The structures contained in the root of each lung are the branches and tributaries of (I) the pulmonary artery, (2) the pulmonary veins, (3) the bronchi, (4) the bronchial arteries which supply the substance of the lung, (5) the bronchial veins, (6) the bronchial lymphatic vessels and glands, (7) the pulmonary plex uses of nerves. Of these the first three are the largest and, in dividing the root from in front, the veins are first cut, then the arteries and last the bronchi. As the bronchi become smaller and smaller by repeated division, the cartilage completely surrounds them and tends to form irregular plates instead of rings—they are therefore cylindrical, but when the terminal branches (lobular bronchi) are reached, the carti lage disappears and hemispherical bulgings called alveoli occur (fig.
6 A). At the very end of each lobular bronchus is an irregular chamber, the atrium (fig. 6 B), and from this a number of thin walled sacs, about I mm. in diameter, open out. These are called the infundibula (fig. 6 C), and their walls are pouched by hemi spherical air-cells or alveoli like those in the lobular bronchi. Each lobular bronchus with its atrium and infundibula forms a lobule of the lung, and these lobules are separated by connective tissue, and their outlines are evident on the surface of the lung.
The muscular tissue, which in the larger tubes was confined to the dorsal part, forms a complete layer in the smaller; but when the lobular bronchi are reached, it stops and the mucous membrane is surrounded by the elastic layer. In the lobular bronchi, too, the lining epithelium gradually changes from the ciliated to the stratified or pavement variety, and this is the only kind found in the infundibula and alveoli. Surrounding each alveolus is a plexus of capillary vessels so rich that the spaces between the capillaries are no wider than the capillaries them selves, and it is here that the exchange of gases takes place between the air and the blood.
partition stops and here the two tubes communicate. The caudal end of the respiratory tube buds out into the two primary bronchi, and the right one of these, later on, bears three buds, while the left has only two; these are the secondary bronchi, which keep on dividing into two, one branch keeping the line of the parent stem to form the stem bronchus, while the other goes off at an angle. By the repeated divisions of these tubes the complex "bronchial tree" is formed and from the terminal shoots the infundibula bud out. The alveoli only develop in the last three months of foetal life. The thyroid cartilage is probably formed from the fourth and fifth bronchial bars, while the cricoid seems to be the enlarged first ring of the trachea. Before birth the lungs are solid and much less vascular than after breathing is established. (For further details see Quain's Anatomy, vol. i., Lond. 1908.) Comparative Anatomy.—In the lower vertebrates respira tion is brought about by the blood vessels surrounding the gill clefts (see PHARYNX). In the higher fishes (Ganoids and Teleos teans) the "swim bladder" appears as a diverticulum from the dorsal wall of the alimentary canal, and its duct sometimes remains open and at others becomes a solid cord. In the former case it is probable that the blood is to some extent oxygenated in the vascular wall of this bladder. In the Dipnoi (mud-fish) the open ing of the swim bladder shifts to the ventral side of the pharynx and the bladder walls become sacculated and very vascular, so that, when the rivers are dried up, the fish can breathe altogether by means of it. In the S. American and African species of mud fish the bladder or lung, as it may now be called, is divided by a longitudinal septum in its posterior (caudal) part into right and left halves. In this sub-class of Dipnoi, therefore, a general agreement is seen with the embryology or ontogeny of Man's lungs. In the Amphibia the two lungs are quite separate though they are mere sacculated bags without bronchi. A trachea, however, appears in some species (e.g., Siren) and a definite larynx with arytenoid cartilages, vocal cords and complicated muscles is established in the Anura (frogs and toads). In most of the Reptilia the bag-like lungs are elaborated into spongy organs with arborizing bronchi in their interior. From the crocodiles upward a main or stem bronchus passes to the caudal end of the lung, and from this the branches or lateral bronchi come off. The larynx shows little advance on that of the Anura.