359. The apparatus consists of a vessel to carry the air to the blood; a vessel to carry the blood to the air; an organ in which the air and the blood meet; and an organization by which both fluids are put in motion. The vessel that carries the air to the blood is the windpipe (fig. [CXXXV]. 1, 2); the vessel that carries the blood to the air is the pulmonary artery (fig. [CXL]. 7); the organ in which the blood and the air meet is the lung (fig. [CXXXV]. 5); the organization which puts the air in motion, is the structure of bones, cartilage and muscles, called the thorax (figs. [CXLI]. and [CXLVI]. ), and the engine that communicates motion to the blood is the right ventricle of the heart (fig. [CXL]. 5).
360. The windpipe is a tube which extends from the mouth and nostrils to the lung (figs. [CLIII]. 1, 9, and [CXXXV]. 2, 5). It is attached to the back part of the tongue (fig. CLII. 2, 9), and passes down the neck immediately before the esophagus, or the tube which leads to the stomach (fig. [CLIII]. 9, 12).
361. In the different parts of its course the windpipe is differently constructed, performs different offices, and receives different names according to the diversity of its structure and function. The first division of it is called the larynx (fig. [CXXXV]. 1.), the second the trachea (fig. [CXXXV]. 2), the third the bronchi (figs. [CXXXV]. 3, 4, 7, and [CXXXVII]. ), and the fourth the air vesicles or cells (figs. [CXXXV]. 8, and [CXXXVIII]. 2).
Fig. CXXXVI.—Posterior View of the Larynx and Trachea.
1. The os hyoides. 2. Thyroid cartilage. 3. Cricoid cartilage. 4. Arytenoid cartilages, separated from each other. 5. Epiglottis. 6. Opening of the glottis. 7. Termination of the cartilaginous rings of the trachea. 8. The ligamentous portion of the trachea. 9. Trachea laid open, showing its internal mucous surface and follicles, with the anterior portion of the cartilaginous rings appearing through it.
362. The first portion of the windpipe called the larynx (figs. [CXXXV]. and [CXXXVI]. ), constitutes the organ of the voice. It is situated at the upper and fore part of the neck (fig. [CLIII]. 7, 9), immediately under the bone to which the root of the tongue, called the os hyoides (figs. [CLIII]. 6, and [CXXXVI]. 1), is attached. The larynx forms a very complex structure, and is composed of a variety of cartilages, muscles, ligaments, membranes, and mucous glands (fig. [CXXXVI]. 2, 3, 4, 5). At its upper part is a narrow opening of a triangular figure called the glottis (fig. [CXXXVI]. 6), by which air is admitted to and from the lung. Immediately above this opening is placed the cartilage, which obtains its name from its situation, epiglottis (fig. [CXXXVI]. 5), which is attached to the root of the tongue (fig. [CLIII]. 6, 7), and which may be distinctly seen in the living body by pressing down the tongue.
363. The Epiglottis is highly elastic, and is an agent of no inconsiderable importance in respiration, deglutition, and speaking. In respiration it breaks the current of air which rushes to the lungs through the mouth and nostrils, and prevents it from flowing to the delicate air cells with too great a degree of force. During the action of deglutition the epiglottis is carried completely over the glottis (fig. [CLIII]. 6, 7, 8), partly because it is necessarily forced backwards, when the tongue passes backwards in delivering the food to the pharynx (fig. [CLIII]. 6, 7, 8, 10), partly because it is carried backwards by certain minute muscles which act directly upon it, and perhaps also partly in consequence of its own peculiar irritability. The moment the action of deglutition has been performed the epiglottis springs from the aperture of the glottis, partly by its own elasticity, and partly by the return of the tongue to its former position. During the act of speaking the column of air which is expelled from the lung, which rushes through the glottis, and which thus forms the voice, strikes against the epiglottis, and the voice becomes thereby in some degree modified.
