The stiff, cartilaginous rings, so noticeable in the rough surface of the trachea and the bronchi, disappear as we reach the smaller bronchial tubes, so that while the former are kept constantly open for the free admission of air, the latter are provided with elastic fibers by which they may be almost closed.
WRAPPING OF THE LUNGS.—The lungs are invested with a double covering—the pleura—one layer being attached to the lungs and the other to the walls of the chest. It secretes a fluid which lubricates it, so that the layers glide upon each other with perfect ease. [Footnote: These pleural sacs are distinct and closed; hence, when the ribs are raised, a partial vacuum being formed in the sacs, air rushes in, and distends the pulmonary lobules.] The lungs are lined with mucous membrane, exceedingly delicate and sensitive to the presence of anything except pure air. We have all noticed this when we have breathed any thing offensive.
FIG. 31.
[Illustration: A, the heart; B, the lungs drawn aside to show the internal organs; C, the diaphragm; D, the liver; E, the gall cyst; F, the stomach; G, the small intestines; H, the transverse colon.]
THE CILIA.—Along the air passages are minute filaments (cilia, Fig. 32), which are in constant motion, like a field of grain stirred by a gentle breeze. They serve to fan the air in the lungs, and produce an outward current, which is useful in catching dust and fine particles swept inward with the breath.
HOW WE BREATHE.—Respiration consists of two acts—taking in the air, or inspiration, and expelling the air, or expiration.
FIG. 32.
[Illustration: B, a section of the mucous membrane, showing the cilia rising from the peculiar epithelial cells on the outside of the mucous membrane lining the tubes; A, a single cell more highly magnified.]
1. Inspiration.—When we draw in a full breath, we straighten the spine and throw the head and shoulders back, so as to give the greatest advantage to the muscles. [Footnote: If we examine the bony cage of the thorax or chest in Fig. 8, we shall see that the position of the ribs may alter its capacity in two ways.
1. As they run obliquely downward from the spine, if the sternum or breastbone be lifted in front, the diameter of the chest will be increased.