The effects of these variations on the tone of the voice have been hitherto little under stood. It has always appeared incomprehen sible why the vocal tube should apparently increase in length in the production of the acute tones, and shorten in the grave ; a cir cumstance which theoretically presents an acoustic paradox. Dodart and many others have conceived the elevation of the larynx to be merely for the purpose of shortening the vocal tube in the supra-laryngeal cavity, and have considered the trachea as producing no effect on the pitch of the tone. Majtmlie has also pointed out the shortening of this part of the tube. In order to ascertain the effect of these changes, the following experi ments were made on the dead body. Having laid bare the vocal organs of an adult male, I raised the larynx to the position it would occupy by the elevation of the voice to an octave, being about half an inch, and at the same time minutely observed the position of the lowest ring of the trachea with reference to the sternum. By this operation I found the trachea was raised out of the chest, nearly to the same extent as the larynx had been elevated towards the base of the skull. The next step was to examine whether any change had taken place in the diameter of the tube. For this purpose, having measured the diame ter of the trachea in its natural position, the larynx was again elevated to the same extent as before, when the diameter was found diminished one-third. These experiments prove that, contrary to the general precon ception, the elevation of the larynx shortens the tube independently of the contraction between the thyroid cartilage and os-hyoides, and at the same time lessens its diameter. The same effects may easily be detected dur ing life by placing the finger on the trachea immediately above the sternum during the elevation of the larynx, when the trachea is found to ascend out of the chest, and after wards to return to its former position ; a movement in which the lungs and bronchi participate. The alteration of the tube in diameter may also be perceived by grasping the trachea with the finger and thumb during the elevation and depression of the larynx.* These movements are so striking as to lead irresistibly to the conclusion, that there exists a constant adaptation between the tension and the vibrating length of the thyro-arytenoid ligaments and the walls of the vocal tube, in the production of tones of the ordinary re gister; for we have seen that the variations of the vocal cords, at least as far as relates to the modulation of sound, are perfectly in dependent of the length of the vocal tube, and consequently the changes in its length which we have just described are not at all necessary for that purpose. Again, the vocal tube is so short, that, as has been ascertained by Weber and others, it could not, were it rigid, affect the pitch of the note produced by the glottis. As however this tube is com posed of flexible materials, its effects are similar to those observed in M. Savart's ex periments ; that is, the relaxed state of the parietes compensates for its want of length, and enables it to vibrate synchronously, and therefore to give forth sounds equally grave with those of the glottis, thereby reinforcing the tone which would indeed be produced, though with much less intensity, without this aid.
The Falsetto, or yam de testa, has always been considered a most embarrassing subject of re search, and its peculiar quality has excited the attention both of the physiologist and of the musician. Its most remarkable characteristic consists in its being less reedy in tone, and partaking nearly of the quality of the har monic sounds of stringed and wind instru ments. The change produced in the voice when passing from the falsetto into the com mon tone, or the reverse, is in some persons very sensible to the ear, whilst in others it is almost imperceptible. Some individuals, more over, have the faculty of producing in the same pitch as many as eight or ten tones, possessing either the falsetto or the common character. The falsetto has been generally ascribed to some particular adaptation of the upper liga ments of the larynx. Dodart* has attempted to prove that it is a supra-laryngeal function, and that the nose becomes the principal tube of sound instead of the cavity of the mouth.
Bennati t also considered these tones as being modulated by the supra•laryngeal cavity alone. This hypothesis, however, is untenable, since it supposes the column of air not to be in fluenced by the trachea, which is contrary to experience. In order to detect some of the movements of the larynx while the voice is passing from the first to the second, or falsetto register, it is only necessary to place the point of the finger in the crico-thyroid chink, when it is found that at the moment the transition from the primary to the secondary register takes place, this chink, which was closed dur ing the production of the highest note of the ordinary register, suddenly opens on the pro duction of the first note of the falsetto register, and consequently the thyro-arytenoid liga ments are relaxed at the same moment the larynx falls, and the vocal tube is lengthened, although during these changes the tones become more acute. As soon as this has taken place, the larynx again rises as the voice becomes more acute. In a mezzo-soprano voice en dowed with a double falsetto, or third register consisting of several tones of each register, with the power of producing tones of the same pitch either of the ordinary or the fal setto quality, we observed that the larynx fell at the commencement of each register, and that the thyro-arytenoid ligaments were twice relaxed, but in a much smaller degree. These observations have since been verified by many musical persons.
In order to explain the phenomena as con nected with the production of falsetto tones, we must remember that at the highest note of the primary register the crico-thyroid mus cles are contracted as much as possible in closing the crico-thyroid chink, and therefore that no further tension of the vocal cords can take place. In this state of things, the thyro arytenoid muscles are at their maximum of elongation, and their transverse section is a minimum; consequently neither can a higher note be produced by an extension of the liga ments, nor are these muscles in a condition to affect the dimensions of the glottis ; hence the necessity of some alteration in the state of the larynx in order to effect the scale of the fal setto, which is an octave above the ordinary register, and to prevent the mere repetition of the same series of sounds. This alteration might be produced in two ways ; one of these is a partial closing of the aperture of the glottis caused by the action of the thyro-arytenoid muscles when they have returned to their ordinary condition, and are in a favourable state to produce that effect under the influence of the laryngeal nerves. For, let us suppose the larynx to be in the same state as at the commencement of the primary register, except that the chink of the glottis is half closed ; the consequence will be that as only half the length of the ligaments can be made to vibrate, the octave of the lowest note in that register will result from the same tension which pro duced that note, and this will manifestly be repeated in consecutive notes of the range of the falsetto. This range is limited in general to a few notes, owing probably to the chink being soon completely closed by the stretching of the vocal cords. It is also owing to this partial closing of the glottis that a much less quantity of air is required for the falsetto than for the ordinary scale, which is proved by our being able to sustain a given note in the fal setto to a much longer time than we can sus tain the corresponding note in the primary register. The partial closing of the glottis was observed by Majendie in his experiments on the dog, and by Mayo in the human subject. Another explanation was suggested by Gott fried Weber, namely, that the falsetto range is caused by a nodal division of the vocal cords producing harmonics of the fundamental notes, by which means the glottis acquires the same pitch as if it were half closed If we consider the glottis as a reed, it is evident that since the number of vibrations must in this case be the same for the same note as when we sup pose the ligaments to obey the laws of cords, the axis of vibration or the breadth of the liga ment must be duly diminished, which may be brought about by the rotation of the thyro arytenoid muscle on its axis.