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). [Illustration: 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.] 370. In the insect, for the reason assigned (351), these vesicles are diffused over the system, aërating every point of the body; in man they are concentrated in the lung; yet by their minuteness, and by the mode in which they are arranged, they present in the small space occupied by this organ, so extended a surface that Hales, representing the size of each vesicle at the 100dth part of an inch in diameter, estimates the amount of surface furnished by them collectively at 20,000 square inches. Keil estimating the number of the vesicles at 174,000,000, calculates the surface they present, at 21,906 square inches. Leiberkuhn at 150 cubic feet; and, according to Monro, it is thirty times the surface of the human body. [Illustration: Fig. CXL.