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Vocal cord tension tested experimentally
Journal o f Voice
Vol. 5, No. 4, pp. 360-363 © 1991 Raven Press, Ltd., New York
Voice: A Historical Perspective
Vocal Cord Tension Tested Experimentally by Dr. R6thi Docent for Laryngology and Rhinology at the Imperial and Royal University of Vienna (with two text-figures.) From the Physiological Institute of the Imperial and Royal University of Vienna (Translated by Donald S. Cooper from the Sitzungsberichte der kaiserlichen Akademie der Wissenschaften. Mathematisch-naturwissenschaftlich'e Classe. Vienna 1897, CVI Band, VI Heft, Abtheilung III: 244-252)
At the 66th Meeting of German Natural Scientists and Physicians in Vienna in 1894, I presented a preliminary communication on the measurement of vocal fold tension, which was based on a series of experimental determinations; I tested individual laryngeal muscles as to whether and to what degree they contribute during their contraction, individually and conjointly with other muscles, to the tension of the vocal cords. I undertook the experiments on dogs, and I used an instrument for them that was constructed in the mechanical workshop of the Physiological Institute according to the instructions of Prof. Sigm. Exner. The principle of the instrument consists in the fact that the vocal cords are pressed by an elastic force from inside outward as far as a certain limit, and the resistance that the vocal cords oppose to this pressure, i.e. the force that is necessary for it, is marked and then evaluated empirically. The instrument essentially consists of two movable main branches b (Figs. 1 and 2) with a hinged joint a (Fig. 1)1; their anterior ends c, intended for introduction into the glottis, are a little curved and tapered for the sake of easier manipulation. Externally, there is closely fitted to the latter in their anterior section a pair of fine contact branches d d, which are compressed by the springs m m, and the latter are connected on both sides with an electric bell through the wires e e. The scissors-like device
is connected at a and n to a handle, which for its part contains the guide for the screw device g. The external contact branches can be separated by this screw device from the main branches b b by means of the wires p (Fig. 2) and the wedges f f . If the contact branches fit closely to the inner main branches, the bell circuit is completed; if, on the other hand, they are lifted off by the wedges by means of the screw g and the wires p, then the circuit is interrupted and the electric bell becomes quiet. Later I used a tangent galvanometer instead of the bell because of its greater sensitivity, and I inserted, appropriate resistances in order not to be misled by the layer of liquid (mucus) that may be present between the inner and outer branches. Under these circumstances, the needle became immobile when the metal contact had been eliminated. The branches are supplied with a mark by a dye applied on their outside, so that they can always be introduced to the same depth. The application of the instrument is now as follows: The animal is tracheotomized, it is curarized to avoid active muscle movements, and after the splitting of the thyrohyoid membrane and the hyoid bone, the epiglottis, the ventricular folds, and a part of the thyroid cartilage located above the level of the glottis are removed. Then the contact scissors are opened far enough with the screw k that the distance between the anterior ends corresponds to the width of the glottis midway between the anterior commissure and the vocal process, and they are introduced at this point up to the mark. Next the instrument is opened by means of the screw k to a defined extent which is
1 This figure shows the instrument from the inferior side, while Fig. 2 shows it from the superior side. Address correspondence and reprint requests to Dr. D. S. Cooper, Department of Otolaryngology and Head and Neck Surgery, University of Southern California, Los Angeles, CA 90033, U.S.A.
360
VOCAL CORD TENSION
~;..
,
.,
,
.,
/
/
..it
FIG. 1. Inferior side.
visible in terms of the graduation h, as a consequence of which the vocal cords are pressed from the inside outward, and the screw device g, which is supplied with the graduation i (Fig. 2), is set in motion until the contact branches d d are lifted off the mairr branches b b, and thereby the circuit is interrupted. Thus, the excursion, up to which the vocal cord is pressed by the opening of the contact scissors, is arbitrarily selected, however, within each experiment it is always of equal magnitude, while the force with which the external branches must be lifted, until the contact is broken, depends on the degree of vocal cord tension; the greater the tension, the greater is the force that must be expended for this purpose, and this force is expressed in terms of the graduation of the screw device i. ff the tension is to be measured during stimulation of the muscle, whether the glottis is closed or wide open, the procedure is quite similar: the instrument is previously adjusted for the current glottal width, the n u m b e r for which the contact branches still directly touch the main branches is read on scale i, the instrument is introduced, the vocal folds are pressed outward from the inside by the opening of the scissors with the screw k, up to a measurement that is read from the graduation h, the screw device g is set into motion until the contact is broken and then a reading is taken at i again. The difference in numbers which results at the two readings corresponds to the degree of tension. If the glottis is completely or partially closed, the
FIG. 2. Superior side.
361
thickness of the anterior end of the scissors at the point where they have been introduced is subtracted from the arbitrarily chosen excursion, and the scissors are opened the same amount less after their introduction into the glottis, as their thickness amounts to anteriorly. For instance, if the glottis is completely closed, if the arbitrary excursion amounts to one scale division, and if the thickness of the scissors measures half of a scale division, then they are opened only half of a scale division; if the excursion amounts to half of a scale division, then the closed instrument is simply introduced up to the mark. The closed scissors had in the anterior section, at the depth to which they are supposed to be introduced, a thickness of 1.5 ram, which corresponded to half a scale division. However, that is how only preliminary measurements are obtained. I obtained the absolute values empirically, by repeating the same procedure with the same adjustments of the pincers on a model that had been prepared from silk threads. The threads, which have been subjected to tension by means of weights, are adjusted at the same distance from one another as the glottal width resulting during the measurement, and after they have been indented and displaced outward by the elastic branches, they are loaded until the contact of the outer branches with the inner ones is restored. The weights that have been imposed for loading indicate the current real degree of tension and stiffness 1 of the vocal cords. Since the animals had been curarized, the contraction of the laryngeal muscles was evoked by direct tetanic muscle stimulation, specifically through insertion of electrodes or by conduction of the currents to lamellae of tin foil placed on them in such a manner that the current had to affect all of the fibers of a muscle to the degree possible, and current loops to other muscles were avoided. Every electrode was terminated by two needles, and the needles with the same polarity were inserted in the analogous ends of the muscles to be tested on both sides. If the intention was to stimulate by means of strips of tin foil, then electrodes split like forks were placed in electrical contact with strips disposed symmetrically on the muscles. These arrangements were necessary, since the nature of the measurement presupposes the synergic I T h e G e r m a n word translated here and elsewhere as stiffness is Festigkeit, which c a n indicate mechanical strength or resistance to fracture or deformation. (Translator's note.)
Journal of Voice, Vol. 5, No. 4, 1991
362
L. R E T H I
TABLE
Experiment number
Weight of animal in kilograms
Vocal cord length in millimeters
1
8
17
2
7
16
3
4
6.5
7.5
21
18
1.
Current state of laryngeal muscles With relaxed vocal cords With stimulation of cricothyroid muscles Stimulation of cricothyroid muscles after removal of posterior cricoarytenoid muscles With relaxed vocal cords With stimulation of cricothyroid muscles With stimulation of posterior cricoarytenoid muscles With stimulation of internal thyroarytenoid muscles With stimulation of cricothyroid muscles and internal thyroarytenoid muscles With relaxed vocal cords Cricothyroid stimulation Stimulation of the internal thyroarytenoid muscles Stimulation of the posterior cricoarytenoid muscles Stimulation of the cricothyroid and posterior cricoarytenoid muscles With relaxed vocal cords With stimulation of internal thyroarytenoid muscles With stimulation of cricoarytenoid and internal thyroarytenoid muscles With stimulation of cricothyroid and posterior cricoarytenoid muscles
Glottic width before insertion of scissors
With free scissors"
Limit of contact With inverted scissors b
Difference c
0.5 0.5 0.5
0.5 0.5 0.5
0.7 1.6 1.5
0.2 1.1 1.0
10 480 320
0.5 0.4 2.25
0.5 0.5 0.5
0.9 1.6 1.25
0.4 1.1 0.75
15 480 130
0.5
0.5
i..3
0.8
150
0.5
0.5
1.7
1.2
580
0.5 0.4 0.5
0.5 0.5 0.5
0.75 1.4 1.2
0.25 0.9 0.7
20 230 120
1.6
0.5
1.1
0.6
90
0.5
0.5
1.7
1.2
580
0.5 0.5
0.5 0.5
0.7 1.4
0.2 0.9
15 230
0.5
0.5
1.7
1.2
580
1.5
0.5
1.85
1.35
1000
Corresponds to a weight in grams of about
a That is, position of the branches, expressed in scale units of i, with free scissors, when the main and contact branches are in direct contact. b That is, position of the branches, expressed in scale units of i, with inverted scissors, when the main and contact branches are in direct contact, after the scissors have been opened the arbitrarily selected amount of 0.5 on scale h. c Difference of readings of scale i, that is, between the two preceding numbers.
action of both halves of the larynx. The stimuli were always maximal. In the following discussion, some tables from the series of experiments will now be adduced, in which the weight of the animal and the length of the vocal cord are taken into consideration, and the degree of vocal cord tension with flaccid vocal cords and during the stimulation of individual, as well as during the simultaneous stimulation of multiple laryngeal muscles, is expressed in terms of weights. Thus, for example, in the first experiment adduced in Table 1, the animal weighs 8 kg, the vocal cord length amounts to 17 mm, glottal width in the middle with flaccid cords amounts to 0.5 on scale h, and the limit, at which the contact was lost for the nonintroduced pincers, amounted to 0.5 on scale i. Now the instrument was introduced and opened as far as 0.5 on scale h, and the screw device on the handle was set in motion. The contact was lost at 0.7 on scale i, the difference of the two readings on this scale accordingly resulted in 0.2, which on evaluation corresponded to a weight of 10 g. Likewise, the numbers during the stimulation of different musJournal of Voice, Vol. 5, No. 4, 1991
cles and muscle groups during several experiments can be read from this table. I adduce only four experiments here in Table 1, and in all four experiments, at the point of insertion of the instrument, a widening of about 0.5 on scale h always resulted. In a further table, the pull of different muscles individually (Table 2) or together with others, and the stiffness of the vocal folds, are expressed in grams (Table 3), therefore it is only the vocal cord
TABLE
Muscles Cricothyroid muscles
Internal thyroarytenoid muscles
Posterior cricoarytenoid muscles
2. Weight of a n i m a l in kilograms
Muscle p u l l in grams
6.5 7 8 6.5 7 7.5 6.5 7
210 465 470 100 135 215 70 115
VOCAL CORD TENSION T A B L E 3.
Muscles
Weight of animal in kilograms
Muscle pull and stiffness of vocal cord in grams
Cricothyroid and internal thyroarytenoid muscles Cricothyroid and posterior cricoarytenoid muscles
7 7.5 6.5 7.5
565 565 560 985
tension during maximal contraction of the muscles in question. As can be seen from this table, the absolute numbers for different experiments often differ quite significantly from one another, but it is not so much the weight of the experimental animal, and also not only the different vocal cord length, but rather the often varyingly compact structure of the vocal cords and different thickness of the muscles that have the greatest part in producing these differences. On the whole, however, one obtains fairly pronounced and often significant quantitative differences for the activity of different muscles, and the results of the investigations reflect well current
363
notions of the influence of individual muscles on the tension of the vocal cords. The numbers obtained with stimulation of cricothyroid can be reconciled well with what we know about this muscle and its influence on vocal cord tension, since it is the tensor par excellence. The numbers become larger during simultaneous stimulation of the internal thyroarytenoid muscle, since the latter increases the stiffness of the vocal cord within the muscular portion during its contraction. Likewise the numbers become larger during simultaneous coactivation of the posterior cricoarytenoid muscle, which during its contraction pulls lengthwise on the vocal cord with one component. The cricoid cartilage is pulled forward toward the thyroid cartilage, as is now generally accepted, and of which one can easily convince oneself during the stimulation of the cricothyroid muscle; a pull is exerted from behind by means of the arytenoid cartilage, and this pull is strengthened by one component of the posterior cricoarytenoid muscle, when it contracts during electrical stimulation. In fact, the numbers obtained during simultaneous stimulation of the cricothyroid muscle and the posterior cricoarytenoid muscle are the largest ones as a rule.
Journal of Voice, Vol. 5, No. 4, 1991
Vol. 5, No. 4, pp. 360-363 © 1991 Raven Press, Ltd., New York
Voice: A Historical Perspective
Vocal Cord Tension Tested Experimentally by Dr. R6thi Docent for Laryngology and Rhinology at the Imperial and Royal University of Vienna (with two text-figures.) From the Physiological Institute of the Imperial and Royal University of Vienna (Translated by Donald S. Cooper from the Sitzungsberichte der kaiserlichen Akademie der Wissenschaften. Mathematisch-naturwissenschaftlich'e Classe. Vienna 1897, CVI Band, VI Heft, Abtheilung III: 244-252)
At the 66th Meeting of German Natural Scientists and Physicians in Vienna in 1894, I presented a preliminary communication on the measurement of vocal fold tension, which was based on a series of experimental determinations; I tested individual laryngeal muscles as to whether and to what degree they contribute during their contraction, individually and conjointly with other muscles, to the tension of the vocal cords. I undertook the experiments on dogs, and I used an instrument for them that was constructed in the mechanical workshop of the Physiological Institute according to the instructions of Prof. Sigm. Exner. The principle of the instrument consists in the fact that the vocal cords are pressed by an elastic force from inside outward as far as a certain limit, and the resistance that the vocal cords oppose to this pressure, i.e. the force that is necessary for it, is marked and then evaluated empirically. The instrument essentially consists of two movable main branches b (Figs. 1 and 2) with a hinged joint a (Fig. 1)1; their anterior ends c, intended for introduction into the glottis, are a little curved and tapered for the sake of easier manipulation. Externally, there is closely fitted to the latter in their anterior section a pair of fine contact branches d d, which are compressed by the springs m m, and the latter are connected on both sides with an electric bell through the wires e e. The scissors-like device
is connected at a and n to a handle, which for its part contains the guide for the screw device g. The external contact branches can be separated by this screw device from the main branches b b by means of the wires p (Fig. 2) and the wedges f f . If the contact branches fit closely to the inner main branches, the bell circuit is completed; if, on the other hand, they are lifted off by the wedges by means of the screw g and the wires p, then the circuit is interrupted and the electric bell becomes quiet. Later I used a tangent galvanometer instead of the bell because of its greater sensitivity, and I inserted, appropriate resistances in order not to be misled by the layer of liquid (mucus) that may be present between the inner and outer branches. Under these circumstances, the needle became immobile when the metal contact had been eliminated. The branches are supplied with a mark by a dye applied on their outside, so that they can always be introduced to the same depth. The application of the instrument is now as follows: The animal is tracheotomized, it is curarized to avoid active muscle movements, and after the splitting of the thyrohyoid membrane and the hyoid bone, the epiglottis, the ventricular folds, and a part of the thyroid cartilage located above the level of the glottis are removed. Then the contact scissors are opened far enough with the screw k that the distance between the anterior ends corresponds to the width of the glottis midway between the anterior commissure and the vocal process, and they are introduced at this point up to the mark. Next the instrument is opened by means of the screw k to a defined extent which is
1 This figure shows the instrument from the inferior side, while Fig. 2 shows it from the superior side. Address correspondence and reprint requests to Dr. D. S. Cooper, Department of Otolaryngology and Head and Neck Surgery, University of Southern California, Los Angeles, CA 90033, U.S.A.
360
VOCAL CORD TENSION
~;..
,
.,
,
.,
/
/
..it
FIG. 1. Inferior side.
visible in terms of the graduation h, as a consequence of which the vocal cords are pressed from the inside outward, and the screw device g, which is supplied with the graduation i (Fig. 2), is set in motion until the contact branches d d are lifted off the mairr branches b b, and thereby the circuit is interrupted. Thus, the excursion, up to which the vocal cord is pressed by the opening of the contact scissors, is arbitrarily selected, however, within each experiment it is always of equal magnitude, while the force with which the external branches must be lifted, until the contact is broken, depends on the degree of vocal cord tension; the greater the tension, the greater is the force that must be expended for this purpose, and this force is expressed in terms of the graduation of the screw device i. ff the tension is to be measured during stimulation of the muscle, whether the glottis is closed or wide open, the procedure is quite similar: the instrument is previously adjusted for the current glottal width, the n u m b e r for which the contact branches still directly touch the main branches is read on scale i, the instrument is introduced, the vocal folds are pressed outward from the inside by the opening of the scissors with the screw k, up to a measurement that is read from the graduation h, the screw device g is set into motion until the contact is broken and then a reading is taken at i again. The difference in numbers which results at the two readings corresponds to the degree of tension. If the glottis is completely or partially closed, the
FIG. 2. Superior side.
361
thickness of the anterior end of the scissors at the point where they have been introduced is subtracted from the arbitrarily chosen excursion, and the scissors are opened the same amount less after their introduction into the glottis, as their thickness amounts to anteriorly. For instance, if the glottis is completely closed, if the arbitrary excursion amounts to one scale division, and if the thickness of the scissors measures half of a scale division, then they are opened only half of a scale division; if the excursion amounts to half of a scale division, then the closed instrument is simply introduced up to the mark. The closed scissors had in the anterior section, at the depth to which they are supposed to be introduced, a thickness of 1.5 ram, which corresponded to half a scale division. However, that is how only preliminary measurements are obtained. I obtained the absolute values empirically, by repeating the same procedure with the same adjustments of the pincers on a model that had been prepared from silk threads. The threads, which have been subjected to tension by means of weights, are adjusted at the same distance from one another as the glottal width resulting during the measurement, and after they have been indented and displaced outward by the elastic branches, they are loaded until the contact of the outer branches with the inner ones is restored. The weights that have been imposed for loading indicate the current real degree of tension and stiffness 1 of the vocal cords. Since the animals had been curarized, the contraction of the laryngeal muscles was evoked by direct tetanic muscle stimulation, specifically through insertion of electrodes or by conduction of the currents to lamellae of tin foil placed on them in such a manner that the current had to affect all of the fibers of a muscle to the degree possible, and current loops to other muscles were avoided. Every electrode was terminated by two needles, and the needles with the same polarity were inserted in the analogous ends of the muscles to be tested on both sides. If the intention was to stimulate by means of strips of tin foil, then electrodes split like forks were placed in electrical contact with strips disposed symmetrically on the muscles. These arrangements were necessary, since the nature of the measurement presupposes the synergic I T h e G e r m a n word translated here and elsewhere as stiffness is Festigkeit, which c a n indicate mechanical strength or resistance to fracture or deformation. (Translator's note.)
Journal of Voice, Vol. 5, No. 4, 1991
362
L. R E T H I
TABLE
Experiment number
Weight of animal in kilograms
Vocal cord length in millimeters
1
8
17
2
7
16
3
4
6.5
7.5
21
18
1.
Current state of laryngeal muscles With relaxed vocal cords With stimulation of cricothyroid muscles Stimulation of cricothyroid muscles after removal of posterior cricoarytenoid muscles With relaxed vocal cords With stimulation of cricothyroid muscles With stimulation of posterior cricoarytenoid muscles With stimulation of internal thyroarytenoid muscles With stimulation of cricothyroid muscles and internal thyroarytenoid muscles With relaxed vocal cords Cricothyroid stimulation Stimulation of the internal thyroarytenoid muscles Stimulation of the posterior cricoarytenoid muscles Stimulation of the cricothyroid and posterior cricoarytenoid muscles With relaxed vocal cords With stimulation of internal thyroarytenoid muscles With stimulation of cricoarytenoid and internal thyroarytenoid muscles With stimulation of cricothyroid and posterior cricoarytenoid muscles
Glottic width before insertion of scissors
With free scissors"
Limit of contact With inverted scissors b
Difference c
0.5 0.5 0.5
0.5 0.5 0.5
0.7 1.6 1.5
0.2 1.1 1.0
10 480 320
0.5 0.4 2.25
0.5 0.5 0.5
0.9 1.6 1.25
0.4 1.1 0.75
15 480 130
0.5
0.5
i..3
0.8
150
0.5
0.5
1.7
1.2
580
0.5 0.4 0.5
0.5 0.5 0.5
0.75 1.4 1.2
0.25 0.9 0.7
20 230 120
1.6
0.5
1.1
0.6
90
0.5
0.5
1.7
1.2
580
0.5 0.5
0.5 0.5
0.7 1.4
0.2 0.9
15 230
0.5
0.5
1.7
1.2
580
1.5
0.5
1.85
1.35
1000
Corresponds to a weight in grams of about
a That is, position of the branches, expressed in scale units of i, with free scissors, when the main and contact branches are in direct contact. b That is, position of the branches, expressed in scale units of i, with inverted scissors, when the main and contact branches are in direct contact, after the scissors have been opened the arbitrarily selected amount of 0.5 on scale h. c Difference of readings of scale i, that is, between the two preceding numbers.
action of both halves of the larynx. The stimuli were always maximal. In the following discussion, some tables from the series of experiments will now be adduced, in which the weight of the animal and the length of the vocal cord are taken into consideration, and the degree of vocal cord tension with flaccid vocal cords and during the stimulation of individual, as well as during the simultaneous stimulation of multiple laryngeal muscles, is expressed in terms of weights. Thus, for example, in the first experiment adduced in Table 1, the animal weighs 8 kg, the vocal cord length amounts to 17 mm, glottal width in the middle with flaccid cords amounts to 0.5 on scale h, and the limit, at which the contact was lost for the nonintroduced pincers, amounted to 0.5 on scale i. Now the instrument was introduced and opened as far as 0.5 on scale h, and the screw device on the handle was set in motion. The contact was lost at 0.7 on scale i, the difference of the two readings on this scale accordingly resulted in 0.2, which on evaluation corresponded to a weight of 10 g. Likewise, the numbers during the stimulation of different musJournal of Voice, Vol. 5, No. 4, 1991
cles and muscle groups during several experiments can be read from this table. I adduce only four experiments here in Table 1, and in all four experiments, at the point of insertion of the instrument, a widening of about 0.5 on scale h always resulted. In a further table, the pull of different muscles individually (Table 2) or together with others, and the stiffness of the vocal folds, are expressed in grams (Table 3), therefore it is only the vocal cord
TABLE
Muscles Cricothyroid muscles
Internal thyroarytenoid muscles
Posterior cricoarytenoid muscles
2. Weight of a n i m a l in kilograms
Muscle p u l l in grams
6.5 7 8 6.5 7 7.5 6.5 7
210 465 470 100 135 215 70 115
VOCAL CORD TENSION T A B L E 3.
Muscles
Weight of animal in kilograms
Muscle pull and stiffness of vocal cord in grams
Cricothyroid and internal thyroarytenoid muscles Cricothyroid and posterior cricoarytenoid muscles
7 7.5 6.5 7.5
565 565 560 985
tension during maximal contraction of the muscles in question. As can be seen from this table, the absolute numbers for different experiments often differ quite significantly from one another, but it is not so much the weight of the experimental animal, and also not only the different vocal cord length, but rather the often varyingly compact structure of the vocal cords and different thickness of the muscles that have the greatest part in producing these differences. On the whole, however, one obtains fairly pronounced and often significant quantitative differences for the activity of different muscles, and the results of the investigations reflect well current
363
notions of the influence of individual muscles on the tension of the vocal cords. The numbers obtained with stimulation of cricothyroid can be reconciled well with what we know about this muscle and its influence on vocal cord tension, since it is the tensor par excellence. The numbers become larger during simultaneous stimulation of the internal thyroarytenoid muscle, since the latter increases the stiffness of the vocal cord within the muscular portion during its contraction. Likewise the numbers become larger during simultaneous coactivation of the posterior cricoarytenoid muscle, which during its contraction pulls lengthwise on the vocal cord with one component. The cricoid cartilage is pulled forward toward the thyroid cartilage, as is now generally accepted, and of which one can easily convince oneself during the stimulation of the cricothyroid muscle; a pull is exerted from behind by means of the arytenoid cartilage, and this pull is strengthened by one component of the posterior cricoarytenoid muscle, when it contracts during electrical stimulation. In fact, the numbers obtained during simultaneous stimulation of the cricothyroid muscle and the posterior cricoarytenoid muscle are the largest ones as a rule.
Journal of Voice, Vol. 5, No. 4, 1991