller space than that, which it fills; and, that it consequently, exerts a degree of traction on every part of the parietes of the thorax. This traction has but little effect upon the ribs, which cannot yield; but upon the diaphragm it is considerable. It is, indeed, in his opinion, the cause why that muscle is always tense, and drawn so as to be vaulted upwards; and when the muscle is depressed during contraction, it is compelled to draw down the lungs towards the base of the chest; so that they are stretched; and, by virtue of their elasticity, have a more powerful tendency to return upon themselves, and to draw the diaphragm upwards.
Many physiologists have pointed out three degrees of inspiration, but it is manifest, that there may be innumerable shades between them:—1. Ordinary gentle inspiration, which is owing simply
to the action of the diaphragm; or, in addition, to a slight elevation of the chest. 2. Deep inspiration, where, with the depression or contraction of the diaphragm, there is evident elevation of the thorax; and, lastly, forced inspiration, when the air is strongly drawn in, by the rapid dilatation, produced by the action of all the respiratory muscles, which elevate the chest directly or indirectly.
Many trials have been instituted for determining the quantity of air, taken into the lungs at an inspiration; and considerable diversity, as might be expected, exists in the evaluations of different experimentalists. We have just remarked, that, in the same individual, the inspiration may be gentle, deep, or forced; and in each case the quantity of air inspired will necessarily differ. There is, likewise, considerable diversity in individuals, as regards the size of the chest; so that an approximation can alone be attained. The following table sufficiently exhibits the discordance of authors on this point. Many, however, of the estimates, which seem so extremely discrepant, may probably be referred to great imperfection in the mode of conducting the experiment, as well as to the causes abovementioned:
In passing through the mouth, nasal fossæ, pharynx, larynx, trachea, and bronchi, the inspired air acquires pretty nearly the temperature of the body; and, if the air has been cool, the same quantity by weight occupies a much larger space in the lungs, owing to its rarefaction in those organs. In its passage, too, it becomes mixed with the halitus, which is constantly exhaled from the mucous membrane of the air-passages; and, in this condition, it enters the air-cells, and becomes mixed with the residuary air in the lungs after expiration.
Expiration. An interval, scarcely appreciable, elapses after the accomplishment of inspiration, before the reverse movement of expiration succeeds; and the air is expelled from the chest. The