The human voice is produced by an expiratory blast of air being forced through the narrow opening at the top of the windpipe, called the glottis. This glottis, which lies in the lower part of the larynx, is bounded on each side by the edges of thin, elastic, membranous folds that project into the air passages. These membranous folds, called the vocal cords, are set vibrating by the current of air from below, and in turn communicate their vibrations to the air in the air passages situated above them.
The vocal apparatus produces sound in the same manner as a musical instrument of the reed-pipe variety. If we compare it with the pipe of an organ, we find all the parts of the latter represented. The lungs within the moving thorax act as the bellows. The bronchi and trachea are the supply pipes and air box. The vocal cords are the vibrating tongues; while the larynx, pharynx, mouth and nose act as the accessory or resonating pipes. The blast of air is produced and regulated by the respiratory muscles; and special intrinsic muscles of the larynx change the conditions of the vocal cords so as to alter the pitch of the notes produced. Other sets of muscles, by altering the conditions of the resonating pipes, give rise to many modifications in the vocal tones, and thus produce what is called speech.
The larynx, which may be regarded as the special organ of voice, is made up of four cartilages, viz., the cricoid, thyroid and two arytenoids, jointed together so as to allow of considerable motion. Of these the inferior, the cricoid, is attached to the trachea, which it joins to the others. It forms a ring, which is thin in front, but deep and thick behind, owing to a peculiar projection upward of its posterior part. The thyroid consists of two side wings so bent as to form the greater part of the anterior and lateral boundaries of the voice box, and can be felt easily in the front of the throat. It is articulated to the sides of the cricoid by its two inferior and posterior extremities, so that the upper part of the cricoid cartilage can move backward and forward. The arytenoid cartilages are little three-sided pyramidal masses placed on the upper surface of the posterior part of the cricoid, to which they are attached by a loose joint. They are so placed that one surface looks inward, the second backward, and the third forward and outward, while the inferior surface rides on the cricoid. One point looks forward, and to it is attached the vocal cord on each side, hence it has been called the vocal process. The apex, which lgoks outward and backward, gives attachment to some of the intrinsic muscles, and hence has been called the muscular process.
The thyroid cartilage is connected with the cricoid below, and with the hyoid bone above by ligaments and tough membranes, which hold the parts together, fill in the intervals, and complete the skeleton of the larynx.
The vocal cords are composed of small strands of elastic tissue, which are stretched between the anterior processes of the arytenoid cartilages and the inferior part of the thyroid, where they are attached side by side to the posterior surface of the angle formed by the junction of the two lateral parts or alae of the thyroid. The mucous membrane which lines the larynx is thin, ind closely adherent over the vocal cords. The surface of the laryngeal cavity is smooth and even, the lining membrane passing over the cartilages and muscles so as to obliterate all ridges except the vocal cords and two others, less sharply defined, called the false vocal cords, which lie parallel to and above the true vibrating cords. Between these is the cavity known as the ventricle of the larynx.
Fig.196. Anterior half of a transverse vertical section through the larynx near its middle, seen from behind. More is cut away on the upper part of the right side. 1. Upper division of the laryngeal cavity; 2. Central portion; 3. Lower portion continued into 4, trachea; e, epiglottis; e', its cushion; t, thyroid cartilage seen in section, vl, true vocal cord at the rima glottidis; s, ventricle of larynx; s', saccule. (A. Thomson).