([Fig. 105], 5; [Fig. 106], a).—The trachea is that part of the air-passage which extends from the larynx to the bronchi ([Fig. 106]). It is a straight tube composed of a lining mucosa with ciliated epithelium, and a connective-tissue covering which encloses supporting cartilages. Each tracheal cartilage is incomplete dorsally where it lies against the œsophagus, the gap between the two free ends of each ring being filled with muscular and connective tissue. As a result of this the diameter of the trachea is not fixed, but can be increased and diminished. The first ring is broader than the others. Where the œsophagus leaves the median line, the dorsal surface of the trachea lies against the longus colli muscles ([Fig. 72], g″). Its ventral surface is against the sternohyoid ([Fig. 65], e) and sternothyroid ([Fig. 65], g′) muscles. Its lateral surfaces are partly covered by the thyroid gland ([Fig. 96], 6), and are in close relation with the carotid artery ([Fig. 119], a, [page 284]), the vagus and sympathetic nerves ([Fig. 156], i), and the internal jugular vein ([Fig. 119], b). In the thoracic cavity the great vessels coming from the heart lie against the ventral surface of the trachea ([Fig. 129], 2). At about the level of the sixth rib the trachea divides into the two main bronchi ([Fig. 106]). Each bronchus is supported by incomplete rings of cartilage like those of the trachea and has otherwise in general the structure of the trachea. In the lungs the bronchi become divided into many branches ([Fig. 106]), in the manner [described] in the account of the lungs.
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.
Fig. 106.—Ramifications of the Bronchi, with Outlines of the Lobes of the Lungs, Ventral View.
1-4, lobes of the right lung; 1′-3′, lobes of the left lung. a, trachea; b, eparterial bronchus.
The left lung is divided into three main lobes (1′-3′); the two cranial ones (1′-2′) are, however, partly united at the base, so that they may be considered subdivisions of but a single lobe; thus the left lung has but two distinctly separated lobes.
Each lung is attached to the aorta, vertebral column, and diaphragm by a fold of pleura, the pulmonary ligament. This is broadest at the caudal lobe of each lung. Each pulmonary ligament is double, being formed of two sheets of the pleura.
