Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/498

480 RESPIRATION Laughing is caused by a series of short expiratory blasts which provoke a clear sound from the vocal chords kept tense for the purpose, and at the same time other inarticulate but very character- istic sounds from the vibrating structures of the larynx and pharynx. The face has a characteristic expression. This act is essentially involuntary, and often is beyond control; it can only be imitated very imperfectly. Yawning is a long deep inspiration followed by a shorter expiration, the mouth, fauces, and glottis being kept open in a characteristic fashion. It is involuntary, but may be imitated. Hiccough is really an inspiration suddenly checked by closure of the glottis; the inspiration is due to a spasmodic contraction of the diaphragm. The closure of the glottis generally leads to a characteristic sound. Innermtion of the Respiratory Movements. The respiratory actions are seen from the above description to be very complicated : their regular performance depends upon the coordination of a great number of factors; and, inasmuch as the respiratory movements do not happen invariably after one stereo- typed pattern, but admit of various modifications, such as sighing, hiccoughing, &c., we must infer that the coordination is such as to admit of corresponding variations. The coordination of the inspiratory and expiratory movements is brought about through the nervous system, the connexion of which with the organs of respiration already described must now be stated. Speaking very generally, there is a nervous centre in the cerebro-spinal axis from which certain nerve fibres proceed to the muscles of respiration; these are called the efferent or centrifugal fibres. There are other fibres which run from the peripheral parts of the body towards the same centre and exert a control over it; these are called afferent or centripetal. The impulses to movement start from the centre and travel down the efferent fibres to the muscles, while impressions arising in the periphery of the body are carried along the afferent fibres to the centre and modify its action. The Respiratory Centre. There is a portion of the medulla oblongata destruction of which causes immediate and permanent cessation of all respiratory movements. This spot has been found to lie in the grey substance near the tip or nib of the calamus scriptorius; it extends on both sides of the middle line, and has re- ceived the name of " le noeud vital " (Flourens). If the spinal cord be severed immediately below the spot, all respiratory movements in the parts of the body below the level of section cease at once, while movements of respiration in parts situated above the level (e. g. , in the face) continue momentarily. If the severance be made above the vital knot, the facial movements of respiration cease, while those of the trunk continue. The nervous centre is bilateral, each half serving its proper side of the body; if the medulla oblongata be split longitudinally in the middle line, through the nceud vital, respiration goes on unchecked; but if one lateral half of the vital area be destroyed, respiration at once ceases on the same side of the body. While the above hypothesis of a respiratory centre in the medulla presiding over the movements of respiration is in all probability substantially correct, it must be stated that, in certain circum- stances of experiment (e.g., when the animal is young and has been poisoned with strychnia, or is kept in a warm chamber), respira- tory movements do occur even when the medulla oblongata with the noeud vital has been removed. That is to say, there are centres in the cord of a lower order than the medullary respiratory centre which are under ordinary circumstances dependent on the main centre but which may act independently. The respiratory centre must be regarded as the seat of origin of the impulses which cause the muscular movements of inspiration and expiration. During the whole of intra-uterine life the centre is inactive, but almost immediately after the child is born, or the placental circulation is interrupted by compression of the umbilical blood-vessels, the centre becomes quickened, and fails not to yield the appropriate stimulus at short intervals during the whole after- period of life. It is impossible to resist the conclusion that the immediate cause of the activity of the respiratory centre is the impure and impoverished state of the blood, which it is the function of respiration to remedy. So long as the placenta performs the function of purifying the blood of the foetus and supplying it with oxygen the respiratory centre is quiescent; the moment the placenta becomes incapable of purifying the foetal blood, as, for example, when the mother is suffocated or asphyxiated, the respiratory centre prepares to act and respiratory movements follow. If it is true that the presence of impure or venous blood in the respiratory centre is the cause of the first respiratory act of an animal, it is also true that the presence of highly arterialized blood is sufficient to render the centre absolutely inactive again. If air or oxygen gas be driven through the lungs of an animal and allowed to escape through holes made in the chest walls, the blood becomes so rapidly and perfectly purified that the whole vascular system is filled with the pure blood which is commonly found in the arteries only. In this case the movements of respira- tion cease until such time as the blood has again lost its purity, when respiration begins, as one might say, da capo. There is no doubt that the essential cause of the inactivity of the respiratory centre in this experiment is the presence in excess of oxygen gas, and that the absence of a certain proportion of oxygen in the blood circulating in the body endows the blood with a power of stimulat- ing directly or indirectly the respiratory centre. To this subject we shall return. The Efferent Fibres. The fibres proceeding from the respiratory centre run down the spinal chord and emerge for the most part in the cervical and dorsal region. Some fibres are collected into a nerve trunk which takes its rise in the third, fourth, and fifth cervical nerves; this is the phrenic nerve, which supplies that half of the diaphragm lying on the corresponding side of the body. Section of one phrenic nerve causes paralysis of the corresponding half of the diaphragm. Other fibres run in the intercostal nerves for the supply of the intercostal muscles, levatores costarum, &c.; others again run in the cervical nerves for the supply of the sterno- mastoid, scaleni, &c. Section of these nerves leads to paralysis of the muscles supplied by them. The facial muscles of respiration are supplied by efferent fibres from the seventh cranial nerve, and the laryugeal muscles by the laryngeal branches of the vagus. In addition to the nerves which supply the respiratory muscles, there are efferent fibres which run in the course of the vagus to the muscles supplying the larynx and the broncllial tubes. The vagi (also called the pneumogastric nerves) are two important trunks which arise from the medulla oblongata, and, after receiving communications for neighbouring nerves, run down by the side of the windpipe to reach the thorax and abdomen. By means of appropriate apparatus, of which several varieties have been devised , contraction of the small bronchial tubes may be readily made sensible and even graphically recorded. A study of the experi- mental results obtained after electrical stimulation of the pneumo- gastric nerves leaves no doubt that these nerves contain motor fibres for the bronchial tubes. The exact distribution of the superior and inferior laryngeal branches of the vagus to the muscles of the larynx is described under ANATOMY. The Afferent Fibres. Many nerve fibres have been ascertained by physiological experiment to have communication with the respira- tory centre. For example, the nerves of the skin of the chest, .when stimulated by the application of cold water, cause a gasping inspiration; the nerves of the skin of the sides of the body and of the soles of the feet, when stimulated by tickling, cause that peculiar series of spasms of the diaphragm which constitute laugh- ing; and so on. But the vagus nerve is that which contains the afferent fibres of chief importance to the respiratory movements. The vagus fibres proceeding from larynx, trachea, lung, and stomach certainty, and possibly also those proceeding from other organs of the abdomen, are all capable of influencing the activity of the main respiratory centre. If the trunk of the vagus nerve on each side of the neck be laid bare and divided in an animal such as a rabbit, the respiratory movements become much less frequent but at the same time deeper. If the end of the nerve above the point of section be carefully stimulated by a weak electric current, respiration again becomes quicker and proportionately shallow until in fact the natural type is restored. If the strength of stimulus be still further increased, the rapidity and the shallowness of breath- ing become still more remarkable, until a degree of stimulation is reached when there is no longer any rhythmical character in respiration, which is reduced to a feeble inspiratory spasm. These facts are conveniently grouped and explained by the following hypothetical statement. The vagus nerve contains fibres running up to the respiratory centre which have the function of modifying the native tendencies of the centre in such a manner as to accelerate its explosions of activity and at the same time render them less formidable. These fibres are constantly in action during healthy life; hence, on dividing the vagus trunk including these fibres, the respiratory centre reverts to its natural type of slow and deep action. On stimulating these fibres artificially they may be brought to spur on the centre once more, or even carry the accelerating process to the point of producing continuous inspiration. Experiment has, however, shown that the influence of the vagus nerve is not so simple as is here supposed. There are other fibres in the vagus which seem to arise in the larynx and run in the trunk of the two laryngeal (but chiefly in the superior laryngeal) branches of the nerve; these have powers exactly opposed to those ascribed to the main-trunk fibres. If the superior laryngeal nerve be divided and the higher end at the point of division be stimu- lated, respiration becomes less frequent and more powerful. These fibres of opposite tendencies run side by side in the vagus trunk, and must be alike excited when an electrical stimulus is thrown into the nerve. The fact that the total result of stimulation is in favour of the first-described accelerating nerves may be hypothetically explained by supposing the accelerating fibres to be more numerous, or more potent with the given stimulus, than the slowing fibres. Nature of the Activity of the Respiratory Centre. Various interest-