1902 Encyclopedia > Voice

Voice




VOICE is produced by the vibrations of the vocal cords, two ligaments or bands of fibrous elastic tissue situated in the larynx. It is to be distinguished from speech, which is the production of sounds intended to express ideas. Many of the lower animals have voice, but none have the power of speech in the sense in which man possesses that faculty. There may be speech with-out voice, as in whispering, whilst in singing a scale of musical tones we have voice without speech. Regarding speech, see PHONETICS and SPEECH-SOUNDS ; also the articles on the various letters of the alphabet.

1. Physiological Anatomy.—The organ of voice, the larynx, is situated in man in the upper and fore part of the neck, where it forms a well-known prominence in the middle line. It opens below into the trachea or windpipe, and above into the cavity of the pharynx, and it consists of a framework of eartilages, connected by elastic membranes or ligaments, two of which constitute the true vocal cords. These cartilages are movable on each other by the action of various muscles, which thus regulate the position and the tension of the vocal cords. The trachea conveys the blast of air from the lungs during expiration, and the whole apparatus may be compared to an acoustical con-trivance in which the lungs represent the wTind chest and the trachea the tube passing from the wind chest to the sounding body contained in the larynx. Suppose two tight bands of any elastic membrane, such as thin sheet india-rubber, stretched over the end of a wide glass tube so as to leave a narrow chink between the free borders of the membrane, and that a powerful blast of air is driven through the tube by a bellows. The pressure would so distend the margins of the membrane as to open the aper-ture and allow the air to escape; this would cause a fall of pressure, and the edges of the membrane would spring back by their elasticity to their former position; again the pressure would increase, and again the edges of the membrane would be distended; and those actions would be so quickly repeated as to cause the edges of the mem-brane to vibrate with sufficient rapidity to produce a musical tone, the pitch of which would depend on the number of vibrations executed in a second of time. The condensation and rarefaction of the air thus produced are the chief cause of the tone, as Von Helmholtz has pointed out, and in this way the larynx resembles the syren in its mode of producing tone (see ACOUSTICS). It is evident also that the intensity or loudness of the tone would be determined by the amplitude of the vibrations of the margins of the membrane, and that its pitch would be affected by any arrangements effecting an increase or de-crease of the tension of the margins of the membrane. The pitch might also be raised by the strength of the current of air, because the great amplitude of the vibra-tions would increase the mean tension of the elastic mem-brane. With tones of medium pitch, the pressure of the air in the trachea is equal to that of a column of mercury of 160 mm.; with high pitch, 920 mm. ; and with notes of very high pitch, 945 mm.; whilst in whispering it may fall as low as that represented by 30 mm. of water. Such is a general conception of the mechanism of voice.

Fio. 1.—Cartilages and ligaments of the larynx seen from the front; half natural size. 1, epiglottis; 2, hyoid hone ; 3, small eornu of hyoid bone ; 4, middle thyro-hyoid ligament; 5, great eornu of hyoid hone ; 6, small nodules of cartilage {airtilmjo tritkea) ; 7, the lateral thyro-hyoid ligament; 8, left lamina or wing of thyroid cartilage ; 9, cricoid cartilage ; 10, lower eornu of thyroid cartilage; 11, part of cricoid united to the thyroid by the middle crico-thyroid ligament; 12, second ring of trachea. (From Krause.)

FIG. 2.—Cartilages and ligament of larynx seen from behind ; half natural size. 1, epiglottis; 2, lesser eornu of hyoid bone; 3, greater eornu of hyoid; 4, lateral thyro-hyoid ligament; 6, cartilago triticea ; 6, upper eornu of thyroid ; 7, thyro-epiglottic ligament; 8, cartilages of Santorini; 9, arytenoid cartilages ; 10, left lamina of thyroid ; 11, muscular process of arytenoid cartilage ; 12, inferior eornu of thyroid ; 13, first ring of trachea; 14, posterior mem-branous wall of trachea ; 15, lamina of cricoid cartilage. (From Krause.)

cornicula laryngis, the cuneiform cartilages, and the apices of the
arytenoids are composed of yellow or elastic fibro-cartilage, whilst
the cartilage of all the others is of the 1

hyaline variety, resembling that of the costal or rib cartilages. These carti-lages are bound together by ligaments, some of which are seen in tigs. 1 and 2, whilst the remainder are represented in tig. 3. The ligaments specially concerned in the production of voice are the inferior thyro-arytenoid liga-ments, or true vocal cords. These are composed of fine elastic fibres attached behind to the anterior projection of the base of the arytenoid cartilages, processus vocalis, 3 in fig. 3, and in front to the middle of the angle be-tween the wings or lamina; of the thyroid cartilage. They are practi-cally continuous with the lateral crico-thyroid ligaments, 6 in fig. 3.

The cavity of the larynx is divided
into an upper and lower portion by 6 5
from a vertical and slightly ob-lique section; two-thirds natural size. 1, epiglottis; 2, arytenoid cartilage ; 3, processus vocalis of arytenoid ; 4, cricoid carti-lage ; 5, capsular thyro-hyoid ligament ; 6, lateral crico-thy-roid ligament; 7, posterior crico-thyroid ligament ; 8, inferior tliyro - arytenoid ligament, or true vocal cord ; 9, thyroid car-tilage ; 10, superior thyro-ary-tenoid ligament, or false vocal cord; ll,thyro-ary-epiglottideus muscle; 12, middle thyro-hyoid ligament; 13, hyo-epiglottic ligament ; 14, body of hyoid bom' ; 15, smaller cornu of hyoid bone, (From Krause.)
the narrow aperture of the glottis or Fig. 3.—Eight half of thejarynx, chink between the edges of the true vocal cords, the rima glottidis. Im-mediately above the true vocal cords, between these and the false vocal cords, there is on each side a recess or pouch termed the ventricle of Mor-gagni, and opening from each ventricle there is a still smaller recess, the laryngeal pouch, which passes for the space of half an inch between the superior vocal cords inside and the thyroid cartilage outside, reaching as high as the upper border of that car-tilage at the side of the epiglottis. The ventricles no doubt permit a free vibration of the true vocal cords (Quain). The upper aperture of the glottis is triangular, wide in front and narrow behind ; and, when seen from above by means of the laryngoscope, it presents the view represented in fig. 4. The aperture is bounded in front by the epiglottis, e, behind by the summits of the arytenoid carti-lages, ar, and on the sides by two folds of mucous membrane, the aryteno-epiglottic folds, ae. The rounded elevations corresponding to the cornicula laryngis and cuneiform cartilages, c, and also the cushion of the epiglottis, e, are readily seen in the laryngoscopic picture. The glottis, o, is seen in the form of a long narrow fissure, bounded by the true vocal cords, ti, whilst above them we have the false vocal cords, ts, and between the true and false cords the open-ing of the ventricle, v. The rima glottidis, between the true vocal cords, in the adult male measures about 23 millimetres, or nearly an inch, from before backwards, and from 6 to 12 millimetres across its widest part, according to the degree of dilatation. In females and in males before puberty the antero - posterior diameter is about 17 millimetres and its transverse diameter about 4 millimetres. The vocal cords of the adult male are in length about 15 millimetres, and of the adult female about 11 milli-metres. The larynx is lined with a layer of epithelium, which is closely adherent to underlying structures, more especially over the true vocal cords. The cells of the epithe-lium, in the greater portion ofFl,0,f; the larynx, are of the columnar ciliated variety, and by the vi-bratory action of the cilia mucus is driven upwards, but over the true vocal cords the epithelium is squamous. Patches of squa-mous epithelium are also found in the ciliated tract above the glottis, on the under surface of the epiglottis, on the inner surface of the arytenoid cartilages, and on the free border of the upper or false cords. Numerous mucous glands exist in the lining membrane of the larynx, more especially in the epiglottis. In each laryngeal pouch there are sixty to seventy such glands, surrounded by fat.

We are now in a position to understand the action of the muscles of the larynx by which the vocal cords, forming the rima glottidis, can be tightened or relaxed, and by which they can be approximated or separated. Besides certain extrinsic muscles—sterno-hyoid, omo-hyoid, sterno-thyroid, and thyro-hyoid— which move the larynx as a whole, there are intrinsic muscles which move the cartilages on each other. 15 Some of these are seen in fig. 5. These muscles are (a) the crico-thy- 11 roid, (b) the posterior crico-arytenoid, ig (c) the lateral crico-arytenoid, (d) the thyro-arytenoid, (e) the aryten- 12 oid, and (/) the aryteno-epiglot-tidean. Their actions will be readily understood with the aid of the dia-grams in fig. 6. (1) The crico-thyroid is a short thick triangular muscle,





the larynx, seen from within ; two-thirds natural size. 1, hyo-epi-glottic ligament, seen in profile ; 2, epiglottis ; 3, aryteno-epiglottic muscle ; 4, Santorini's car-tilage ; 5, oblique arytenoid muscle ; 6, transverse arytenoid muscle, seen in profile ; 7, posterior crico-arytenoid ; S, lateral crico-arytenoid ; 9, lower cornu of thyroid cartilage cut through ; 10, insertion of posterior portion of crico-thyroid muscle ; 11, left lamina of thyroid cartilage cut through ; 12, long thyroepiglottic muscle (a variety); 13, inferior thyro-arytenoid: 14, thyroepi-glottic ; 15, superior thyro-arytenoid ; 16, median thyro-hyoid ligament. (From Krause.)
its fibres passing from the cricoid Fio. 5.—Muscles of the left side of cartilage obliquely upwards and out-wards to be inserted into the lower border of the thyroid cartilage and to the outer border of its lower horn. When the muscle contracts, the cri-coid and thyroid cartilages are ap-proximated. In this action, how-ever, it is not the thyroid that is depressed on the cricoid, as is gener-ally stated, but, the thyroid being fixed in position by the action of the extrinsic muscles, the anterior border of the cricoid is drawn upwards, whilst its posterior border, in con-sequence of a revolution around the axis uniting the articulations be-tween the lower cornua of the cricoid and the thyroid, is depressed, carrying the arytenoid cartilages along witli it. Thus the vocal cords are stretched. (2) The thyro-arytenoid has been divided by anatomists into two parts — one, the internal, lying close to the true vocal cord, and the other, external, immediately within the ala of the thyroid cartilage. Many of the fibres of the anterior portion pass from the thyroid cartilage with a slight curve (concavity inwards) to the processus vocalis at the base of the arytenoid cartilage. They are thus parallel with the true vocal cord, and when they contract the arytenoids are drawn forwards, carrying with them the posterior part of the cricoid and relaxing the vocal cords. Thus the thyro-arytenoids are the antagonists of the crico-thyroids. Ludwig has pointed out that certain fibres (portio-ary-vocalis) arise from the side of the cord itself and pass obliquely back to the processus vocalis. These will tighten the parts of the cord in front and relax the parts behind their points of attachment. Some of the fibres of the outer portion run obliquely upwards from the side of the crico-thyroid membrane, pass through the antero-posterior fibres of the inner portion of the muscle, and finally end in the tissue of the false cord. These fibres have been supposed to render the edge of the cord more prominent. Other fibres inserted into the processus vocalis will rotate slightly the arytenoid outwards, whilst a few passing up into the aryteno-epiglottidean folds may assist in depressing the epiglottis

FIG. 6.—Diagrams explaining the action of the muscles of the larynx. The dotted lines show the positions taken by the cartilages and the true vocal cords by the action of the muscle, and the arrows show the general direction in which the muscular fibres act. A, Action of crico-thyroid: 1, ci'icoicr cartilage; 2, arytenoid cartilage; 3, thyroid cartilage; 4, true vocal cord; 5, thyroid cartilage, new position; 6, true vocal cord, new position. B, Action of arytenoid: 1, section of thyroid; 2, arytenoid ; 3, posterior border of epiglottis ; 4, true vocal cord; 5, direction of muscular fibres; 6, arytenoid, new position; 7, true vocal cord, new position. C, Action of lateral crico-arytenoid ; same description as for A and B; 8, posterior border of epiglottis, new position; 9, arytenoid in new position. D, Action of posterior crico-arytenoid ; same description. (From Beaunis and Bouchard.)

(3) The posterior and lateral crico-arytenoid muscles have antagon-istic actions, and may be considered together. The posterior arise from the posterior surface of the cricoid cartilage, and passing up-wards and outwards are attached to the outer angle of the base of the arytenoid. On the other hand, the lateral arise from the upper border of the cricoid as far back as the articular surface for the arytenoid, pass backwards and upwards, and are also in-serted into the outer angle of the base of the arytenoid before the attachment of the posterior crico-arytenoid. Imagine the pyra-midal form of the arytenoid cartilages. To the inner angle of the triangular base are attached, as already described, the true vocal cords ; and to the outer angle the two muscles in question. The posterior crico-arytenoids draw the outer angles backwards and in-wards, thus rotating the inner angles, or processus vocalis, outwards, and, when the two muscles act, widening the rima glottidis. This action is opposed by the lateral crico-thyroids, which draw the outer angle forwards and outwards, rotate the inner angles inwards, and thus approximate the cords. (4) The arytenoids pass from the one arytenoid cartilage to the other, and in action these cartilages will be approximated and slightly depressed. (5) The aryleno-cpi-glottidcan muscles arise near the outer angles of the arytenoid; their fibres pass obliquely upwards, decussate, and are inserted partly into the outer and upper border of the opposite cartilage, partly into the aryteno-epiglottic fold, and partly join the fibres of the thyroarytenoids. In action they assist in bringing the arytenoids to-gether, whilst they also draw down the epiglottis, and constrict the upper aperture of the larynx. The vocal cords will be also relaxed by the elasticity of the parts.

2. General Physiological Characters.—As already stated, the intensity or loudness of voice depends on the amplitude of the movement of the vocal cords. Pitch depends on the number of vibrations per second; and the length, size, and degree of tension of the cords will determine the number of vibrations. The more tense the cords the higher the pitch, and the greater the length of the cords the lower will be the pitch. The range of the human voice is about three octaves, that is from fax (87 vibrations per second) to S0I4 (768 vibrations). In men, by the development of the larynx, the cords become more elongated than in women, in the ratio of 3 to 2, so that the male voice is of lower pitch and is usually stronger. At the age of puberty the larynx grows rapidly, and the voice of a boy " breaks" in consequence of the lengthening of the cords, generally falling an octave in pitch. A similar change, but very much less in amount, occurs at the same period in the female. At puberty in' the female there is an increase of about one-third in the size of the glottis, but it is nearly doubled in the male, and the adult male larynx is about one-third greater than that of the female. In advanced life the upper notes of the register are gradually weakened and ultimately disappear, whilst the character of the voice also changes, owing to loss of elasticity caused by ossifica-tion, which first begins about middle life in the thyroid cartilage, then appears in the cricoid, and much later in the arytenoid. Eunuchs retain the voices of childhood; and by careful training it is possible in normal persons to arrest the development of the larynx so that an adult male can still sing the soprano parts sometimes used in cathedral choirs. The ranges of the different varieties of voice are shown in the following diagram, where the dotted lines give the range of certain remarkable voices, and the figures represent vibrations per second.

== DIAGRAM ==

There is thus a range for ordinary voices of nearly two octaves, and certain rare voices may have a range of three and a half octaves. A basso named Gaspard Forster passed from fa _ j to la3; the younger of the sisters Sessi had a contralto voice from do2 to fa5; the voice of Catalani ranged three and a half octaves; a eunuch singer, Fari-nelli, passed from la, to re6; Nilson, in // Flauto Magico, can take fa6; and Mozart states that he heard in Parma in 1770 a singer, Lucrezia Ajugari, range from sol2 to do6, which she gave purely, whilst she could execute trills on re5. The latter is the most highly pitched voice referred to in musical literature, an octave and a half above the highest ordinary soprano. The range of these voices is shown in dotted lines in the accompanying diagram, and the number of vibrations per second is also noted, taking the middle C of the piano as 256 vibrations per second. It will be observed that the lowest note of Gaspard Forster's voice is not much above the pitch at which the perception of musical tone begins, and that from this note to the upper note of Lucrezia Ajugari there is a range of nearly six octaves, whilst the extreme range of ordinary voices, from the lowest bass to the highest soprano, is a little over three octaves. It is also interesting to observe in connexion with this that the range of the human ear for the perception of musical tone is from do_j to do10, or from about 32 to 33,768 vibrations per second,—eleven octaves.

== TABLE ==

The quality of the human voice depends on the same laws that determine the quality, clang-tint, or timbre of the tones produced by any musical instrument. Musical tones are formed by the vibrations of the true vocal cords. These tones may be either pure or mixed, and in both cases they are strengthened by the resonance of the air in the air-passages and in the pharyngeal and oral cavities. If mixed—that is, if the tone is compounded of a number of partíais—one or more of these will be strengthened by the cavities above the cords acting as a resonator; and so strongly may these partials be thus reinforced that the fundamental one may be obscured, and a certain quality or timbre will be communicated to the ear. Further, Von Helmholtz has shown that special forms of the oral cavity reinforce in particular certain partials, and thus give a character to vowel tones,—indeed to such an extent that each vowel tone may be said to have a fixed pitch. This may be proved by piutting the mouth in a certain form, keeping the lips open, and bringing various tuning forks sounding feebly in front of the opening. When a fork is found to which the resonant cavity of the mouth corresponds, then the tone of the fork is intensified, and by thus altering the form and capacity of the oral cavity its pitch in various conditions may be determined. Thus, according to Von Helmholtz, the pitch corresponding to the vowels may be expressed :—

== TABLE ==

Donders has given a third result, differing from each of the above; and there is little doubt that much will depend on the quality of tone peculiar to different nationalities. By means of Koenig's manometric flames with revolving mirror the varying quality of tone may be illustrated: with a pure tone, the teeth in the flame-picture are equal, like the serrations of a saw, whilst usually the tone is mixed with partials, which show themselves by the unequal serra-tions. It is thus certain that quality of voice depends, not merely on the size, degree of elasticity, and general mobility of the vocal cords, but also on the form of the resonating cavities above, and there can be no doubt that very slight differences in these may produce striking results.





3. Condition of the Larynx in the Various Registers.—In singing, one can readily observe that the tone may appear to come chiefly from the chest, from the throat, or from the head, or it may show the peculiar quality of tone termed falsetto. Authorities differ much in the nomenclature applied to these varieties of the voice. Thus the old Italian music masters spoke of the voce di petto, voce di gola, and voce di testa. Madam Seiler describes five conditions, namely, the first series of tones of the chest register, the second series of tones of the chest register, the first series of tones of the falsetto register, the second series of tones of the falsetto register, and the head register. French writers usually refer to two registers only, the chest and the head ; whilst Behnke gives three registers for male voices (lower thick, upper thick, and upper thin), and five for the voices of women and children (lower thick, upper thick, lower thin, upper thin, and small). These distinctions are of more importance practically than as implying any marked physiological differences in the mechanism of the larynx during the production of the tones in the different registers. By means of the laryn-goscope it is possible to see the condition of the rima glottidis and the cords in passing through all the range of the voice.

In 1807 Bozzini first showed that it was possible to see into the dark cavities of the body by illumining them with a mirror, and in 1829 Babington first saw the glottis in this way. In 1854 Garcia investigated his own larynx and that of other singers, and three years later Tiirck and especially Czermak perfected the construction of the laryngoscope. In 1883 Lennox Browne and Emil Behnke obtained photographs of the glottis in the living man. The laryn-goscope is a small mirror, about the diameter of a shilling, fixed to the end of a long handle at an angle of 125° to 130°. This mirror is gently pushed towards the back of the throat, and if sufficient light be thrown into the mouth from a lamp, and if the eye of the observer be in the proper position, by angling the small mirror it is not difficult to get a view of the glottis. The light from the lamp is reflected by the mirror down on the glottis, from this it is reflected back to the mirror, and then by the mirror it is finally reflected to the eye of the observer. Usually the observer has in front of his eye a mirror by which a powerful beam of light can be thrown from a lamp into the mouth and throat. In the centre of the mirror there is a small hole through which the eye of the observer sees the image in the small mirror at the back of the throat. By placing a second plane mirror in front of the face, an observer can easily study the mechanism of his own larynx.

Suppose the picture of the larynx to be examined in the small mirror at the back of the throat, an image will be seen as in fig. 4. During calm breathing, the glottis is lance-shaped, between the yellowish white cords. A deep inspiration causes the glottis to open widely and in favourable circumstances one may look into the trachea. When a sound is to be made, the vocal cords are brought close together, either along their whole length, as in fig. 7, or only along the ligamentous portion, the space between

Fig. 7. Fig. 8.
FIG. 7.—Arrangement of glottis previous to emission of a sound, b, epiglottis ;
rs, false cord ; ri, true vocal cord ; ar, arytenoid cartilages. (From Mandl.) FIG. 8.—Closure of the ligamentous portion of glottis, b, epiglottis ; rs, false
cord ; ri, true vocal cord ; or, space between arytenoids; ar, arytenoid
cartilages ; c, cuneiform cartilages; ray, ary-epiglottic fold; ir, iuter-ary-
tenoid fold. (From Mandl.)

the arytenoids being still open, as in fig. 8. Then when the sound begins the glottis opens (fig. 4), the form of the opening influencing the kind of voice, whilst the degree of tension of the cords will determine the pitch.

During inspiration the edges of the true vocal cords may occasionally be close together, as in sobbing, and it has been pointed out by various observers that during inspiration the false cords are easily separated, even when they touch, and during expiration, owing to dilatation of the ventricles, they come together and may readily close. Thus, from the plane of the cords, the true cords are most easily closed during inspiration and the false cords during expiration. Wyllie clearly showed in 1865 that the false vocal cords play the chief part in closure of the glottis during expiration (Edin. Med. Jour., 1866). Lauder Brunton and Cash have confirmed Wyllie's results, and have shown further that the function of the false cords is to close the glottis and thus fix the thorax for muscular effort; and they adduce many facts from comparative anatomy in favour of this view, these cords being strongly developed in those animals whose habits render fixation serviceable, whilst, on the other hand, they are absent or weakly developed in animals where fixation is of little or no service (Jour. Anat. and Physiol., vol. xvii.).

During the production of the chest voice, the space between the arytenoid cartilages is open, and between the vocal cords there is an ellipsoidal opening which gradually closes as the pitch of the sound rises (see figs. 9, 10, 11).

Fig. 9. Fig. 10.
FIG. 9.—Chest voice, deep tone. 6, epiglottis ; or, glottis ; rs, false vocal' cord ; ri, true vocal cord ; rap, ary-epiglottidean fold ; ar, arytenoid carti-lages. (From Mandl.)
FIG. 10.—Chest voice, medium tone, orl, ligamentous portion of glottis ;. ore, portion of glottis between arytenoids ; remaining description as in fig. 7. (From Mandl.)
During head voice, the opening between the arytenoids is completely closed ; the portion between the vocal cords is-

Fig. 11. Fig. 12.
FIG. 11.—Chest voice, high tone ; description same as for tigs. 7 and 8. (From Mandl.)
FIG. 12.—Head voice, deep tones. Z, tongue; e, epiglottis ; pe, pharyngo-epi-glottidean folds; ae, ary-epiglottic folds; rs, false cords ; ri, true vocal cords; g, pharyngo-laryngeal groove ; ar, arytenoid cartilages ; c, cuneiform carti-lages ; o, glottis; r, inter-arytenoid folds. (From Mandl.)

open, but in place of being almost a narrow straight slit as in chest voice, it is wide open so as to allow an escape of more air (see fig. 12). The condition of the cords during falsetto is, according to Miiller, one in which the cords can only vibrate at their margins, and especially in the middle, in consequence of the false cords pressing-downwards upon them. Oertel, on the other hand, states-that in falsetto the cords vibrate throughout their length, " but that they form nodal lines parallel to the free borders-of the loops or bellies of vibration " (Beaunis). Probably in these circumstances the membranes become much thinner. Oertel also found that during the falsetto voice the epiglottis became erect, the apices of the arytenoids were directed backwards, and the whole larynx became narrower but longer from before backwards. Behnke says that, in. the production of the "small register," the mechanism " consists in the formation of an oval orifice in the front part of the glottis which contracts the more the higher the voice ascends, the vocal ligaments being, in the hinder part, pressed together so tightly that scarcely any trace of a slit remains" (Lennox Brown and Behnke, op. cit.). Illing-worth is of opinion that falsetto (and even the ordinary voice) is produced in the "same way as the mouth is used, in whistling" (see Edin. Med. Jour., 1876). This view may be true to some extent as regards falsetto, but it will not hold good for the ordinary voice.

The crico-thyroid muscle is supplied by the superior laryngeal branch of the pneumogastric nerve, and all the other muscles by the inferior or recurrent laryngeal branch of the same nerve. The superior laryngeal is also the sensory nerve of the larynx. Stirling has found ganglionic ocells in the course of this nerve. Paralysis of the motor ofibres causes aphonia, or loss of voice. If one cord is paralysed the voice may be lost or become falsetto in tone. Sometimes the cords may move in breathing or during ocoughing, but be motionless during an attempt at the pro-duction of voice. Rarely, incomplete unilateral paralysis of the recurrent nerve, or the existence of a tumour on oeach cord, thus making them unequal in length, may cause a double tone, or diphthongia (Turck). Hoarseness is caused by roughness or swelling of the cords.

On the history of the theories of voice production, see Longet's Traité de Physiologie, 1869, vol. ii. p. 733, and Gavarret's Phonation et Audition, 1877, ,p. 541. An excellent bibliography is given in Beaunis's Physiologie Humaine, 1881, vol. ii. p. 946; also in Quain's Anatomy, 1882, vol. ii. p. 538. (J. G. 51.)


Footnotes

A cheap and efficient form of auto-laryngoscope was constructed by the late Dr David Foulis, and may be had from Messrs. W. B. Hilliard & Son, 65 Renfield Street, Glasgow.




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