RESPIRATION 475 interchange of oxygen and carbon dioxide takes place at the surface of the organism, so far as we know, continu- ously. In the higher animals, constituted as they are of a vast number-of structural units accurately packed together, each resembling more or less in its physiological instincts the unicellular amoeba, respiration presents a much more intricate problem. The fine interstices which exist between the structural elements do indeed contain a small quantity of a fluid medium which serves the function of the water bathing the amoeba ; but the store of oxygen in the medium would speedily become exhausted, and the emitted carbon dioxide would quickly accumulate to a dangerous degree, if the medium were not continually restored to its original purity. This revival is effected by the circulating blood which is brought by its capillaries into the neighbour- hood of the remotest cells of the body. But even the mass of the blood is small compared with that of the cells it nourishes ; unless it be itself purified and restored in turn the interstitial juices which depend upon it for their purification must soon fail to support the respiration of the cells. Such restoration of the blood takes place in certain organs called lungs or gills, where the blood acquires a fresh store of oxygen and parts with its excess of carbon dioxide. Respiration in the higher animals may therefore be divided into (1) internal respiration, or the interchange of oxygen and carbon dioxide between the cells of the body and the fluid drenching them, and (2) external respiration, or the gaseous interchange taking place in the special re- spiratory organs (lungs, gills). The first is really a part of NUTRITION (q.v.) ; the second, or respiration proper, is the subject of the present article. It will be evident on reflection that the process of respiration naturally falls to be described under two divisions, the first of which is concerned with the move- ments of the chest in inspiration and expiration and the manner in which they are brought about, and the second with the interchange of gases which takes place between the blood and the air in the lungs. THE MOVEMENTS OF RESPIRATION. Structure of the Organs of ^Respiration. In order to understand the movements it is necessary first to know the structure of the air passages and thorax. Anatomy of the Air Passages. The essential organs of respira- tion consist of an air tube called the trachea, communicating at its upper end with the mouth and bifurcating below into two bronchi, one on the right hand and one on the left. Each bronchus divides and subdivides, diminishing in calibre at every division until a diameter of about 1 mm. is attained ; such a diminutive bronchial tube is called a bronchiole. Every bronchiole is a cylindrical tube which divides dichotomously and rapidly several times, and finally terminates in irregular alveolar jmssages. The sides of the alveolar passages, and of the subdividing bronchioles in less abundance, are studded with hemispherical dilatations called air cells or alveoli. The terminal portion of an alveolar passage, with its air cells, is sometimes spoken of as an infundibulum a term we may wisely forget at once, since it points to a distinction where no essential difference exists. The alveoli cluster in great abundance about the alveolar passages, and, although we have spoken of them as hemispherical, they are in reality made polygonal by mutual com- pression. They are surrounded by connective tissue of a very elastic quality, which gives to their delicate walls a firm support, and is so disposed about them that all the alveoli derived from one bronchiole are more closely knit together than they are bound to those of a neighbouring bronchiole ; hence we may speak of a bronchiole with the assemblage of its members as a lobule, a term of peculiar importance since it will be evident on reflection that each lobule contains all the essential parts of a lung is in fact a lung in miniature. By connecting tissue the lobules are compacted to form lobes, of which two on the left side and three on the right go to make lip the respective lungs. The trachea and larger bronchi are composed of a series of super- posed crcscentic pieces or imperfect rings of hyaline cartilage which with a tough fibrous and elastic membrane form a tube ; in the trachea the incomplete cartilage-rings are so placed that their hiatuses are at the back. In the bronchial tubes, especially the smaller ones', the cartilages are fewer, and less regular in shape, being in fact mere nodules in the bronchial walls. In the bronchioles there is no cartilage. There is an external fibrous and elastic layer investing the trachea and bronchi ; and stretching from tip to tip of the imperfect cartilaginous rings are bands of involuntary muscular fibres. The function of the cartilages is doubtless to maintain the patency of the primary air passages by resisting external compression ; that of the muscular fibres is pro- bably to resist unusual distension of the tubes, as in the strain of coughing, &c. The mucous membrane of the trachea consists of a mucosa of fine connective tissue, mixed with some tissue of the ade- noid sort, and with elastic fibres, disposed in longitudinal bundles. Beneath the mucosa is the slight submucosa, which supports large blood-vessels, lymphatics, and mucous glands, and unites the mucosa to the cartilages. Above the mucosa lies the basement membrane, supporting a stratified epithelium, the upper cells of which are columnar and ciliated and may include amongst them the well-known goblet-cells, while the deeper layers are squamous and capable of regenerating the upper stratum if this be lost. The epithelium is pierced by the ducts of glands. Blood-capillaries, lymphatics, and fine nerves have all been shown to exist in the tracheal mucosa. The same structural elements as are found in the trachea con- tinue to be found in all the series of bronchial tubes above 1 mm. in diameter ; they are arranged in much the same order, save that the muscular fibres are relatively increased and that they surround the whole tube (within the zone occupied by the cartilaginous nodiiles) somewhat like the tunica media of an artery. The mucous membrane is still ciliated, but its surface-cells are squat. On the other hand the bronchioles and alveolar passages have a different structure : cartilage and glands are no longer found ; the epithelium is flat and ciliated ; and the muscular fibres gradually thin out on the walls of the alveolar passages. It is, however, the air-cells which have most interest for the physiologist. They are about roirth inch in diameter, and their walls are made up of a delicate film-like basement membrane, on tho outside of which are numerous elastic fibres. Elastic connective tissue intervenes between neighbouring alveoli, and mingled with it are not a few involuntary muscular fibres. The alveoli are lined with flat cells in a single layer, some cells being large, clear, and polygonal, while others are small and granular, and are found singly or in groups of two or three inserted between the edges of the clear sort. The cells are united together by cement in which fine holes are sometimes to be seen ; through these holes, or pseudt- stomata, migratory leucocytes can make their way into and out of the alveoli. FKI. 1. Histology of the hing-ve*icles. V, V, blood-vessels bordering on the alveoli; c, c, blood capillaries of an alveolus; /, /, alveolar epithelium shown separately; E,E, relative position of alveolar epithelium and blood-capillaries ; e, e, elastic texture of lung-substance. (From Hermann's Handbuch der Physiologic.) The blood-vessels of the lung are of two sorts, nutritive and functional, i.e., concerned in the function of the organs. The former are called bronchial, and arise from the aorta or intercostal arteries. They serve to nourish the tissues of the lung, and the blood they contain finds its way, in part into the bronchial veins and thence into the vena azygos, intercostal vein, or superior vena cava, and in part into veins of the functional system. The latter system of vessels consists of the pulmonary arteries, which arise in the
