The consonants, or short sounds, may also be made without interrupting the current of air, by various modifications of the vocal organs. In sounding singly any one of the letters, we can detect its peculiar requirements. Thus _m_ and _n_ can be made only by blocking the air in the mouth and sending it through the nose; _l_ lets the air escape at the sides of the tongue; _r_ needs a vibratory movement of the tongue; _b_ and _p_ stop the breath at the lips; _k_ and _g_ (hard), at the back of the palate. Consonants like _b_ and _d_ are abrupt, or, like _l_ and _s_, continuous. Those made by the lips are termed _labials_; those by pressing the tongue against the teeth, _dentals_; those by the tongue, _linguals_. The child gains speech slowly, first learning to pronounce the vowel _a_, the consonants _b, m_, and _p_, and then their unions --_ba, ma, pa_. DESCRIPTION OF THE ORGANS OF RESPIRATION.--Beneath the larynx is the windpipe, or _trachea_ (see Fig. 29), so called because of its roughness. It is strengthened by C-shaped cartilages with the openings behind, where they are attached to the œsophagus. At the lower end, the trachea divides into two branches, called the right and left _bronchi_. These subdivide in the small bronchial tubes, which ramify through the lungs like the branches of a tree, the tiny twigs of which at last end in clusters of cells so small that there are six hundred million in all. This cellular structure renders the lungs exceedingly soft, elastic, and sponge-like. [Footnote: The lungs of slaughtered animals are vulgarly called "lights," probably on account of their lightness. They are similar in structure to those of man. They will float on water, and if a small piece be forcibly squeezed between the fingers (notice the creaking sound it gives), it will retain sufficient air to make it buoyant.] FIG. 30. [Illustration: _Bronchial Tubes, with clusters of cells._] 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.