the living vocal organs, but in a state of dis ease, the other on the organs after death.
Variations in the hygrometric and thermo metric states of the air exert very powerful influence on the pitch of the voice ; during the prevalence of a cold moist state of the atmosphere, especially in England, the voices of singers become lower by two or three notes, and regain their usual pitch when the air be comes dry.* In thus tracing out the analogy be tween the laws of stretched cords and those of the vocal ligaments, it is not intended that those ligaments should be considered as stringed instruments, but only that this analogy is accurate as far as relates to the velocity with which an impulse is propagated along them. Dodart supposed the tension of the vocal cords to be merely subservient to an altera tion in the size of the aperture of the glottis, and that the difference of of a fibre of silk, or of a hair in the dimensions of that aperture, was sufficient to alter the pitch of the voice ; but this has been so completely refuted by more recent physiologists, and is so directly at variance with acoustic prin ciples, that we need not give illustrations of its fallacy. M. Savart considered that the action of the air in its passage through the ventricles of the larynx, between the su perior and inferior ligaments, is really the source of sound, and analogous to the me chanism of the birdcall or dog-whistle.t There is certainly a great resemblance in the structure of that instrument to the space above mentioned in many of the higher ani mals, which might easily have led to this in genious hypothesis ; but, as we find neither superior ligaments nor ventricles of Morgagni in many of the order Ruminantia, in which the voice is very sonorous, this theory (as Muller remarks) is untenable.
We next come to the consideration of the alleged analogy between the action of the vocal ligaments and that of the reeds of mu sical instruments. This opinion is maintained by MM. Biot, Cagniard Da la Tour, Majendie, Malgaigne, Miller, and several other distin guished scientific men. It is opposed prin cipally by M. Savart, who observes that the essential principle of the action of reeds con sists in the periodical opening and shutting of the orifice through which the stream of air passes, but that this is wanting in the glottis ; and that were the latter a reed, the edges of the thyro-arytenoid ligaments which form the sides of the chink would be alternately forced asunder by the column of air in the larynx, and brought together by their tension ; whereas he found by experiment that air blown through the glottis produced sound although its edges were from one-sixth to one-fourth of an inch asunder. M. Savart has however clearly mis taken the circumstance wherein the essential principle of reeds consists, since those of the clarionet, bassoon, hautboy, &c. do not en
tirely close the apertures through which the breath passes ; and this is likewise the case with the natural reed formed by the lips of players on the flute and horn. There is in all probability a double action of the vocal cords in the production of sound ; the one being a vibratory motion throughout their length similar to that of a musical string, and the other an oscillation like that of a reed, forming a partial opening and closing of the glottis. The author is led to adopt this view of the functions of the vocal orgal,s from considering that every circumstance which he has established in his previous investiga tion of their action when treated as cords, is perfectly consistent with the hypothesis of their vibrating like the tongues of reeds ; for let us now suppose them to be simply mem branous tongues. In this case the axis of motion is the edge of the ligament attached to the thyro-arytenoid muscle ; the vibrations take place in a plane perpendicular to the axis of that muscle, and the length of the tongue is the breadth of the ligament. The author has observed in repeated experiments on the larynx after death, that the chink of the glottis was partially opened and closed in the production of sound, and Milner found that by decreasing the breadth of the liga ment he rendered the note more acute ; but as this breadth is so small, being in its ordi nary state in an adult generally less than one tenth of an inch, it is extremely difficult to measure the variations corresponding with different notes ; and the author cannot learn that any one has yet succeeded in determin ing these lengths with sufficient ac curacy to form data for the application of the mathematical formula of elastic vibrating tongues.* We know that the number of vi brations made by the same tongue in a given time varies inversely as the square of its length. If, therefore, a tongue whose length is only '1 inch give any note, the length ne cessary to produce the octave will be •07 inch, that is, the variation will he only •03 inch ; we see then how minute must be the changes answering to the intermediate notes, and consequently how much more difficult it is to determine them in the vocal ligament when considered as a tongue than as a stretched membrane or cord. It is moreover observable that the extension and relaxation of the vocal cord, which, as we have seen, are analogous to those of a musical string, produce a corresponding shortening and elon gation of its axis, regarded as a tongue ; and lastly, since one tone only is produced at a time, the vibrations resulting from the double action which appears to exist in the vocal apparatus must be synchronous.