478 RESPIRATION dilatatores naris. In some people this movement 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 respira- tion, 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 during 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 ordinary breathing, but are very well marked in all during forced respiration. The Mechanics of Respiration. In the description of the anatomy of the thorax it has been shown that the thorax is practically a closed box entirely filled by the lungs, heart, and other structures contained within it. If we were to freeze a dead body until all its tissues were rigid, and then were to remove a portion of the chest wall, we should observe that every corner of the thorax is accurately filled by some portion or other of its contents. If we were to perform the same operation of removing a part of the chest wall in a bsdy not first frozen we should find, on the other hand, that the contents of the thorax are not by any means in such circumstances bulky enough to fill up the space provided for them. If we were to measure the organs carefully we should find that those which are hollow and whose cavities communicate with the regions outside the thorax are all larger in the frozen corpse than in that which was not frozen. In other words, the organs in the thorax are distended somewhat in order that they may completely fill the chest cavity ; and the nature of this curious and important condition may best be illustrated by the simple diagrams, figs. 3 and 4 (from Hermann's Physiologic des t 1 t Fig. 3 Fig. 4. Menscken), where t is the trachea, I the lung, v the auricle of the heart, k the ventricle, i an intercostal space with its flexible membranous covering. When the interior of the vessel is rendered vacuous by exhaustion through the tube o, the walls of the lungs and heart are expanded until the limits of the containing vessel are accurately filled, while all flexible portions of the walls of the vessel (corre- sponding to the intercostal membranes and the diaphragm of the thorax) are sucked inwards. From this description it follows that the lungs, even when the thorax is most contracted, are constantly over- distended, and that, when the cause of this over-distension is removed, the lungs, being elastic, collapse. It further follows that if the thorax is dilated, the flexible hollow organs it contains must perforce be still more distended, a distension which in the case of the lungs is followed l>y an indrawing of air through the trachea in all cases where the trachea is open. Thus, as the act of respiration is primarily a dilatation of the thorax, the part played by the lungs is, as Galen knew, a purely passive one. How is dilatation of the thorax effected 1 It has been pointed out that the rib-planes decline from the horizont in two directions, viz., from behind forwards, and fror the antero-posterior mesial plane outwards ; a glance fig. 5 will make this double sloping clear to the reader. It has, moreover, been explained that the diaphragm arches upwards into the thorax in such a manner that the lateral parts of the arch are vertical and in contact with the inner face of the tho- racic walls (see fig. 2). This being the structure of the thorax, the enlargement of its cavity is brought about (1) by raising the rib- planes until they ap- proach the horizontal, and (2) by depressing the diaphragm and making its rounded dome more cone-like in outline. A moment's consideration will show how these actions en- large the boundaries of the thorax. (a) When the postero-an- terior slope of the rib- planes is diminished by the raising of the anterior ends of the ribs, the whole ster- num is thrust upwards and forwards, and the antero- posterior diameter of the thorax is increased, (b) When the lateral slope of the rib-planes is diminished by the ribs being moved upwards about an axis passing through their sternal and vertebral extremities, it is evident that the lateral diameter of the thorax must be increased, (c) When the muscular portion of the diaphragm contracts, the curves of its dome-like shape are straightened, the whole diaphragm comes to look more conical on section, and the apposition of its lateral parts to the inner surface of the thorax is destroyed ; the two apposed surfaces are drawn apart much as the leaves of a book might be, and a space is formed between them, into which some portion of the lung slips, (d) When the diaphragm descends it draws with it the whole contents of the thorax ; inasmuch as the contents as a whole are conical in shape with the apex upward and are fitted into the conical space of the thoracic cavity, it is clear that the descent of the contents will tend to create a space between them and the thoracic walls ; for each stratum of lung, &c., which is adapted to fit a certain level of thorax, will thereby be brought into a lower and (as the thorax is conical) a more spacious level. Hence the descent of the diaphragm causes a much greater enlargement of the thorax than is measured by the mere elongation of the vertical diameter. In this manner the thorax is distended and air is drawn into the lungs. The contraction of the thorax in expiration is brought about by the return of the ribs and diaphragm to their original position of rest. FIG. 5. Showing slope of ribs. (From Hermann's Handbuch.)
