Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Todd, Robert Bentley
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. 
We have seen how nearly, when we take 
into account the delicacy and difficulty of the 
experiments, their results agree with the 
theory that the vocal cords are subject to the 
same laws as other stretched laminæ, and it 
would be highly interesting to compare these 
results with the simultaneous variations which 
they undergo transversely, and thus discover 
how far the laws of vibrating elastic tongues 
may be applied to them. It might possibly 
be objected to the idea of this twofold action, 
that the production of sound by the vocal 
cords is sufficiently accounted for by suppos¬ 
ing them to vibrate merely as elastic tongues ; 
but then it is found by experiment, that by 
artificially dividing their length into two ven¬ 
tral segments, there results the octave of the 
fundamental note, which proves that at all 
events they vibrate as cords. In conclusion, 
we must ever bear in mind the vast difference 
between natural and artificial mechanism, and 
however complicated a problem it may be to 
determine that constitution of the vocal ap¬ 
paratus, by which the thyro-arytenoid liga¬ 
ments may simultaneously obey the laws of 
cords and tongues, yet to a physiologist who 
is accustomed to meet with the most admir¬ 
able contrivances and combinations in the 
animal frame, the difficulty of finding a strictly 
mathematical solution is, in such a case, no 
objection to its truth, wlien the facts, as far 
as they have been observed, are decidedly 
favourable to its reality. Were the move¬ 
ments of the glottis independent of any tube 
or column of air, the study of the functions 
of the vocal organs would be much more 
simple ; but we find it situated nearly in the 
centre of the vocal tube of which the trachea 
and bronchi are the inferior, and the upper 
part of the larynx, pharynx, nose and mouth, 
the superior portion ; we have therefore to 
consider the influence of this tube, and of its 
inclosed column of air in the production of 
In order to investigate the mutual relations 
between a reed and a pipe, two methods 
may be adopted : one of these is to vary the 
pitch of the reed w'hile the length of the pipe 
remains constant, and the other to vary the 
length of the pipe with a reed sounding one 
tone only when detached from the tube. In 
the construction of reeded pipes for musical 
purposes, it is incumbent on the mechanician 
to adjust the length of the tube to the pitch 
of the reed. When a free reed is used on 
the principle of Kratzenstein or Grenié, it is 
found that, if the pipe be not in perfect unison 
with the reed, the purity of the tone de¬ 
creases within certain limits, as the discord¬ 
ance between the reed and pipe increases. 
The researches of MM. Biot, Weber, Willis 
and Miiller have greatly enlarged our know¬ 
ledge on this subject. We learn from their ex¬ 
periments how great an influence is mutually 
exerted between a pipe and its reed, when 
the pitch of the one is made to vary while 
the other remains constant, and we may con¬ 
clude that analogous effects are produced 
between the vocal tube and the glottis. The 
slightest knowledge of acoustics is sufficient 
to inform us that the pitch of any pipe, such 
as the organ, the flute, the trumpet, in short 
of all musical tubes vibrating in a similar 
manner, depends on the velocity of an im¬ 
pulse propagated in the air within, and is 
determined by the length of the pipe. As 
long as the tubes of musical instruments re¬ 
main rigid, the nature of the materials which 
compose them does not affect the pitch of 
the sound, but merely influences the quality 
of the tone, and it is indifferent whether we 
employ metal, wood, or paper in their con¬ 
struction ; each of these substances will yield 
a tone of a particular timbre, or quality, de¬ 
pending on the nature of the motions pro¬ 
duced among its particles by the friction of 
the air on its surface; but the pitch will be 
the same in each, if the lengths of the pipes 
be equal, proving that the air itself is the 
source of sound. When, however, the sides 
of the tube are composed of flexible mem¬ 
branes, the inclosed air has a vibratory mo¬ 
tion, conjointly with, and subordinate to, that 
of the parietes of the tube, whereby the pitch 
of the sound is affected, as well as its quality. 
M. Savart* found that by taking tubes com¬ 
posed of layers of paper of constant length, 
but varying in thickness, graver sounds were 
produced as the parietes became thinner, and 
that the gravity of the sound was increased 
by moistening and relaxing the sides of the 
tubes. We shall presently see the application 
of these facts to the vocal apparatus. 
We find the flexibility of the trachea and 
bronchi capable of being varied by the opera¬ 
tion of two forces, the one longitudinal or 
parallel to the axis of the tube, the other 
transverse. The first of these comprises the 
muscles which elevate and depress the larynx; 
the latter, the cartilaginous segments of rings 
perpendicular to the axis of the tube having 
muscular fibres attached to their posterior 
extremities, the contraction and elongation of 
which regulate the diameter of the trachea. 
The pharynx, mouth and nasal cavities, which 
form the superior extremity of the vocal tube, 
are also provided with muscles to modify the 
tension of that part of the tube so that it 
may vibrate synchronously with the rest. 
The necessity for this change in the dimen¬ 
sions of the tube, in order that it may vibrate 
in unison with the glottis, is in accordance, 
not only with the joint system of pipe and 
reed above described, but also with what 
actually takes place in the vocal organs of 
living animals. When the voice is raised in 
the scale from grave to acute, a corresponding 
elevation takes place in the larynx towards 
* Annales de Chemie et de Physique, tom. xxxii.


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