The oral, nasal, and pharyngeal cavities exercise an important influence on the quality of sounds after their production by the larynx. Further effects are ascribed by Bennati to the arches of the palate, the uvula, and velum, all of which appear to contract with the acute, and relax with the grave tones, and are in constant motion during the modulstion of the voice. The contraction of these parts during the production of acute sounds has also been observed by Fabricius ab Aquapendente, Meyer, Gerdy, and Dzondi. Bennati con ceived, as has been already mentioned, that the falsetto notes, which lie calls notes " snr laryngiennes," are produced exclusively in the superior part of the vocal tube; but it has been shown that this hypothesis is contrary to acoustic principles, and that the same motions of the palate are also equally observable during the production of acute tones of the ordinary register. Muller also states that the arches of the palate may be touched by the finger with out altering the pitch, which could not be the ease on the hypothesis of Bennati. It is to he remarked that neither Muller nor Bennati mentions the opening of the crico-thyroid chink on sounding the first note in the falsetto re gister; neither do they mention the simul taneous falling of the larynx, and they deny the existence of a third register. According to the hypothesis of Lehf'eldt and Muller, any increased intensity of vocal sound ought to raise the pitch of the voice; but if this were the case, the performance of prolonged vocal sounds on the same note, but of variable in tensity, would be rendered impossible without asimultaneous adjustment between the tension of the vocal ligaments and the current of air ; whereas, by examining the state of ttie erico thyroid chink during the utterance of these sounds, it is found that no such adjustment takes place. The exquisite quality of the sounds of the larynx, when modified by.the oral and nasal cavities, renders the human voice far superior to any artificial musical in strument ; since its tones glide through all the enharmonic intervals between successive notes, an effect which no such instrument can perfectly imitate. Dodart estimates the num ber of tones which can be produced by the voice and appreciated by the ear in thy com pass of an octave, at three hundred : a striking proof of the complete control exercised by the laryngeal nerves over the vocal apparatus.
The action of the vocal organs in producing speech is a distinct branch of the physiology of voice, which the author has elsewhere in vestigated. It is well known that the vowel sounds have been imitated by Kratzenstein, • * Vide "On Articulate Sounds, and on the Causes and Cure of Impediments of Speech. London. 1851." De Kempelen, and Willis, by means of me chanism, and that the principles on which they depend have been successfully analysed by the latter : but this is a subject which would require a very lengthened examination to render it the justice which its importance demands.
Having now completed the investigation of the physiological character of the human organs of voice, and having for the sake of simplicity considered them in three distinct lights, namely, as membranous ligaments obey ing the laws of musical strings, as a reeded instrument, and as a membranous pipe with a column of air vibrating within it, the, results of the various experiments which have been noticed would certainly seem to warrant the conclusion that each of these views is cor rect; for it cannot be denied that these ex periments clearly show the vocal apparatus to be influenced by the air expelled from the chest in precisely the same way as if it were a stretched cord, a reed, or a vibrating tube. Why then should we hesitate to adopt the obvious conclusion that the vocal organs do iu fact combine the properties of these various instruments, and are themselves the perfect types of which these instruments are only im perfect imitations? The error of those who have preceded the author in this inquiry seems to consist in viewing the organs of voice, not as a complex, but as a simple tip paratus; with some the favourite hypothesis has accordingly been that of musical strings, with others that of a reed, while experiments are equally in favour of both.
It cannot be expected that in this brief treatise, a subject, wherein, notwithstanding the attention hitherto bestowed on it for many years by men of the highest philosophical talent, so little comparatively has been effected, should be at once exhausted, and all its diffi culties removed ; but the inductive method, the only satisfactory mode of reasoning on such subjects, has been most scrupulously pursued; and whatever explanations have been offered of the phenomena of the voice are at least founded on facts which are incontro vertibly established.