The larynx is the organ of voice. It is situated between the trachea and the base of the tongue, at the upper and back part of the neck, where it forms a considerable projection in the middle line, called Adam’s apple. The trachea is a cylindric tube, which extends from the larynx downward about 4½ inches, when it divides into the right and left bronchial tubes. The bronchial tubes, on entering the lungs, divide and subdivide, until finally they terminate in a lobule which is composed of air-cells and intercellular passages. In inspiration the cavity of the thorax is enlarged by an active contraction of the muscles, in consequence of which the pressure of air within the lungs becomes less than that of the air outside of the body, and this difference of pressure causes a rush of air through the trachea into the lungs, until an equilibrium of pressure is established between the outside air and that within the lungs. This constitutes _inspiration_. Upon the relaxation of the respiratory muscles, the elasticity of the chest-walls and lungs, aided perhaps, to some extent, by the contraction of certain muscles, causes the chest to return to its original size. In consequence of this, the pressure within the lungs now becomes greater than that outside, and the air rushes out of the trachea, until the equilibrium is once more established—_expiration_. During quiet respiration all parts of the lungs are not equally expanded; it is chiefly the apices of the lungs, reaching up into the region of the neck, and the central parts of the lungs, which undergo the least change of volume. This lack of a thorough distention and aëration of every part of the lungs is a cause of weakness of the lungs as well as of the entire body, for it is precisely those parts of the lungs which are the least active that are most prone to become the seat of tuberculosis. In forced inspiration the cavity of the thorax is increased from 2 to 3 inches, partly by the elevation of the ribs and partly by the descent of the diaphragm, due to the contraction of its muscular fibers. In contracting, the diaphragm presses upon the abdominal viscera, pushing them downward about 3 inches, so that a projection of the flaccid abdominal walls occurs. The movements of the diaphragm are less extensive in women than in men, which is believed to be due to the corsets and general manner of dress. A perfectly free mobility is necessary for change in the size of the chest and lungs, in which the respiratory movements take place from sixteen to twenty times a minute.