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. [Illustration: Fig. CXXXVII. View of the trachea, showing, first, the division of the tube into the right and left bronchus, and the subdivision of the bronchi into the bronchial tubes; and secondly, the membranous and cartilaginous tissues of which the organ is composed.] 364. The second portion of the windpipe termed the trachea (fig. CXXXV. 2), commences at the under part of the larynx (fig. CXXXV. 1), and extends as far as the third dorsal vertebra, opposite to which it divides into two branches which are termed the bronchi (fig. CXXXV. 3, 4, and CXXXVII.). One of these branches, called the right bronchus, goes to the right lung; the other branch, called the left bronchus, goes to the left lung (fig. CXXXV. 3, 4). 365. The trachea of man, like the tracheæ of the air-breathing insect (351), is composed of three tissues. These tissues differ essentially from each other in nature, and are widely different in form and arrangement. They consist of membrane, muscle, and cartilage. 366. The membranous portion of the human trachea consists of three coats, the cellular (fig. CXXXVII.), the ligamentous (fig. CXXXVI. 8), and the mucous (fig. CXXXVI. 9). From the cellular and ligamentous coats the tube receives its strength, and in some degree its elasticity; and the mucous coat constitutes the chief seat of the respiratory function. Between the ligamentous and mucous coats are placed two sets of muscular fibres; the first, the external set, passes in a circular direction around the tube; the second set, placed immediately beneath the circular, is disposed longitudinally, and collected into bundles. The office of the circular fibres is to diminish the calibre of the tube, and that of the longitudinal is to diminish its length.