If the larynx of an animal be dissected out, and the vocal cords be stretched, they will vibrate like a piece of caoutchouc or leather in a current of air. In conducting these ex periments, it is necessary to secure the same conditions as those which are required in the laminm above mentioned ; for instance, the inner edges of the glottis must be turned towards each other till they are in the same plane and parallel to one another before they will produce any sound ; hence we infer, that when the tension of the arytenoid ligaments takes place in the living animal, they turn upon their axes till their planes (which in the state of relaxation are inclined to the axis of the vocal tube) become perpendicular to it, and as the edges of the glottis approximate, and its chink is nearly or entirely closed up, they acquire the true vibrating position. The production of the most simple tones of voice requires the associated actions of a most extensive range of organs ; and it is calculated that in the ordinary modulation of the voice, more than one hundred muscles are brought into action at the same time.
The lungs having been first supplied with air by the act of inspiration, and the air in the chest and trachea having subsequently been condensed by the muscles of expiration, a portion of the edges of the glottis yields to its pressure, and is curved upwards, so as to form an angle with the axis r f the vocal tube, leaving between them a narrow aperture through which the air escapes. The tension and elasticity of the vocal ligaments tend to restore them to the plane of the vibrating position ; the air having been rarefied below the glottis during their elevation, becomes condensed on their depression, and the ne cessary force is again accumulated to re elevate the vocal ligaments, and thus an oscilla ting movement, consisting of a partial opening and closing of the glottis, takes place, which being communicated to the contiguous air, the sounds of the voice are produced.
The relative length of the vibrating edge of the glottis is regulated by the pressure of the column of air in the trachea, and the resistance of the vocal ligaments. The intensity of the voice in the same medium, and under similar collateral circumstances, depends on the pressure of the column of air in the trachea, and the range of motion performed by the vibrating edges of the glottis. The vocal ligaments do not vary the pitch of the voice by their tension alone, but by their variations in length and tension conjointly. The author has learnt this from his own experiments on the vocal functions of the larynx, which have been confirmed both by Majendie and Mayo ; the former having observed in the larynx of a dog that a longer portion of the ligaments of the glottis vibrated during the utterance of grave tones, and that the length was dimi nished as the tones became acute. The latter
had an opportunity of inspecting the move ments of the glottis in a man who had made an attempt to destroy himself by cutting his throat. In this case the larynx was divided immediately above the vocal cords, and in consequence of the oblique direction of the wound, the arytenoid cartilage and the vocal cord on one side were injured. During respiration the glottis was observed to assume a triangular form, but when a sound was uttered, the chordw vocales became nearly parallel, and the rima glottidis of a linear form. The posterior part of the aperture did not appear to be closed. In a second case of this kind, he observed that the aryte noid cartilages, as long as the vocalization of the breath continued, maintained the position which they had assumed when the glottis was closed entirely.* The vibrations of the thyro-arytenoid ligaments are considered by Ferrein t to be analogous to those of strings ; hence he denominated these ligaments (though improperly) chordw vocales. He imagined that the longitudinal tension of these cords alone governed the pitch of the voice. Mr. WillisT has embraced the hypothesis of Ferrein ; he observes, that to obtain the various notes of the glottis, it is only ne cessary to vary its longitudinal tension after the ligaments have been placed in the proper position ; but M. Biot § remarks, " Qu'y a-t-il en effet dans la glotte qui ressemble une corde vibrante? Comment pourroit-on en tirer jamais des sons d'un volume com parable h ceux que l'homme produit ? Les plus simples notions d'acoustiques suffisent pour faire rejeter cette etrange opinion." On inspecting the larynx from above, we see two very nearly rectangular-shaped la minae, one on each side of the chink of the glottis, hut nothing resembling an isolated cord. The mucous membrane which lines the thyro-arytenoid ligaments (to which it closely adheres), as well as the rest of the vocal tube, must be considered as forming a part of the weight of the vibrating surface upon which the air acts ; the thyro-arytenoid ligaments confer on this membrane the re quisite tension and resistance during vocaliza tion, and it is this membrane which gives the sides of the glottis their laminated figure. The vocal ligaments, with their lining mem brane, are stretched by the thyro-cricoid muscles, not all round like a drum, but in one direction only, namely, in that of their length, being attached on three sides, leaving one only free to vibrate. The vocal cords are, as has been seen, rectangular-shaped membranes, and from experiments made on the larynx after death by Ferrein, and others (which the author has repeatedly verified), are found to vibrate like cylindrical cords ; we will therefore apply to the former the well-known formulm which regulate the vibrations of the latter.