Page:The American Cyclopædia (1879) Volume X.djvu/727

 LUNGS 721 performed by these animals. In others, as in the frogs, and still further in turtles and sau- rians, the lungs are more important and more complicated in structure. They are still air sacs ; but their cavities are divided into a con- siderable number of secondary spaces or areolse, by incomplete partitions, of a thin membranous texture, projecting from their internal surface. This increases very much the extent of the respiratory membrane ; and as the blood ves- sels penetrate everywhere the substance of the partitions, as well as the rest of the pulmo- nary walls, the mutual action of the air in the sacs and the blood in the vessels is greatly in- tensified, and the same result is accomplished in a comparatively short period. Thus pul- monary respiration in these animals is more active in proportion to the increased involu- tion of the pulmonary membrane. This com- plication of the structure of the lungs is car- ried to the greatest extent in the warm-blooded animals and in the human species. In man the respiratory apparatus commences at the larynx, which communicates by the opening of the glottis with the pharynx. The larynx is continuous below with the trachea, a cylin- drical tube, four inches in length and nearly one inch in width, running down the anterior part of the neck, on the median line, to the top of the chest. It is composed of a fibrous membrane, strengthened by a series of cartila- ginous rings, complete in front but open pos- teriorly, which serve to maintain the cylindri- cal form of the trachea, and to keep its cav- ity open for the free passage of the air. It is lined with a mucous membrane covered with ciliated epithelium. At the upper part of Arrangement of Air Passages in the Human Lungs. a. Larynx. 6. Trachea, c, d. Bronchi, e. Bronchial tubes. f. Lobules. the chest the trachea divides into two main branches, of a structure similar to its own, the right and left bronchi, each of which passes to the root of its corresponding lung. Here the bronchi begin to divide, subdivide, and ramify, radiating in smaller and more numer- Two of the Lobules of the Human Lung. a. Small bronchial tube, a ter- minal branch of which goes to each lobule. &. Bounded air vesicles, projecting from the sur- iace of the lobule. ous divisions to every part of the lungs, and becoming constantly more delicate and mem- branous in their texture. They are now called the bronchial tubes ; and their smaller ramifi- cations lose altogether their cartilaginous ele- ment, and consist only of a fibro-elastic tubu- lar membrane. Each terminal bronchial tube finally goes to a special division of the pulmo- nary tissue, termed a lobule. This is an air sac of a more or less conical form, the bron- chial tube opening into it at its apex, while its base is turned in the opposite direction. The interior of the lobule, like the entire lung of the frog, is imperfectly divided by membranous partitions into secon- dary cavities of a cup- like form, all of which, though separated from each other, communi- cate with the central cavity of the lobule. These terminal cavities, into which the air finally penetrates, are called the air vesicles. The air vesicles are rounded cavities, about -fa of an inch in diameter, lined with a single layer of pavement epithelium. They are sur- rounded by a tissue containing a large propor- tion of elastic fibres, in which also ramify the capillary blood vessels. Owing to the small size and excessive multiplication of these air vesicles, the entire extent of respiratory mem- brane in the human lung is very great. It has been estimated by Lieberkuhn at not less than 1,400 sq. ft. ; and there is reason to believe that this estimate is not an exaggeration, The tissue of the lungs is thus of a spongy nature; that is, it contains a mul- titude of minute cav- ities, filled with air, disseminated through a soft, moist, fibrous, and vascular texture. Accordingly, unlike any of the other in- ternal organs, the lungs are lighter than water, and float upon its surface when sep- arated from the body. They also retain the air entangled in their substance with such ob- stinacy that it cannot be expelled by any com- pression or violence short of absolutely disin- tegrating the pulmonary substance; and the lungs, if healthy, will still float upon the sur- face of water even after they have been bruised Air Vesicles of the Human Lung, partially cut across, and highly magnified. a. Fibrous tissue intervening between the vesicles. 6. Layer of epithelium cells, lining the internal surface of a vesicle.