How is dilatation of the thorax effected? It has been pointed out that the rib-planes decline from the horizontal in two direc tions, viz., from behind for wards, and from the antero posterior mesial plane outwards; a glance at fig. 6 will make this double sloping clear to the read er. It has, moreover, been ex plained that the diaphragm arches upwards into the thorax in such a manner that the lateral parts of the arch are vertical and in con tact with the inner face of the thoracic walls. This being the structure of the thorax, the en largement of its cavity is brought about (I) by raising the rib planes until they approach the horizontal, and (2) by depressing the diaphragm and making its rounded dome more cone-like in outline. A moment's considera tion will show how these actions enlarge the boundaries of the thorax.
Nervous Mechanism of Respiration.—The chest, then, for purposes of respiration, consists of a box which dilates and con tracts rhythmically; the actual rhythm is supplied by the nervous system. Physiologists are not agreed as to the precise role which the brain plays, but the following conception has much to rec ommend it. There is in the medulla oblongata a "centre," i.e., something analogous to a telephone exchange from which rhyth mical messages pass down the nerves which connect it with the muscles of respiration. The principal nerves in question are the phrenic (4th cervical) nerves which supply the diaphragm and the intercostal nerves which supply the intercostal muscles. If the mechanism consisted merely of this centre and the motor nerves which it operates, respiration would be of a very curious type; for, apart from controlling influences, the natural rhythm of the centre is one which produces a series of gasps at slow intervals. Pursuing the analogy of the telephone exchange, the particular centre immediately responsible for the primitive gasping type of respiration, and known as the "respiratory" centre, is of the na ture of a local exchange and is governed by two other centres in the brain, each of which modifies the natural gasping rhythm. One such centre imparts an inspiratory bias to the gasping rhythm, so that the respiration of an animal possessing these two centres, and these only, consists of infrequent respiratory efforts between which the lung is distended and therefore full of air. The third centre in the brain imparts the smoothness and a rate which gives respiration a more normal character.
Even so, there are other influences which conspire to impart the usual rhythm to breathing. They come from without the central nervous system, and the most important of them arrive from the lung itself, along the vagus nerve.
As has been shown by Hering and Breuer, and by Head, at each phase of respiration a message is sent from the lung up the vagus. The precise nature of this message is unknown, whether it merely demands the termination of that particular phase, or whether it demands the initiation of the next, or both, is uncer tain; the certain thing is that the change from inspiration to ex piration (and vice versa) which would take place in time apart from vagus influences, is accelerated by them, so that respiration is more rapid and less deep with the vagi intact than with them cut. Animals will, however, live for a long time without their
vagi, and when they die it is not because the power of respiration is deficient. Of the other nerves which lead to the brain, that which most influences respiration is the fifth cranial nerve, as is shown when strong ammonia is placed beneath the nose. The sensory nerves from almost any part of the skin, too, can influence respiration, as when cold water is suddenly dashed on to the sur face of the body.
Types of Respiration.—The visible characters of respiration in man vary considerably accord ing to age and sex. In men, while there is a moderate degree of upheaval of the chest, there is a considerable, although not pre ponderating, degree of excursion of the abdominal walls. In women the chest movements are de cidedly most marked, the excur sion of the abdominal walls being comparatively small. Hence we may distinguish two types of respiration, the costal and the abdominal, according to the pre ponderance of movement of one or the other part of the body wall. In forced respiration the type is costal in both sexes, and so it is also in sleep. The cause of this difference between men and women has been variously ascribed (a) to constriction of the chest by corsets in women, (b) to a natural adaptation to the needs of childbearing in women, and (c) to the greater relative flexibility of the ribs in women permitting a wider displacement under the action of the inspiratory muscles.
In healthy breathing the mouth should be closed and the ingoing current should all pass through the nose. When this happens the nostrils become slightly expanded with each inspiration, probably by the action of the M. dilatores naris. In some people this move ment is hardly perceptible unless breathing be heavy or laboured. As the air passes at the back of the throat behind the soft palate it causes the velum to wave very gently in the current ; this is a purely passive movement. If we look at the glottis or opening into the larynx during respiration, as we may readily do with the help of a small mirror held at the back of the throat, we may notice that the glottis is wide open during inspiration and that it becomes narrower by the approximation of the vocal chords dur ing expiration. This alteration is produced by the action of the laryngeal muscles. Like the 'movements of the nostril, those of the larynx are almost imperceptible in some people during or dinary breathing, but are very well marked in all during forced respiration.