abl3 grs. - (4.) 2/NQ In an adult male I found that the two vocal ligaments, when divested of mucous mem brane, weigh one grain, which is scarcely one half their weight by theory; hence it appears that a considerable portion of mucous mem brane is connected with the vocal cord in the production of sound, which agrees with the anatomy of these parts.
It is now necessary to offer some explana tion respecting the vital state of the vocal ligaments. The state of repose is the ordi nary condition of the vocal ligament in the living subject, when the voice is not exercised ; but we must not therefore conclude it to be incapable of further contraction. In fact, the state of repose during life is a state of tension, for the ligaments being connected with the thyro-arytenoid muscles, not in a few points, but continuously throughout their whole length, must obey the motion of these muscles, which, like all other muscles, are in a state of tension during repose. We also know by experience that when we produce a sound lower than the usual pitch of our voice, the crico-thyroid chink is opened principally by the contraction of the same muscles, and the ligaments'must therefore at the same time be relaxed. It appears, then, both from the anatomy and physiology of the human larynx, that the ordinary state of the vocal cords is one of considerable tension, which admits of being lessened, and thereby produces the range of lower notes. If we suppose the glottis to be partially closed when we are talking, that is, at the ordinary pitch of our voice, and to be more opened as the tones become graver, this of course will co-operate with the relaxation of the vocal cords. in the production of the higher notes, the crico thyroid chink closes, and the thyro-arytenoid muscles, and consequently the ligaments, are elongated. Since, therefore, the vocal liga ments have been proved to extend and con tract fbr acute and grave tones respectively, and after death vibrate in a great measure like musical strings, we think it may be fairly inferred that they likewise obey, to a certain extent, during life, the laws of the vibrations of such strings, and that the con clusions which we have derived from the foregoing formulae are not far removed from the truth. A further confirmation of these views may be derived from the following con siderations. The length of a cord of invariable
weight varies directly as the tension, and in versely as the square of the number of vibra tions. Now, if we assume the length of the vocal cord, which gave under a tension of 32 loths to be 91 inch, which is the mean length of the male vocal cord in its greatest tension, according to the first table, and which gave the notes Al Cl, under the tension 8 and 2 loths respectively, the corresponding lengths of that cord, according to the formula, will be 83 inch and 58 inch*; but -58 inch is less than the least length in repose in the table. This result is, however, quite consis tent with the theory here proposed ; because after death the thyro-arytenoid muscle be comes of itself elongated, and consequently the vocal ligament attached to it, and there fore the length of the ligament must be greater in this state than when it is in that which we have defined to be the state of repose before it has lost its vitality.
In experiments made on the larynx by stretching the vocal ligaments with given weights, and by forcing a current of air through the glottis, care must be taken to keep the organs moist, and of the same temperature as they possess during life. The amount of condensation of the air in the vocal tube has been ascertained by Cagniard De la Tour, and Midler, the former in the living, and the latter in the dead subject. In a person who had an opening in the windpipe after the operation of tracheotomy, Cagniard De la Tour found that the tension of the air in the vocal tube, while blowing the clarionet, was equal to a column of water of thirty centimetres in height, and that to produce a simple vocal sound in the same person a tension of sixteen centimetres was necessary. Miller found that he could pro duce sound in a larynx artificially by a tension of 3.1 centimetres; but for very loud sounds an increased tension was requisite. The dis crepancy between the experiments of Cag niard De la Tour and Millier may be ascribed to the circumstance that the one operated on .vfs I\ 2 P * N varies as / varies as then -91 inch 32, length of cord for Al (818 8the length of the cord for Al =- -83 inch ; and the length for Ct = 58 inch.