It is generally held that this economy of breath can be secured, without strain on the vocal cords, only by opposing the action of the inspiratory muscles to the action of the muscles of expiration. Instead of allowing the inspiratory muscles to relax completely at the beginning of the expiration these muscles are to be held on tension throughout the expiration. By this means both the force and the speed of the expiration can be regulated at will; no undue pressure is exerted on the vocal cords, and the tone is prolonged steadily and evenly so long as the expiration lasts. II [Illustration: FIGURE I] The main passage from the lungs to the outer air is the trachea or windpipe, a tube formed of from sixteen to twenty rings of cartilage, united by tendons and muscular fibres, and lined with mucous membrane. At the top of the trachea is situated the larynx, the organ of phonation, strictly speaking. The larynx is built up of two cartilages, the thyroid and cricoid, which represent developments of what once were the uppermost two rings of the trachea. The cricoid cartilage, forming the base of the larynx, is shaped like a seal ring, with the bezel to the back. The thyroid cartilage, just above the cricoid, has the form of an open book, V-shaped in horizontal section. In the interior of the larynx are two tiny cartilages, the right and left arytenoids. These rest on top of the rear portion of the cricoid, on which they rotate freely. Figure I shows two views, front and rear, of the cartilages of the trachea and larynx. Immediately above and connected with the thyroid cartilage is the hyoid bone, which serves as the attachment of the base of the tongue. It is shaped like a horseshoe, the ends pointing to the rear. [Illustration: FIGURE II] Figure II presents a section of the interior of the larynx, all the muscular tissues having been removed. In this cut the position of the arytenoid cartilages is plainly shown. By feeling the outside of the throat with the thumb on one side and the forefinger on the other, the hyoid bone and the thyroid and cricoid cartilages can easily be located. The space between the two cartilages in front is covered by the crico-thyroid membrane. The muscles of the larynx are considered in two groups, the extrinsic, those which connect the larynx with the other parts of the body, and the intrinsic, those which belong to the larynx strictly speaking. By the extrinsic muscles the larynx is held in its place in the throat. These muscles are usually held to have no direct office in phonation. [Illustration: FIGURE III] The intrinsic muscles of the larynx are nine in number, as follows: 2 Thyro-arytenoids, right and left. 2 Crico-thyroids, right and left. 2 Lateral crico-arytenoids, right and left. 2 Posterior crico-arytenoids, right and left. 1 Arytenoideus.