ACOUSTICS. 83 
vibrations in the shape of sound figures, a few of which are represented in 
pl. 19, figs. 85—90. These were produced by stretching a membrane 
over a wooden hoop or glass bell, sprinkling it with fine sand, and 
causing in it a sympathetic vibration, by means of an approximated 
tuning-fork or organ pipe. ‘The whole series of figures here answers to one 
and the same tone, their different forms being produced by making the tone 
higher or lower. 
1. Voice and Hearing. 
For a description of the organs of voice and hearing existing in the animal 
body, we must refer our readers to the section Anthropology, and confine our- 
selves here to the consideration of the more strictly physical part of the sub- 
ject, how a tone is produced and modified by the larynx. The larynx consists 
of four cartilages: the cricoid, the thyroid, and the two arytenoid, which are 
intimately connected with the windpipe and form its continuation, contract- 
ing to a mere slit, the glottis. This may be opened or closed by means of 
muscles attached to the cartilages forming its walls. Over this glottis lie 
two sack-like cavities, the ventriculi morgagnii, whose upper edges form a 
second glottis half an inch above the first. The whole is covered by the 
epiglottis, which prevents solid particles of food from entering the trachea, 
while passing through the cesophagus to the stomach. Various individuals, 
as Biot, Ferrain, and Cagniard de la Tour, have instituted experiments with 
caoutchouc on the formation of tones by the organs of voice; the most 
satisfactory, however, are those of Miiller, performed with separated 
larynges. PJ. 19, fig. 98, represents such a larynx attached to a board, f, the 
larynx terminating with the chordz vocales, which are stretched between 
aandb. ais one of the arytenoid cartilages (the other is behind it), bis the 
under side of the thyroid cartilage, d the inner membrane of the larynx 
which ends in the chorde vocales, which are stretched between a and 6b. 
The upper parts are not represented, for the sake of greater clearness of the 
figure. If such a larynx be blown through by means of the air-tube, w, it 
gives a tone precisely similar to that of the human voice, which is strength 
ened, not altered, by the upper parts, which vibrate and intonate at the same 
time. The change of tone is produced merely by the greater or less tension 
of the chorde vocales, this being effected by the action of special muscles 
in approximating or separating the cartilages. This motion is imitated by 
the strings x and y, which are loaded with weights. In this manner Miller 
was enabled to produce all the tones of the human organ, the higher by 
drawing zx, the lower by means of y. In animals, the organs of voice are 
constructed on the same plan, but with different modifications. 
The organ of hearing consists of three parts: the external ear, the cavity of 
the tympanum, and the labyrinth. The external ear serves by means of the 
concha to catch external vibrations and to convey them through the meatus 
externus to the tympanum, which separates the outer from the inner cham- 
ber. This tympanum is a membrane stretched over a long hoop, and to its 
ICONOGRAPHIC ENCYCLOP£DIA.—VOL. I. LG 257 
