PHYSIOLOGY. 
blogists, that the air, expelled from the limgs 
in expiration, striking against the sid(4 of the 
riiiia glottidis (chord® vocales) constitutes 
the voice. But it is necessary that tlie 
opening should be placed in some condition 
produced by an exertion of the will; for 
although air is constantly passing to and 
fro, the voice is not formed unless by an 
express effort for that purpose; neither 
is it formed during sleep ; nor after the 
muscles of the arytenoid cartilages have 
been paralysed by dividing their nerves. 
The manner in which the voice is chang- 
ed fronf acute to grave, and vice versa, has 
been much disputed : whether it arise from 
dilatation and contraction ofthe aperture, or 
from tension and relaxation of the chord® 
vocales. On the former supposition the 
human larynx may be compared to a wind 
instrument, in which the enlargement of 
the aperture renders the sound grave, and 
its diminution acute. By the latter ex- 
planation it resembles a stringed instru- 
ment. After considering the arguments on 
both sides, we should be inclined to admit 
the operation of both causes. The change 
of the voice from acute to grave at the time 
of puberty, when the larynx undergoes a 
remarkable developement, as well as its 
acuteness in females, whose glottis is Ittss 
by one third than that of man, shew that 
the size of the aperture has a great in- 
fluence. Observing on the other hand that 
the vocal chords admit of considerable 
tension and relaxation, we must allow that 
these variations will render them suscepti- 
ble of executing, in a given time, vibrations 
more or less extensive and rapid. And 
although they are neither dry, stretched, 
nor isolated, which are necessary condi- 
tions to the production of sound in those 
stringed instruments to which the larynx 
has been compared, yet they are analogous 
to vibrating bodies placed at the top of 
wind instruments, as the reed in hautboys, 
the mouth-piece in flutes, &c. and equally 
contribute to the formation and varied in- 
flexions of vocal sound. That all the changes 
and conditions of the vocal organs, of what- 
ever description, necessary to the produc- 
tion and modification of sounds, are pro- 
duced by the muscles of the part, is render- 
ed obvious by the elegant experiment, in 
which the ligature or section of one or both 
recurrent nerves, or paria vaga, either sig- 
nally impairs, or entirely destroys, the vocal 
powers of the animal. 
The modifications of the voice are also 
^ected by thp length of the trachea ; hence 
the larynx is manifestly drawn up in the 
neck, in the utterance of acute sounds, and 
as plainly descends when a grave sound is 
produced. In singing, where these effects 
take place in a greater degree, the head is 
thrown back upon the neck in the former 
case, and brought forwards on the chest ira 
the latter. 
The voice is stronger in proportion to the. 
capacity of the thorax ; hence it is weaker 
after meats, when the stomach, distended 
by food, prevents the descent of the dia- 
phragm, and in consumptive persons, where 
the capacity of the lungs is diminished by 
disease. It acquires more force and in- 
tensity, and becomes more sonorous, by its 
reflections in the mouth and nasal canals. 
Hence it is disagreeably altered when it* 
passage in this direction is stopped by dis- 
ease, as by polypus, and it is then com- 
monly, but quite erroneously, said that per- 
sons speak through the nose. 
Whistling, which is common to man with 
singing-birds, is produced in the latter by 
their double larynx; but in the former it is 
effected by a contraction and corrugation 
of the lips, in imitation of the effect pro- 
duced by birds. 
In singing, the voice runs through the 
different degrees ofthe harmonic scale with 
more or less rapidity, changing from acute 
to grave, and vice versa, witli an expression 
of the intermediate notes. It requires much 
more exertion than speech. Tlie glottis 
enlarges and contracts, the larynx is elevat- 
ed or depressed, the neck elongated or 
shortened, inspirations are accelerated, pro- 
longed, or retarded ; expirations are long, 
or short, and abrupt. The power of sing- 
ing is peculiar to man, and forms the great 
prerogative ofhis vocal organs. Whistling is 
common also to birds ; which are often^taught 
to pronounce words without any great diffi- 
culty. On the other hand, parrots are said, 
in two or three instances, to have been 
taught, by vast labour, to produce a kind of 
imitation of singing ; but no barbarous tribe 
has been hitherto met with, which has not 
been accustomed to employ singing as the 
natural expression of them feelings and pas- 
sions. 
Speech is a peculiar modification of the 
voice, formed during the expulsion of the 
air from the chest, chiefly by means of the 
tongue, wliich is applied to the neighbour- 
ing parts, as the palate and teeth, assisted 
by the various motions of the lips. A voice 
is common to brutes with man ; it exists 
already in the newly-born child, and hat 
