280 RESPIRATORY APPARATUS OF MAMMALS. this cavity, by their summit or apex; and are covered by a serous membrane termed the pleura, which also lines the thorax, being reflected from one surface to the other precisely in the manner of the pericardium (§ 43). Thus the pleura of the outer surface of the lung is continually in contact with that which forms the inner wall of the thorax; they are both kept moist by fluid secreted from them; and they are so smooth, as to glide over one another with the least possible friction. d The lungs themselves are very minutely subdivided; and thus expose a vast extent of surface in proportion to their size. The air-cells of the human lung, into which the air is conveyed by the branches of the wind-pipe, and on the walls of which the blood is distributed, do not average above the 1-100th of an inch in diameter. In the accompanying figure is represented, on one side, the lung, d, presenting its natural appearance; and on the other, the ramifications of the airpassages or bronchial tubes, c, e, by which air is conveyed into every part of the Fig. 162.-AIR-TUBES AND LUNG OF MAN. lungs. The trachea or wind pipe, b, opens into the pharynx or back of the mouth, by the larynx, a. The construction of this is especially destined to produce the voice, and will be explained under that head (Chap. XIII.); but it may be here mentioned that the entrance from the pharynx into the larynx consists of a narrow slit, capable of being enlarged or closed by the separation or approximation of its lips, which form what is called the glottis. The aperture of the glottis is regulated by the muscular apparatus of the larynx; the actions of which are not under the direct control of the will, but are automatic, like those concerned in swallowing (§ 194); and the purpose of this provision is to RESPIRATORY APPARATUS OF MAMMALS. 281 prevent the entrance of anything injurious into the windpipe. Thus if we attempt to breathe carbonic acid gas, which would produce an immediately fatal result if introduced into the lungs, the lips of this chink immediately close together, and so prevent its entrance. The contact of liquids or of solid substances, too, usually causes the closure of the aperture, so that they are prevented from finding their way into the windpipe; but this does not always happen, especially when the glottis is widely opened to allow the breath to be drawn-in (§ 193). 329. The larynx, trachea, and bronchial tubes, to their minutest ramifications, in all air-breathing Vertebrata, are lined by a mucous membrane continued from the back of the throat; and this membrane, like the gills of aquatic animals, is covered with cilia, which are in continual vibration. It is obvious, however, that the purpose of this ciliary movement must be here different from that which is fulfilled by the same action on the surface of the gills (§ 319); and it probably serves to get rid of the secretion which is being continually poured out from the surface of the mucous membrane, and which, if allowed to accumulate there, would clog up the aircells, and in time produce suffocation. The vibration of the cilia is observed to be always in one direction,-towards the outlet; and it is in this direction, therefore, that the fluid is gradually but regularly conveyed. The ciliary movement may be seen to take place on the surface of the mucous membrane of the nose; but not on that of the pharynx, where it would be continually interrupted by the passage of food. 330. The constant renewal of the air in the lungs is provided for, in Mammals, by a peculiar mechanism, which accomplishes this purpose most effectually, though itself of the most simple character. We must recollect that the thorax in this class is an entirely closed cavity. It is bounded above and at the sides by the ribs (the space between which is filled up by muscles, &c.), and below by the diaphragm, which here forms a complete partition between the thorax and abdomen. The whole of this cavity, with the exception of the space occupied by the heart and its large vessels (and also by the gullet, which runs down in front of the spine), is constantly filled-up by the lungs. Now the size of this cavity may be made to vary considerably;—in the first place, by the 282 RESPIRATORY MOVEMENTS OF MAMMALS. movement of the diaphragm; and in the second, by that of the ribs. 331. 1. The diaphragm, in a state of rest or relaxation, forms a high arch, which rises into the interior of the chest, as at g, fig. 163; but when it contracts, it becomes much flatter (though always retaining some degree of convexity upwards), and thus adds considerably to the capacity of the lower part of the chest. The under side of the diaphragm is in contact with the liver and stomach, which, to a certain degree, rise and fall with it. It is obvious that, when the diaphragm descends, these organs, with the abdominal viscera in general, must be pushed downwards; and as there can be no yielding in that direction, the abdomen is made to bulge forwards when the breath is drawn-in. On the other hand, when the contraction of the diaphragm ceases, the abdominal muscles press back the contents of the abdomen, force up the RESPIRATORY MOVEMENTS OF MAMMALS. 283 liver and stomach against the under side of the diaphragm, and cause it to rise to its former height. 332. 11. The play of the ribs is rather more complicated. These bones, c c, and c c' (to the number of twelve on each side in Man), are attached at one end by a moveable joint to the spinal column, a; whilst at the other they are connected with the sternum (breast-bone) by an elastic cartilage. Now each rib, in the empty state of the chest, curves downwards in a considerable degree; and it may be elevated by a set of muscles, of which the highest, i, are attached to the vertebræ of the neck and to the first rib, whilst others, e, e, e (termed intercostals), pass between the ribs. The cartilages also curve downwards in the opposite direction, from their points of attachment to the sternum. When the breath is drawn-in, Now the first rib is raised by the contraction of the muscles, i; and all the other ribs, which hang, as it were, from it, would of course be raised by this action to the same degree. But each of them is raised a little more than the one above it, by the contraction of its own intercostal muscle; and thus the lower ribs are raised very much more than the upper ones. by the raising of the ribs, they are brought more nearly into a horizontal line, as are also their cartilages; and since the combined length of the two is greater, the nearer they approach to a straight line, it follows that the raising of the ribs must throw them further out at the sides, and increase the projection of the sternum in front, thus considerably enlarging the capacity of the chest in these directions. When the movement of inspiration is finished, the ribs fall again, partly by their own weight, partly by the elasticity of their cartilages, and partly by the contraction of the abdominal muscles which are attached to their lower border.-For the full understanding of this description, it is desirable that the reader should examine the movements of his own or another person's chest, by placing his fingers upon different points of the ribs, and watching their changes of position during the drawing-in and the expulsion of the breath. 333. Now the cavity of the thorax is itself perfectly closed; so that, if it were not for the expansion of the lungs, a void or vacuum would be left when the diaphragm is drawn down and the ribs elevated. The atmosphere around presses to fill that vacuum; but this it can only 284 RESPIRATION IN MAN. do by entering the lungs through the windpipe, and inflating them (or blowing them out), so as to increase their size in proportion to the increase of the space they have to fill. In this manner the lungs are made constantly to fill the cavity of the chest, however great may be the increase in the latter. But if we were to make an aperture through the walls of the chest, the air would rush directly into its cavity, when the movements of inspiration are performed, and the lung of that side1 would not be dilated. The same thing would happen if there were an aperture in the lung itself, allowing free communication between one of the larger bronchial tubes and the cavity of the chest; for the air, although still drawn-in by the windpipe, would pass directly into the cavity of the chest, rather than dilate the lung, which would thus become entirely useless. Such an aperture is sometimes formed as the result of disease; and if the action of both lungs were thus prevented, death must immediately take place from suffocation. 334. The extent of the respiratory movements varies considerably; but in general it is only such as to change about the seventh part of the air contained in the lungs. (It may be generally noticed, that every fifth or sixth inspiration in Man is longer and fuller than the rest.) Their rate varies according to age, and to the state of the nervous system; being faster in infants and in young persons than in adults; and more rapid in states of mental excitement, or irritation of the bodily system, than in a tranquil condition. In a state of rest, from 14 to 18 inspirations take place every minute in an adult, and at each about 20 cubic inches of air are drawn-in; but both the depth and frequency of the inspirations are considerably increased by exercise. Taking an average alternation of activity and repose, it appears that about 360 cubic feet of air pass through the lungs every twenty-four hours, or 15 cubic feet every hour; and as the air which has once passed through the lungs contains about 1-24th part of carbonic acid, about 15 cubic feet of that gas, containing nearly 8 ounces of solid carbon, are thrown-off in the course of twenty-four hours. 335. Now carbonic acid, when diffused through the atmosphere to any considerable amount, is extremely injurious to 1 Each lung has its own cavity; the two being completely separated from each other by the pericardium (§ 43). |