The root of the lung, as it is called, is formed by the bronchi, two pulmonary arteries, and two pulmonary veins. The nerves and lymphatic vessels of the lung also enter at the root. If we only remember that all the bronchial tubes, great and small, are hollow, we may compare the whole system to a short bush or tree growing upside down in the chest, of which the trachea is the trunk, and the bronchial tubes the branches of various sizes. 207. Minute Structure of the Lungs. If one of the smallest bronchial tubes be traced in its tree-like ramifications, it will be found to end in an irregular funnel-shaped passage wider than itself. Around this passage are grouped a number of honeycomb-like sacs, the air cells[35] or alveoli of the lungs. These communicate freely with the passage, and through it with the bronchial branches, but have no other openings. The whole arrangement of passages and air cells springing from the end of a bronchial tube, is called an ultimate lobule. Now each lobule is a very small miniature of a whole lung, for by the grouping together of these lobules another set of larger lobules is formed. Illustration: Fig. 89. A, diagrammatic representation of the ending of a bronchial tube in air sacs or alveoli; B, termination of two bronchial tubes in enlargement beset with air sacs (_Huxley_); C, diagrammatic view of an air sac. a lies within sac and points to epithelium lining wall; b, partition between two adjacent sacs, in which run capillaries; c, elastic connective tissue (_Huxley_). In like manner countless numbers of these lobules, bound together by connective tissue, are grouped after the same fashion to form by their aggregation the lobes of the lung. The right lung has three such lobes; and the left, two. Each lobule has a branch of the pulmonary artery entering it, and a similar rootlet of the pulmonary vein leaving it. It also receives lymphatic vessels, and minute twigs of the pulmonary plexus of nerves. Illustration: Fig. 90.—Diagram to illustrate the Amounts of Air contained by the Lungs in Various Phases of Ordinary and of Forced Respiration. The walls of the air cells are of extreme thinness, consisting of delicate elastic and connective tissue, and lined inside by a single layer of thin epithelial cells. In the connective tissue run capillary vessels belonging to the pulmonary artery and veins. Now these delicate vessels running in the connective tissue are surrounded on all sides by air cells. It is evident, then, that the blood flowing through these capillaries is separated from the air within the cells only by the thin walls of the vessels, and the delicate tissues of the air cells. This arrangement is perfectly adapted for an interchange between the blood in the capillaries and the air in the air cells. This will be more fully explained in sec. 214.