Since the purpose of respiration is to give to the blood the opportunity of renewing its supply of oxygen, and of getting rid of the carbonic acid with which it is charged, it might be supposed that the respiratory exchange would be increased, so far as the intake of oxygen is concerned, by breathing oxygen gas instead of air; but it appears that under normal conditions nothing is gained. When an animal is breathing air, its blood takes up all the oxygen that it wants—all the oxygen, that is to say, for which its tissues are asking. Offering it pure oxygen in place of mixed oxygen and nitrogen does not induce it to take up more. The hæmoglobin is almost saturated with oxygen when the blood leaves the lungs under ordinary conditions. In certain diseases of the lungs, however, in which the blood becomes unduly venous, the respiration of oxygen may be beneficial; but even in these cases the results are disappointing, because the system is suffering much less from deficiency of oxygen than from accumulation of carbonic acid. Substituting oxygen for air does not facilitate the escape of carbonic acid.
The nervous mechanism of respiration has been the subject of much investigation and of many experiments, without, it must be confessed, the development of a quite complete or satisfactory theory. Respiration is a rhythmic process. About seventeen times in a minute the intercostal and diaphragmatic muscles contract. Inspiration is immediately followed by expiration, the falling movement being due, as already explained, to the elasticity of the lungs, which are stretched during inspiration. A slight pause intervenes between the end of expiration and the commencement of the next inspiratory movement. Tranquil respiration is a succession of reflex inspiratory movements, the depth of which varies according to the needs of the body—that is to say, according to the condition of the blood. If the need for aeration of the blood becomes urgent, the depth of inspiration is increased, and expiration also becomes an active movement, certain muscles, especially those of the abdomen, being called into play. In this condition two sets of reflex actions alternate. A large number of nerves are concerned even in tranquil respiration. If a man in falling “breaks his back” at the junction of the cervical and thoracic regions, costal respiration ceases. The series of intercostal nerves which arises from the dorsal spinal cord below the level at which it is injured are thrown out of action. Diaphragmatic respiration still continues, because the nerve of the diaphragm, the phrenic, arises from cervical roots. The lungs are supplied by the vagus nerve. This nerve joins the medulla oblongata as one of a group of three—glosso-pharyngeal, vagus, and spinal accessory—which by a large number of roots enter the groove between the olive and the restiform body. The vagus is the channel along which afferent impulses from the lungs enter the medulla. Such impulses call for respiratory movements. Cutting both vagi, however, does not put an end to respiration. Inspiratory movements continue, but they are much deeper and separated by much longer pauses. Such a form of respiration is inefficient. The blood is not properly aerated. The animal falls into a condition of dyspnœa, which ends in death. When the central end of the cut vagus is stimulated, the movements become more natural. Clearly, the respiratory reflex is not dependent upon the vagus, since it continues after the nerve is cut, although the impulses which pass up this nerve regulate its rhythm. They govern the length of the inspiratory movements, cut them short at the right moment, and secure their succession at proper intervals.
The transfer of afferent impulses into efferent channels occurs in the medulla oblongata. Long ago it was found that if the brain above this level be removed, part by part, respiration is not interfered with until the medulla oblongata is injured. When a cut is made into the floor of the fourth ventricle not far to one side of the middle line, the respiratory movements on that side of the body cease. If the injury be bilateral, even though very limited in extent, respiration stops. This spot was therefore spoken of as the “respiratory centre.” Flourens, who first discovered it, believed that it was a mere spot. He gave to it the fanciful name of nœud vital. It is the place at which the afferent nerves which call for respiration are brought into connection with all the various motor nerves which bring about the respiratory movements of nostrils, larynx, chest, and diaphragm. Possibly the knife in Flourens’ incision divides the tract of fibres which distributes afferent impulses, but whether the junction be a defined tract or no, injury to this region of the medulla throws the nervous mechanism of respiration out of gear. At this particular spot lies the “centre” for respiration—the one part of the nervous system which must be intact if the movements of respiration are to be carried out. There is no reason for thinking that respiratory impulses are generated at this spot. It is a centre in the same sense in which Crewe is a centre for distributing the goods of Lancashire and other parts of England to North Wales. The use of the term “nerve-centre” has been very much abused. Centres were supposed to be collections of cells, each group of which had some prerogative of initiation. Reasoning from the analogy of human institutions, it was thought necessary that the nervous system should be organized into departments severally responsible for the administration of the activities of certain sets of muscles: one centre controlled respiration, another the beat of the heart, another deglutition. The centres were dependent one on another; each regulated lower centres, and was governed by those above it, in this bureaucratic scheme. We know nothing of any function of nerve-cells other than that of transmitting impulses. All that we know about nerve-cells is that they place afferent and efferent routes in communication, and interpose resistance into nerve-circuits. Every nerve-cell of the grey matter of the brain and spinal cord gives off processes which ramify. The ultimate twigs into which a branch divides are in connection with other sets of twigs derived from the end-branchings of nerve-fibres or processes of other nerve-cells. A nerve-fibre is but the axis-cylinder process of a nerve-cell. Impulses encounter resistance in passing along the neuro-fibrillæ ([cf. Fig. 22]) contained in the twig-connections of the ramifying processes of nerve-cells. There is no reason for supposing that anything like the same resistance is offered to the passage of impulses along the fibrillæ where they lie within the stout branches of the cell-processes or within the body of the cell. It is easy to make a pictorial representation of such a mechanism. Imagine a model of the stem of a tree made by binding together a large number of wires; its branches as containing small groups of wires; the ultimate twigs as separate wires. Carry wires from the roots of one tree to the branches of another. Trees so constructed might be taken as representing nerve-cells. We have not as yet succeeded in demonstrating the isolated neuro-fibrillæ as they pass over from the end-twigs of a nerve-fibre to the end-twigs of a nerve-cell branch, but we have abundant reason for believing that they do so pass, and that the resistance to the passage of a nerve-impulse is interposed in this neutral or junctional zone. This resistance has to be overcome. It is overcome by the summation of impulses. All nerve-impulses are vibratory. The first vibrations may fail to get through; but if the vibrations continue, they exert a cumulative effect. After a time they overcome the resistance; sensory impulses flow through the centre into motor channels. In this way we endeavour to explain the rhythmic discharge through the respiratory and other centres. It has not been found possible to determine the source of all the afferent impulses which reach the centre. Respiration continues after all accessible nerves have been cut, including even the posterior roots of the cervical nerves. Probably it is a mistake to look for definite afferent channels in the medulla and the rest of the brain. All parts of the body need aerated blood. From all parts, including nerve-tissue itself, arises the demand for respiration. Possibly nerve-centres have the power, as it were, of storing impulses, and discharging them after the stream of fresh arrivals has ceased to flow. They may acquire a habit.
The resistance in the centre is profoundly affected by the condition of the blood. As the blood becomes more venous, impulses pass across the nerve connections with ever-increasing force. Kept in the first instance to definite channels, they spread as the centre becomes more excitable farther and farther afield, reaching one group of muscles after another, and pressing them into the service of respiration. When, in dyspnœa, every muscle which can in any way help the movements of the chest is doing its best, others which are useless for this purpose receive the reflected impulses and join in, producing general convulsions. The increased activity of the respiratory centre which is produced by slight venosity of the blood is shown in the rapid and deep inspirations which are caused by violent exercise. Perhaps it is justifiable to go a step farther, and to assert that there is something in blood which has been rendered venous by muscular activity which is specially exciting to the respiratory centre. If the blood from a limb be prevented from returning to the general circulation, by compressing or tying its great veins, and if the muscles of the limb be strongly stimulated by an electric current, their activity, so long as the passage through the veins is blocked, has no influence upon respiration. But, on relaxation of the pressure on the veins, respiration may become twice as deep and twice as frequent as it was before the muscles were stimulated, although the limb is now in a condition of perfect rest.
What is the special action of the vagus nerve? Its superior laryngeal branch checks inspiration and induces expiration, as already said. The impulses which pass up its main trunk bring about ordered movements. They are not dependent for their generation upon the condition of the blood in the lungs. When the chest is filled with nitrogen, inspiration and expiration alternate in the usual way, although the blood is growing steadily more venous. The failure of inspiration to bring about aeration of the blood does not lead to a prolongation of the inspiratory effort. Inspiration is cut off and expiration established in regular sequence. In performing “artificial respiration” ([cf. p. 184]) for the purpose of saving life, in cases in which respiration has ceased owing to the lungs being filled with water, or for other reasons, the chest is enlarged by raising the arms above the head, and diminished by pressing the elbows against the sides. Enlargement promotes a tendency to expiration, compression a tendency to a natural inspiratory effort. Evidently there is a connection between the movements of the chest and the stimulation of the respiratory centre. If respiration is being carried on artificially, by forcing air from a bellows into the trachea, the nostrils dilate as the chest is distended, and contract as it is emptied, so long as the vagus nerve is intact, just as they do in normal respiration. This shows that, when the chest is emptied, a message is sent through to the nucleus of origin of the nerve which supplies the dilator muscles of the nostril. When the lungs are full, a message calls upon the nostrils to contract. The only factor which is common to pressing in and pulling out the ribs, and filling and exhausting the lungs with a bellows, is the alteration in the form of the lungs which is produced by the two methods. It is impossible to resist the conclusion that the stretching of the tissue of the lungs stimulates the nerve-endings of the vagus. The impulses thus induced automatically stop inspiration, and lead to an expiratory effort.
There are many indications that the nervous mechanism of respiration is a double one, certain stimuli inducing expiration, with inhibition of inspiration, others inhibiting expiration and inducing inspiration. There are, however, many difficulties in the way of formulating a satisfactory theory of the relation of these antagonistic actions. We may frequently observe indications of such an antagonism between the two phases of the respiratory mechanism. Cold water dashed on the back of the head (when the head is being shampooed) induces a long inspiration with inhibition of expiration. A blow in the pit of the stomach “knocks all the wind out of a man.” Expiration is prolonged until the lungs are unusually empty, and yet the victim of the blow feels as if he would never again be able to draw breath.
Modified Respiratory Movements.—The object of coughing is to expel foreign matter from the windpipe or larynx; of sneezing, to clear the nose. The former action consists of a long deep inspiration; the closure of the glottis; a forcible expiration. The blast of air encountering a closed glottis acquires considerable pressure. When the resistance of the glottis is overcome, the blast rushes through, carrying with it mucus or bread-crumb, or whatever the substance may be which irritated the endings of the superior laryngeal nerve. In sneezing, the back of the tongue is thrust against the palate, closing the aperture of the fauces. Inspiration is prolonged. A strong expiration follows. The blast rushes through the nasal cavities. This reflex is usually provoked by a tickling of the endings of the fifth nerve in the nasal mucous membrane. It is also caused in many persons, through the optic nerve, by a bright light; an apparently purposeless reflex about which we shall have something more to say in a subsequent chapter. Laughing and crying are modified respiratory movements as useless, so far as any immediate purpose is accomplished, as sneezing in response to a bright light. As means of expressing emotions they have been cultivated by the human race. Possibly a case for crying might be made out on physiological grounds. Under certain circumstances it relieves a feeling of distress which, while it lasts, is detrimental to the proper functions of the body. Laughing undoubtedly is beneficial. The rapid movements of the chest quicken the circulation. The shaking of the midriff favours the discharge of digestive secretions, accelerates the movements of the alimentary canal, and generally is beneficial to digestion. But “laugh and grow fat” is not necessarily the order of cause and effect. An efficient digestion and a good capacity for assimilation lead to a sense of bien-être which predisposes to a merry view of life.
Yawning is a deep inspiration with open mouth and larynx. It commences usually at the end of a normal inspiration, a slight pause being followed by further inspiration, deep and prolonged. Its commencement seems to be due to impulses generated by the relaxation of the tone of the muscle which holds up the lower jaw. The masseter goes off duty for a moment, allowing the jaw to fall. A reflex contraction of the muscles which open the mouth immediately follows. Muscles of the neck and head also come into play. Not improbably the yawn ends in a general stretch. If the origin of this reflex is obscure, its usefulness is marked. The circulation is quickened, the blood is changed, nervous system and muscles again become alert.
“Apnœa” is the condition of arrested respiration. If a man about to dive into the water breathe deeply and rapidly half a dozen times, he abolishes for a while the desire to breathe. One is naturally inclined to explain this as due to a surplus of oxygen taken into the blood, but a moment’s reflection shows that this cannot be the cause. In the first place, as we have already pointed out, the blood which leaves the lungs in tranquil respiration is very nearly saturated with oxygen. It can take up but little more. Again, the deep inspirations do not change the air in the air-chambers; time is required for the renewal by diffusion of their gaseous contents. It is improbable that the constitution of the air in the alveoli is sensibly altered by a few deep breaths. Probably the explanation is to be found in the effect upon the nerve-centre of distention of the chest. Stretching the nerve-endings of the vagus in the lungs inhibits inspiration. If the stimulation be excessive, inspiration is inhibited for a considerable time. That this is the right theory of apnœa is proved by repeatedly inflating the lungs of an anæsthetized animal with a pair of bellows. The same arrest of inspiration is induced whether the lungs are inflated with air or with a neutral gas, such as nitrogen, so long as the vagus nerve is intact. If this be cut, inflation with a neutral gas no longer produces apnœa.
“Dyspnœa” is the term applied to the complex conditions and movements which result from deficient aeration of the blood, or, rather, from the distribution of insufficiently aerated blood to the centres in the medulla oblongata. The blood of the rest of the body may be in a satisfactory condition, but if, owing to ligature of the carotid and vertebral arteries or other causes, the blood supplied to the brain be inadequate to its proper nutrition, the phenomena of dyspnœa are as marked as they are when air is prevented from entering the lungs. That the excitability of the nerve-centres in the brain is greatly increased when this organ is supplied with venous blood, and that their tendency to transmit impulses which call for respiration is consequently exaggerated, is remarkably shown by the following experiment: Two rabbits—A. and B.—are placed under the influence of chloroform. Their carotid arteries are cut, and a crossed circulation established by connecting the proximal ends of A.’s arteries with the distal ends of B.’s, and vice versa. The head of each rabbit is now supplied with blood from the heart of the other, the rest of its body by blood from its own heart. A.’s chest is now opened, so that its lungs collapse and cease to take part in respiration. The animal continues to make the movements of respiration in a tranquil manner, whereas B. is thrown into violent dyspnœa. The animal whose brain is receiving aerated blood remains normal, notwithstanding the fact that its lungs and the rest of its body are poisoned with venous blood. The animal whose brain is supplied with venous blood becomes dyspnœic, although its lungs and body are receiving pure arterial blood.