Twelve _air tubules_ have their origin from each common bronchus, or trunk. Four are given off from the internal wall of the main bronchus by a series of openings arranged in a row. Seven are given off from the external wall by a second series similar to that of the first. The twelfth extends from the inferior wall, and immediately takes a course downward and outward and communicates with the posterior diaphragmatic air-sac. This may be considered as the terminal branch of the trunk. All of these secondary canals, except the last, pass toward the periphery of the lung. They divide and subdivide at the periphery, covering it with their ramifications. The canals extending from the inner wall are distributed to the inferior face of the lung. Those extending from the outer wall are distributed to the outer face of the lung. The first constitute the _diaphragmatic_ and the second the _costal bronchial tubes_. The four _diaphragmatic bronchial tubes_ are numbered in the order in which they are given off. The first is carried forward horizontally, the second transversely inward, the third obliquely inward and backward, and the fourth directly backward. They have, by some anatomists, been called the anterior, the internal, and the posterior diaphragmatic bronchial tubes. There are two posterior diaphragmatic bronchial tubes; the larger called the great posterior, and the smaller, which passes directly backward, the small posterior. The _costal bronchial tubes_, seven in number, are numbered from the front backward in the order they are given off. Parallel at their origin, and side by side, like pipes of an organ, they soon spread out in fan-shape like the preceding. They extend from their central origin to the periphery. The first extends obliquely upward and inward to the anterior extremity of the lung. All branches from this bronchus extend from its anterior wall. The first branches are inflected to reach the external border of the lung. The succeeding branches are directed forward and the last forward and inward. They all meet those from the anterior diaphragmatic bronchus, but do not anastomose with them. The second, the third, and the fourth costal bronchi extend in a transverse manner and ramify on the inner border of the lung. The fifth and the sixth are directed toward the posterior extremity of the lung. The seventh, very small, reaches this extremity, where it disappears. The first costal bronchus is the largest; those following it gradually become smaller. At their points of origin they adhere closely to the ribs. They are all imperforate, which is a distinguishing feature from those occupying the opposite face.