The structure of the air-tubes and the lungs themselves next requires consideration. Beginning with the upper portion, we have to consider the trachea, or windpipe, which in the human subject descends in the middle line from the larynx (q.v.) to the level of the third dorsal vertebra, where it divides into the right and left bronchi. It is kept permanently open by front 16 to 20 cartilaginous rings, which surround two-thirds of the tube, and are incomplete behind, where the tube is completed by the same fibrous mem brane which covers and unites the cartilages in front and on the sides. In this fibrous membrane are numerous tracheal glands (which probably furnish much of the vapor of the breath, and may occasion its odor), together with unstriped muscular fiber, to which the term traehealis muscle lms been given. The trachea measures about 44 in. in length, and is about three-quarters of an in. wide. Its mucous membrane is continu ous through the glottis with that of the pharynx or throat, and is covered with ciliated columnar epithelium (q.v.). Of the bronchi, the right is wider, shorter, and more hori zontal than the left. Their walls are composed on the same plan as those of the trachea. Upon entering the lung, each bronchus divides in the method already described. The walls of these bronchial tubes become thinner as they approach the air-cells. The car tilaginous portions which, in the primary divisions of each bronchus, partially retained the annular form, become gradually reduced to mere flakes, and finally cease in tubes of /th or of an in. in diameter. The unstriped muscular fibers occurring in the trachea are continued downward to the minutest tubes, forming a very thin layer, completely surrounding the canal, and the ciliated epithelium extends equally far. The terminal bronchial tube loses its epithelium and muscular coat at about -nth of an in. from the most distant air-cell to which it leads, and is thus reduced to a single coat, consisting of the basement membrane (see Mucous MEMBRANE), with yellow elastic fibers blended with it. Of this structure, the interlobular passages and the air-cells are composed.
The mode in which the blood is perpetually changed in the lungs next demands consideration. The venous or impure blood, collected from all parts of the body in the right side of the heart, is conveyed to the lungs by the pulmonary artery, which is about the size of the aorta, and, like that vessel, is furnished with three semilunar valves at its origin, which prevent the blood from regurgitating into the light ventricle of the heart (see CIRCULATION). The pulmonary artery divides, before entering the lungs, into a right and a left branch, which ramify as far as the lobules in company with the bronchial tubes. At this point, they distribute themselves on the outside of the lobules, in the so-called interlobular fissures, and penetrating between the air-cells, form a capil lary network on and in the walls of the cells and of the lobular passages. This network empties its blood, which is now aerated, into minute venous radicles which converge to form larger veins, and these finally form the four pulmonary veins, which discharge their arterialized blood into the left side of the heart. The walls which support the capillary network of the lungs arc (as Todd and Bowman observe) "for the most part much too thin to inclose the capillaries between the two layers of their substance, and therefore the capillaries project fairly into the air-cells by a great part of their circum ference, being adherent to the wall by a narrow fine only. The capillary wall is thus exposed and bare, in contact with the air of the cell, and nothing besides the delicate membrane of the capillary intervenes between the air and the blood. A capillary fre quently passes through an aperture in the cell-wall, so as first to project into one ca, and further on into a contiguous one, but never becomes altogether free from the wall."—Phys. Anat. v. ii. p. 393. The diameter of these capillaries is about of an in., which is comparatively large. and admits of the passage of blood freely; and the air and the blood may be said to be in contact, since they are only separated by a deli cate capillary wall, less than 20 of an in. in thickness. if the rate of the blood in
the capillaries betaken at an in. and three-quarters per minute (according to the estimate of Valentin, drawn from observation of the frog's foot), it has been calculated that the blood would at each circuit remain in contact with the air about one second and a half. Lr all probability, however, the motion of the blood is quicker in the pulmonary capil• larks of man nod other mammals and of birds than in those of the frog's foot.
In addition to the pulmonary artery and the pulmonary veins, which convey the blood to and from the lungs for the purpose of aeration, there are other vessels, known as the bronchial vessels, for the nutrition of the lung itself, the distribution of which, and their mode of communication with the pulmonary vessels already described. have been subjects of much discussion; but into this we need not enter: The lungs ale sup plied with nerves from the an'terior and posterior pulmonary plexuses, lying at the mot of the organ, and composed of filaments of the pneutnogastrie and sympathetic nerves. The filaments from these plexuses accompany the bronchial tubes, in which they are finally lost. The part which these nerves play in the respiratory process will be con sidered after we have described the movements of respiration, by which the air in the lungs is being perpetually changed.
For a description of the shape and framework of the chest, see CHEST. The chest (or thorax, as it is termed by anatomists) is so constructed as to be capable of enlargement in height (vertically), in depth (or front the front backward), and in width (or from side to side. Its height is increased mainly by the descent of the diaphragm, and to a certain extent by the elevation of the ribs, and the widening of the intercostal spaces; while its depth and width are increased by the elevation of the ribs, which carry forward and elevate the breast-bone (or sternum), especially at its lowest end, and are slightly rotated on an imaginary axis, joining their extremities, by which their central portion is raised, and slightly removed from the mesial plane of the chest. It is only in forced or deep inspiration that all these means of enlarging the chest are called into play. Au ordinary inspiration is attended in men with very slight elevation of the ribs (about one-twentieth of an inch), while in women the elevation is much greater, especially in the upper ribs; the cause of this difference in the sexes probably lying in the narrower waist of the female requiring a compensation in the upper part of the chest. MM. Beau and Maissiat describe three varieties of ordinary respiration—viz.: 1. Abdominal, or that chiefly effected by the diaphragm, and seen in the motion of the walls of the belly; 2. Costo inferior, or that in which the seven lower ribs are observed to act; and 3. Costo-superior, or that effected in a considerable degree by the upper ribs. The first variety occurs in infants up to the end of the third year, and in males generally; the second in boys after the age of three, and in men; and the third in adult females. Our limited space pre cludes a detailed notice of the various muscles which are concerned in respiration. The total power of the respiratory muscles has been measured by several physiologists, among whom Dr. Hutchinson deserves special notice. He finds, as the average of 1500 experiments, that the power of expiration is nearly one-third stronger than that of inspi ration, and he is of opinion that when the expiratory are not stronger than the inspiratory muscles, some disease is present. He ter,ted the force of the two classes of respiratory muscles by causing persons to make the most powerful efforts of which they were capa ble, when breathing through the nose into an instrument termed a spirmneter, and by this means he found that men of 5 ft. 7 or 8 in. in height have the greatest inspi ratory power, it being_ equal. on an everage, to a column of mercury of 2.75 in., while their expiratory power was equal to 3.97 inches. The following table is given by hint as exhibiting the range through which these powers may vary within the limits of health, Power of Power of Inspiration. Expiration.