4. =The Lungs. Pulmones.=--Immediately after division of the trachea the two bronchi enter the lungs (Fig. 106). These are two large, much-lobed organs, which fill the greater part of the thoracic cavity. The main lobes of the lungs are completely separated from each other, except in so far as they are connected by the bronchi and connective tissue; the main lobes may also be partly subdivided into secondary lobes that are not thus completely separated. The two lungs are completely separated from each other, except at the =radix=, where they are united by the bronchi; they lie in the right and left halves of the thoracic cavity, with the mediastinal septum between them. The bronchi on entering the lungs divide in the following manner. Each divides at first into two main branches. The cranial branch on the right side is known as the =eparterial= bronchus (_b_), because it lies craniad of the pulmonary artery. All the others are =hyparterial=; i.e., they lie caudad of the pulmonary artery. The right cranial bronchus does not further subdivide into large bronchi, but gives off numerous small branches. The right caudal bronchus divides into three main branches. There are thus four main branches of the right bronchus. The left cranial bronchus divides into two main branches; the left caudal bronchus continues caudad as a main trunk giving off small branchlets. Of the left bronchus there are thus but three main subdivisions. The main subdivisions of the bronchi on the two sides correspond, as will be seen, with the lobulation of the lungs.
The =right= lung (Fig. 106, 1-4) is slightly larger than the left (1′-3′). It divides into three smaller proximal lobes (1-3), and one large distal one (4). The most cranial one of the proximal lobes (1) is sometimes partly subdivided. The third one of the proximal lobes (3) lies mediad of the others; it is partly subdivided and one-half projects into a pocket in the mediastinum, so that it comes to lie across the middle line, extending a short distance onto the left side. This lobe is frequently called the =mediastinal= lobe (3). The caudal lobe (4) of the right lung is large and flat, containing about half the substance of the lung.