The lungs are united with the heart and with the trachea by a part called the root, which, in each side, is composed of the large bronchus, a branch of the pulmonary artery, two pulmonary veins, and smaller vessels and nerves, the whole being almost completely covered by the pleura. Each lung is divided into lobes, two in number in the left one and three in the other. Each lobe is divided into lobules, which are arranged on the bronchial tubes like grapes on a hunch. Each lobule is surrounded by condensed areolar tissue mixed up with yellow elastic tissue. Each lobule is a fair representation, on a small scale, of a whole lung, as it hangs upon a bron chial tube, a branch of the pulmonary artery, branches of bronchial vessels and nerves. In the roots of the lungs the two large bronchi divide, the right into three, and the left into two bronchial tubes, one for each of the pulmo nary lobes. The primary bronchial tubes are very short, and divide into two or three smaller tubes, each of which gives off two or three divi sions. Before reaching their termination, the tubes branch off four or five times more.
The weight of the lungs varies much accord to age and sex. In adult men the two lungs weigh from 40 to 50 ounces, and in women from 28 to 35 ounces, thus, being the lightest of the organs of the body. The ratio of the weight of the lungs to that of the body is as 1 to 30 or 40. The specific gravity of the lungs is very slight, and, unless the air has been expelled from the cavities of the bronchial tubes and of the alveoli, any part of the lungs dipped into water will rise and float.
As respiratory organs the lungs bear a cer tain average relation to the physical proportions and condition of the individual. The average amount of air in the case of an individual five feet eight inches in height that goes in and out of the lungs at each inspiration and expiration is about 20 cubic inches; this is called the tidal air. This is only about one-seventh of the total capacity of the lungs, and as the ordinary adult takes about 18 breaths a minute, the air of the lungs would be renewed only about twice a minute if it were not for the diffusion of gases.
By means of forced inspiratory movements the ingoing tide may be increased by 120 cubic inches; by means of a forced expiration the outgoing tidal air may be increased by 90 cubic inches. After the most forced expiration pos sible there always remain within the lungs about 90 to 100 cubic inches of air. So that if a person takes as deep a breath as possible, and then makes as forced an expiration as he can, he will drive out 120-1-20+9230 cubic inches of air. This is termed the respiratory capacity. Since the tidal air is only 20 cubic inches, and 180 cubic inches remain in the chest after an ordinary expiration, it follows that the air di rectly changed during the respiration is not that really within the lungs themselves, but is that within the nose, windpipe and larger bronchi, the pipes that result from the branching of the windpipe. Therefore the changes of the air within the essential parts of the lungs are the result of diffusion between it and the purer air of the bronchi, aided by the rush with which the tidal air flows in. The total amount of air inhaled daily by an average person at rest is estimated to be nearly 700,000 cubic inches, while if he is working the amount is of course much larger. This is a basis for the planning of the size and ventilation of schoolrooms and other public places.
The ordinary respiratory movements differ in the two sexes and at different periods of life. In young children the chest is altered in size chiefly by the movements of the diaphragm, and the protrusion of the abdominal wall during inspiration is therefore very marked. In men also it is the diaphragm which is chiefly opera tive, but the ribs are also moved. In women it is the movement of the ribs, especially the upper ones; which is the most extensive. The respiratory rhythm is the relation of the acts of inspiration and expiration to each other as regards time. See ANATOMY; BREATHING AND