We have seen how nearly, when we take into account the delicacy and difficulty of the experiments, their results agree with the theory that the vocal cords are subject to the same laws as other stretched laminx, and it would be highly interesting to compare these results with the simultaneous variations which they undergo transversely, and thus discover how far the laws of vibrating elastic tongues may be applied to them. It might possibly be objected to the idea of this twofold action, that the production of sound by the vocal cords is sufficiently accounted for by suppos ing them to vibrate merely as elastic tongues; but then it is found by experiment, that by artificially dividing their length into two ven tral segments, there results the octave of the fundamental note, which proves that at all events they vibrate as cords. In conclusion, we must ever hear in mind the vast difference between natural and artificial mechanism, and however complicated a problem it may be to determine that constitution of the vocal ap paratus, by which the thyro-arytenoid liga ments may simultaneously obey the laws of cords and tongues, yet to a physiologist who is accustomed to meet with, the most admir able contrivances and combinations in the animal frame, the difficulty of finding a strictly mathematical solution is, in such a case, no objection to its truth, when the facts, as far as they have been observed, are decidedly favourable to its reality. Were the move ments of the glottis independent of any tube or column of air, the study of the functions of the vocal organs would be much more simple; hut we find it situated nearly in the centre of the vocal tube of which the trachea and bronchi are the inferior, and the upper part of the larynx, pharynx, nose and mouth, the superior portion ; we have therefore to consider the influence of this tube, and of its inclosed column of air in the production of voice.
In order to investigate the mutual relations between a reed and a pipe, two methods may be adopted : one of these is to vary the pitch of the reed while the length of the pipe remains constant, and the other to vary the length of the pipe with a reed sounding one tone only when detached from the tube. In the construction of reeded pipes for musical purposes, it is incumbent on the mechanician to adjust the length of the tube to the pitch of the reed. When a free reed is used on the principle of Kratzenstein or Grenie, it is found that, if the pipe be not in perfect unison with the reed, the purity of the tone de creases within certain limits, as the discord ance between the reed and pipe increases. The researches of MM. Biot, Weber, Willis and Muller have greatly enlarged our knoW ledge on this subject. We learn from their ex periments how great an influence is mutually exerted between a pipe and its reed, when the pitch of the one is made to vary while the other remains constant, and we may con clude that analogous effects are produced between the vocal tube and the glottis. The slightest knowledge of acoustics is sufficient to inform us that the pitch of any pipe, such as the organ, the flute, the trumpet, in short of all musical tubes vibrating in a similar manner, depends on the velocity of an im pulse propagated in the air within, and is determined by the length of the pipe. As
long as the tubes of musical instruments re main rigid, the nature of the materials which compose them does not affect the pitch of the sound, but merely influences the quality of the tone, and it is indifferent whether we employ metal, wood, or paper in their con struction; each of these substances will yield a tone of a particular timbre, or quality, de pending on the nature of the motions pro duced among its particles by the friction of the air on its surface; but the pitch will be the same in each, if the lengths of the pipes be equal, proving that the air itself is the source of sound. When, however, the sides of the tube are composed of flexible mem branes, the inclosed air has a vibratory mo tion, conjointly with, and subordinate to, that of the parietes of the tube, whereby the pitch of the sound is affected, as well as its quality. M. Savart* found that by taking tubes com posed of layers of paper of constant length, but varying in thickness, graver sounds were produced as the parietes became thinner, and that the gravity of the. sound was increased by moistening and relaxing the sides of the tubes. We shall presently see the application of these facts to the vocal apparatus.
We find the flexibility of the trachea and bronchi capable of being varied by the opera tion of two forces, the one longitudinal or parallel to the axis of the tube, the other transverse. The first of these comprises the muscles which elevate and depress the larynx; the latter, the cartilaginous segments of rings perpendicular to the axis of the tube haling muscular fibres attached to their posterior extremities, the contraction and elongation of which regulate the diameter of the trachea. The pharynx, mouth and nasal cavities, which form the superior extremity of the vocal tube, are also provided with muscles to modify the tension of that part of the tube so that it may vibrate synchronously with the rest. The necessity for this change in the dimen sions of the tube, in order that it may vibrate in unison with the glottis, is in accordance, not only with the joint system of pipe and reed above described, but also with what actually takes place in the vocal organs of living animals. When the voice is raised in the scale from grave to acute, a corresponding elevation takes place in the larynx towards the base of the cranium. By placing the finger on the pomum Adami this 'notion can be easily felt, and at the same time the thyroid cartilage is drawn up within the os-hyoides, and presses on the epiglottis ; the small space between the thyroid and cricoid closes, the pharynx is contracted, the velum palati is de pressed and curved forward, and the tonsils approach each other : the reverse of these phenomena takes place during the descent of the voice. These are the principal pheno mena common to most manunalia which can be recognised by external observation ; the other changes being, on account of their situ ation, invisible.