In inspiration the pressure of the elastic air in the lungs causes these organs to ex pand, so as to keep their outer surface in contact with the inner surface of the dilating thorax ; and by this the air of the lungs be comes rarified, and a quantity of fresh air rushes along the trachea and bronchial tubes to restore its equilibrium; in expiration, on the other hand, the lungs are compressed, and a portion of air is forced outward along the same passages. In these movements the lungs are not quite passive. The external surface of the lungs, and of the numerous lobes into which they may be divided, is covered with an elastic membrane, and this, conjoined with the weight of their tissues, must favour the expulsion of the air during expiration, and present a certain amount of resistance to its entrance during inspiration.t When the external air is admitted freely into the sac of the pleura, by an opening in the parietes of the thorax sufficiently large to permit the air to pass through it in greater quantities than it can enter the lungs by the trachea, the lung collapses rapidly and is compressed against the spine; and if this take place on both sides of the chest, the respiratory process is arrested, and the indi vidual dies, as from suffocation. When the lungs lose their elasticity, and the air-cells become dilated and their septa partially broken down, as in emphysema, the respiratory mem brane is not only diminished in extent, but expiration is more difficult, and when the chest is laid open after death, the lungs col lapse imperfectly or not at all. It is evident that still more serious evils must follow inter lobular emphysema, or effusion of air into tbe cellular tissue surrounding the smaller lobes of the lungs, if this occurs to a considerable extent.
Though the trachea, the bronchii, and even the smaller bronchial tubes are provided with distinct muscular fibres which can be thrown into contraction by direct excitation, and even, according to some experimenters, by excitation of their nerves, yet the notion entertained by many of the older, and even by some modern physiologists, that the lungs have an active power of contraction and dilatation synchronous with and aiding the movements of inspiration and expiration, is undoubtedly untenable. These tnuscular fibres of the bronchial tubes are endowed with that kind of contractility termed simple contracti lity, which manifests itself by more slow and prolonged contractions and relaxations than that of the voluntary muscles and the heart.* The possession of this property of simple contractility unfits these muscular fibres from acting' simultaneously with the muscles of respiration moving the thorax, but fits them for effecting these changes on the capacity of the air-tubes, which may aid in the expulsion of substances from their interior, as in cough ing. The movements of the cilia placed on the inner surface of the respiratory organs, can assist little, if at all, in renewing the at mospheric air in the lungs. The passage of
the air into and from the lung, has an im portant effect upon the muscular respiratory movements. When a lung, or a considerable portion of it, is prevented from expanding by disease or any other cause, the pressure of the air on the inner surface of that por tion of the chest covering the unexpansihle lung is not now exercised during its dilata tion ; in other words, this portion of the chest in expanding must do so in opposition to the whole of the atmospheric pressure on its outer surface, amounting to 15 pounds on the square inch. This pressure appears to be too great for the muscles of inspiration, acting upon that part of the chest, to overcome, for the ribs are. there motionless or nearly so, and if the lung is in a state of collapse, the walls of the thorax covering it fall in.
The muscular movements of inspiration and expiration are, in the natural and healthy state of the body, performed without the in tervention of volition, and even without our consciousness, and belong to the class of movements which have lately received the appellation of excito-motary. When, how ever, the free aeration of the blood in the lungs is impeded, a sensation, urgent and imperious in its demands, is felt, which in our language is somewhat clumsily designated " the sensation of the want of fresh air in the lungs," and more elegantly in French, le besoin de respirer. These respiratory move ments, therefore, depend upon the transmis sion inwards of certain excitations along afferent nerves to the central organs of the nervous system, whence a motive influence is sent outwards along the motor or efferent nerves. distributed in the muscles to be moved. One of the principal excitor or af ferent nerves of respiration Is the par vagum ; and the medulla oblongata is the portion of the central organs of the nervous system to which all the excitations of the nervous system capable of producing a respiratory muscular movement must be brought. The motor or efferent nerves that convey outwards from the medulla oblongata the motive in fluence which stiinulates the muscles of respi ration to contract are the phrenie, and part of the anterior roots of the dorsal and lumbar spinal nerves, the recurrent laryngeal, the portio dura, the spinal accessory, and some branches of the cervical and upper part of the axillary plexus besides the phrenic, especially the branch distributed in the serratus magnus muscle, termed by Sir Charles Bell the ex ternal respiratory. Some of these efferent nerves, like the muscles in which they are distributed, are habitually engaged in carry. ing on the respiratory, imuseular movements, while others aid these only when the respira tion requires to be carried on mere vigorously than usual.