Chapter XVII: Appendix: I. to Chapter IV

THE VOCAL ORGANS OF ANIMALS.

Comparative anatomy is the foundation of modern physiology: to understand the human organism we must compare it with the organisms of the lower animals. This is as true of the organs of speech as of the organs of locomotion or sensation, and we shall find that, in spite of varying degrees of development, the vocal organs of both man and beast present a general resemblance to each other. Some of the quadrumana have large sacs between the thyroid cartilage and the _os hyoideum_, which have much to do with modifying and increasing the resonance of the voice. The laryngeal sacs possessed by some of the monkeys of Africa cause the acuteness of tone and hoarseness of cry that characterize them. The great intensity of the voice in the American “howlers” is due to the size of the epiglottis and the existence of large cavities in the thyroid cartilage and _os hyoideum_ which communicate with the ventricles of the larynx and the laryngo-pharyngeal sacs. The bray of the ass has been traced to two large sacs existing between the vocal chords and the inner surface of the thyroid cartilage. Some of the marsupials, such as the kangaroo, have membranous vocal chords which stretch upon themselves and so cannot be stretched by the arytenoid muscles. A few of the mammalia, _e.g._ the giraffe, the porcupine, and the armadillo, have no vocal chords, and are therefore mute. This is also the case with the cetacea, the bellowing of the whale being produced by the expulsion of water through the nostrils during the act of exspiration.

Birds possess a superior larynx which differs considerably from that of the mammalia, and has nothing to do with the production of sound. Below this is the inferior larynx at the lower end of the trachea, just before it bifurcates into the two bronchi. This is the organ of voice, and differs a good deal, both in form and structure, in the several species of birds. It is double, except in the parrot and a few other birds, and is almost always symmetrical. It is composed of the lower rings of the trachea united so as to form a tube, at the lower end of which are two protuberances, one in front of the other, and joined together in most birds by a thin rod of bone (the _os transversale_). To the upper edge of this bone is attached a delicate membrane (the _membrana semilunaris_), which is turned upwards, and to the lower edge another membrane (the _membrana tympaniformis_), formed of the membranous wall of the bronchus. The latter membrane is highly developed in singing birds, and still more so in speaking birds, and it can render the first-mentioned membrane (with which it is connected) tense when made to vibrate. In some birds the inferior larynx has as many as five muscles, in others none. It is wanting altogether in vultures. It will be seen that the two membranes correspond to the vocal chords in the mammalia, sounds being produced by the vibration of their margins. The various notes are caused by changes in the degree of tension of the membranes, by differences in the force of the air-current, and by changes in the length and degree of tension of the trachea and other parts. The range of the voice in birds is usually within an octave, but may be much greater.

Serpents have no vocal chords, and their hiss is the result of breath being forcibly driven through a soft glottis. Frogs have no trachea, so that their larynx opens into the bronchial tubes; but the loudness of the croaking of male frogs is due to the distension of two membranous sacs at the sides of the neck. Some frogs have membranous vocal chords: others two reed-like bodies, the anterior ends of which are fixed, while the posterior ends looking into the bronchi are free.

We must wait for the microphone to confute or confirm the statement of M. Langlois, of Freiburg, that ants communicate with one another by means of audible sounds. The recent observations of Sir John Lubbock seem to show the contrary. At all events, the sounds produced by most insects are produced externally and not internally. The stridulation of the cricket or grasshopper is made by rubbing certain file-like organs against the edges of membranous drums on the wings. The pitch of the sounds produced by the cricket is high, consisting of 4,096 vibrations per second. The shriek of the death’s-head moth is produced by the friction of parts connected with the mouth and proboscis, the buzzing of flies and gnats by the rapid vibration of two rudimentary posterior wings called _halteres_. The humming of humble-bees, beetles, and the like is due to the passage of the air through the spiracles.

Fish, with few exceptions, have no special sonorous apparatus. The noise they make when taken out of the water is caused by the sucking or flapping movements of their mouth or gill coverings. It is possible that the air-bladder opening into the pharynx which is possessed by some fish, may enable them to emit sounds.

_Fig. 1._ VIEW OF THE TRACHEA AND LARYNX.]

_Fig. 2._ VIEW OF THE LARYNX FROM ABOVE. 1, Crico-arytenoid ligaments; 2, thyroid cartilage; 3, cricoid cartilage; 4, arytenoid cartilages; 5, chordæ vocales; 6, the right thyro-arytenoideus lateralis; 7, the left crico-arytenoideus lateralis (the right being removed); 8, crico-arytenoid ligaments; 9, arytenoideus transversus (connecting the arytenoids); 10, rima glottidis.

_Fig. 3._ 1, Soft palate (velum pendulum palati); 2, uvula; 3, tongue; 4, hyoid bone; 5, thyroid cartilage; 6, epiglottis; 7, glottis; 8, trachea; 9, cricoid cartilage; 10, pharynx; 11, superior opening of larynx; 12, œsophagus; 13, orifice of Eustachian tube.]

_Fig. 4._ Position for _a_. _Fig. 5._ Position for _e_ (in _hay_). _Fig. 6._ Position for _i_ (in _he_). _Fig. 7._ Position for _u_. _Fig. 8._ Position for _k_, _g_, _ng_. _Fig. 9._ Position for _m_.]

_Fig. 10._ Position for _r_. _Fig. 11._ Position for _t_, _d_, _n_. _Fig. 12._ Position for _y_. _Fig. 13._ Position for _s_, _z_. _Fig. 14._ Position for _th_. _Fig. 15._ Position for _f_, _v_. _Fig. 16._ Position for _p_.]

DIAGRAM SHOWING THE RANGE OF THE HUMAN VOICE.

(From McKendrick’s “Outlines of Physiology,” p. 642.)

Pitch of the vowels, according to Helmholtz.

Pitch of the vowels, according to König.]