CHAPTER IX.
THE LUNGS AND RESPIRATION.
Besides the heart and the great vessels the chest
contains the lungs, the chief organ of respiration, which,
with the rest of the respiratory system, will now be
treated. The nose and mouth, through which the air
first enters the body, have already been spoken of.
From them the air passes through the larynx to the
trachea, thence to the bronchi, and so to the lungs,
where the supply of oxygen for the tissues is taken from
the air by the hemoglobin of the blood.
The Larynx.—The larynx lies in front of the pharynx at the upper and fore part of the neck, where it causes a considerable projection, known as Adam's apple. It is a triangular box, base up, flattened at the back, in front, and at the sides, but becoming cylindrical below. Above it opens into the bottom of the pharynx and below into the trachea. It is lined with mucous membrane. Its opening at the base of the tongue is closed during swallowing by a little door-like valve of fibro-cartilage, the epiglottis, to prevent the entrance of food.
Nine cartilages go to make up the larynx, of which the most important are the thyroid and cricoid cartilages and the epiglottis already mentioned. The thyroid is the largest and is open behind, its two alæ or wings meeting in an acute angle in front and forming the Adam's apple, always more prominent in the male than in the female. It is attached above to the hyoid bone and has cornua or horns on either side, top and bottom. The cricoid or ring-like cartilage resembles a seal ring with the stone placed posteriorly. It is stronger than the thyroid and forms the lower part of
