The stun of the areas of the two bronchi is greater than that of the trachea ; by which adaptation the latter is more readily supplied with air during the vocalization of the breath. In all mammalia, birds, and reptiles, the axes of the bronchi are inclined to that of the trachea at a greater or less angle. With reference to the voice, the larynx is the most important organ in the whole apparatus. The mouth, fauces, tongue, and nasal organs are not necessary to the production of voice ; nevertheless they exer cise a considerable influence on its quality, and are indispensable for the production of arti culate language. The thorax is sufficiently capacious to contain as much air after a full inspiration as will sustain the glottis in a state of vibration, when the tone is of mo derate intensity, during the space of fifteen seconds, which will enable a person to pro nounce in rapid succession from thirty to forty monosyllables at one expiration.* The phenomena of the voice of animals must at a very early period have afforded to physiologists proof of the susceptibility of i membranous structures to enter into a state of vibration ; and it is now generally known that membranes, whether twisted into a cord like the string of a violin, or in the form of a parallelogram stretched in one direction as the vocal ligaments, or in that of discs stretched all round as the head of a drum, are all capable of producing musical sounds when properly exciterL The theory of the vibratory movements of stretched membranous surfaces has occupied the attention of many of the most celebrated mathematicians, such as Euler, Bernoulli, Riccati, Biot, Poisson, Sir John Herschel, and others. It is a subject requiring the most profound analysis, and the solution of pro blems of much greater complexity than those either of strings or bars ; but in order to bring the theory of vibrating membranes within the reach of computation, the membranes are supposed to be homogeneous and of equal thickness and elasticity. Now this hypo thesis will not satisfy the conditions of the vibratory movements of the vocal organs, such as the windpipe for example, which is composed of tissues of variable thickness, density, and elasticity; it would therefore be futile in the present state of the science of acoustics to attempt any mathematical solu tion of the laws of the equilibrium and move ments of the heterogeneous masses of the vocal tube. When, however, a membrane is stretched in one direction only, it obeys the same laws as a string.
Having adapted two lamina of India rubber to a pipe connected with the bellows of an organ, M. Biot caused a current of air to pass over their free edges, by which means he obtained with facility sounds of different pitch. Professor Willis made similar expe riments with leather and caoutchouc, but could not produce with these substances so great a range of tones as the glottis will and therefore concluded that the vocal ligaments possess greater elasticity. Mr. Willis has also investigated the position in which it is necessary that membranous laminae should be placed, in order that they may be excited and sustained in a state of vibration. He has likewise given a satisfactory explana tion of the mode of action of the air on reeds, such as those of the organ pipe, which applies also to free reeds, and every other case where a vibratory motion is maintained by a current of air.
The experiments and theory of Mr. Willis are exceedingly important, for he has shown that in ordinary breathing the vocal cords remain inclined to each other, at an angle which prevents any vibratory motion ; whereas when their surfaces lie in the same plane, the breath immediately excites them into a state of vibration ; the natural position of the vocal cords in these two states is seen in (fig. 890). Muller also made some experiments on stretched membranous bands, both isolated, and in connection with a tube ; from which he concludes that the force of the current of air influences the pitch of the note produced ; so that a strong current will produce a more acute tone than a weak one, and vice versa ; but the author has not found this to be the case in any of the experiments which he has made. To obtain a pure quality of tone when two membranous bands are stretched across a tube, it is necessary that they should be of equal weight and length, and subjected to equal tension, otherwise they cannot vibrate freely in equal periods of time. Ac cording to Cagniard De la Tour, if two mem branous laminm of equal length and weight be stretched by unequal forces, so that there is an interval of a fifth between the notes they yield separately, the note resulting from their combined action is the intervening third. Muller is disposed to doubt the accuracy of De La Tour, but his own views do not differ ma terially from those of the latter, as he says that when one tongue is most readily thrown into vibrations by the current of air, the sound is emitted by it alone, but if the blast is such that it throws them both into motion, they may both vibrate together, and by reciproca tion produce a simple sound intermediate between the fundamental note of the two vibrating separately ; they may also emit two distinct sounds, or the blast being modified, the two sounds may be produced in suc cession. From these researches it appears, that membranous lamina', stretched in imi tation of the thyro-arytenoid ligaments, will not only vibrate readily, but produce a range of musical tones. It has been remarked that sounds are most readily produced when the two laminae are stretched in the same plane, and that a smaller volume of air is required the nearer the edges of the laminae approach each other, and a still smaller one when their edges actually touch. De Kempelen states, that to produce sound, the edges of the glottis must be app4oximated to within or at least of an inch. These experi ments upon artificial vibrating tongues per fectly agree with those the author has made on the larynxes of animals. Owing to the nature of the articulation of the thyroid with the cricoid cartilage, and the manner in which the crico-thyroid muscles act, an equal ten sion of both the thyro-arytenoid ligaments is simultaneously secured, supposing the aryte noid cartilages to be at the same time in corresponding positions, which is a necessary condition for the production of a synchronous vibratory motion in the two lips of the glottis.