View of the trachea, showing, first, the division of the tube into the right and left bronchus, and the subdivision of the bronchi into the bronchial tubes; and secondly, the membranous and cartilaginous tissues of which the organ is composed.
364. The second portion of the windpipe termed the trachea (fig. CXXXV. 2), commences at the under part of the larynx (fig. [CXXXV]. 1), and extends as far as the third dorsal vertebra, opposite to which it divides into two branches which are termed the bronchi (fig. [CXXXV]. 3, 4, and [CXXXVII]. ). One of these branches, called the right bronchus, goes to the right lung; the other branch, called the left bronchus, goes to the left lung (fig. [CXXXV]. 3, 4).
365. The trachea of man, like the tracheæ of the air-breathing insect ([351]), is composed of three tissues. These tissues differ essentially from each other in nature, and are widely different in form and arrangement. They consist of membrane, muscle, and cartilage.
366. The membranous portion of the human trachea consists of three coats, the cellular (fig. [CXXXVII]. ), the ligamentous (fig. [CXXXVI]. 8), and the mucous (fig. [CXXXVI]. 9). From the cellular and ligamentous coats the tube receives its strength, and in some degree its elasticity; and the mucous coat constitutes the chief seat of the respiratory function. Between the ligamentous and mucous coats are placed two sets of muscular fibres; the first, the external set, passes in a circular direction around the tube; the second set, placed immediately beneath the circular, is disposed longitudinally, and collected into bundles. The office of the circular fibres is to diminish the calibre of the tube, and that of the longitudinal is to diminish its length.
367. As the tracheæ of the insect are kept constantly open for the free admission of air by their middle membranous tunic, dense, firm, elastic, and coiled into a spiral ([351]), so, for the accomplishment of the same purpose, there are placed between the membranous coats of the human trachea delicate rings of the more highly organized substance, cartilage (35). These cartilaginous rings amount in the entire course of the tube to sixteen or eighteen in number (fig. [CXXXV]. 2); each cartilage being about a line in breadth, and the fourth of a line in thickness. They never form complete circles, but only a large segment of a circle (fig. [CXXXVI]. 7); the circle is incomplete behind (fig. [CXXXVI]. 7, 9), because there the esophagus is in direct contact with the trachea (fig. [CLIII]. 9, 12), and instead of dense and firm cartilage, a soft and yielding substance is placed in this situation, in order that there may be no impediment to the free dilatation of the esophagus during the passage of the food.
368. The point at which the bronchi enter the substance of the lung is called the root of the lung (fig. [CXXXV]. 3, 4). As soon as the bronchi begin to divide and ramify within the lung each cartilage, instead of preserving its crescent shape, is divided into two or three separate pieces, which nevertheless are still so disposed as to keep the tube open. With the progressive diminution in the size of the bronchial branches, their cartilages become less numerous, and are placed at greater distances from each other, until at length as the bronchi terminate in the vesicles, the cartilages wholly disappear; and with the decreasing number and size of the cartilages, the thickness of the cellular, ligamentous, and muscular coats of the bronchi also lessens, until at the points where the cartilages disappear, the muscular and mucous tunics, now reduced to a state of extreme tenuity, alone remain. The essential constituent of the air vesicles, then, is the mucous membrane; but there is reason to suppose that the muscular tunic is likewise continued over these vesicles.
369. It has been stated that the tracheæ of the insect terminate in the different tissues of its body by minute vesicles of an oblong form. The termination of the bronchi in the human lung presents a strikingly analogous appearance. Malpighi, who with extraordinary talent and success devoted his life to the investigation of the minute structures of the various organs of the human body, represents the mucous membrane of the bronchial tubes as terminating in minute vesicles of unequal size: and Reisseissen, who has more recently resumed the inquiry and examined this structure with extreme care, agrees with Malpighi in stating that the bronchial tubes at their terminal points expand into minute, delicate, membranous vesicles of a cylindrical and somewhat rounded figure (fig. [CXXXVIII]. 2). The bronchial tubes do not divide to any great degree of minuteness (fig. [CXXXVIII]. 1), but terminate somewhat abruptly in the vesicles (fig. [CXXXVIII]. 2), which though minute are large enough to be visible to the naked eye (fig. [CXXXVIII]. 2). Viewed in connexion with the bronchial tubes at their terminal points, the vesicles present a clustered appearance, not unlike clusters of currants attached to their stem (fig. [CXXXVIII]. 2).
Fig. CXXXVIII.—View of the Bronchial Tubes terminating in Air vesicles.
Fig. 138.Fig. 139.
External view.—1. Bronchial tube. 2. Air vesicles. Fig. 139. The same laid open.