The emotions of animals are frequently expressed in various ways by characteristic movements and noises. Male rabbits sound an alarm by pounding the ground with their hind feet, while bucks and bulls send forth a resounding challenge by stamping with their front hoofs. Rattlesnakes shake their caudal castanets and many stridulating insects express themselves audibly by rubbing hard parts together.
Among vertebrates that produce a distinctive noise, the voice apparatus is generally a wind instrument, and consequently a direct part of the respiratory system.
The majority of vertebrates are dumb. Fishes, which outnumber all other vertebrates in species as well as individuals, have only a few representatives, such as drumfishes and “grunters,” that break the piscine vow of silence.
Amphibians, excluding the musical frogs and toads, are practically voiceless, and reptiles also, if a few unusual cases such as hissing snakes, guttural geckos, and bellowing bull alligators are omitted, are prevailingly silent.
When King Solomon ecstatically sang in a springtime of long ago, “The flowers appear on the earth; the time of the singing of birds is come and the voice of the turtle is heard in our land,” he did not refer to the silent reptile of that name, which is quite unable to celebrate the changing seasons vocally, but to the plaintive notes of the turtle dove.
Some moralist has pointed out the fact that probably the first vocal words spoken on this earth were the words of the croaking frogs, which sound like “work, work, work It should be noted in passing that the sedentary frogs, like many other armchair givers of advice, pay no heed to their own exhortations.
Birds as a rule are notably vociferous but there are numerous species, such as the stately storks, that preserve a dignified silence, while it is a curious fact that among mammals gigantic whales have relatively less voice than tiny squeaking mice.
The words in common use to describe sounds produced by mammals indicate a wide variety of distinctive “voices,” with corresponding diversity in the wind instruments involved. For example, the horse “whinnies’; the cow “moos”; the donkey “brays”; the pig “squeals”; the sheep “bleats”; the elephant “trumpets”; the porcupine “grunts”; the lion “roars”; the cat “purrs”; the wolf “howls”; the dog “barks”; the rat “squeaks” ; the lemur “wails”; the monkey “chatters”; and some human beings “sing.”
Since most voices are dependent upon the expulsion of air from the lungs, the vocal apparatus, or larynx, is advantageously located around the glottis, that is, the slitlike entrance to the trachea. Different sounds are produced by modifying the shape of the aperture and the contour of the cavity through which expelled air escapes. The cartilages, membranes, walls, and muscles of the larynx constitute a mechanism for effecting this result.
The mouth cavity in man, as well as the pliable cheeks and flexible tongue, aid greatly in altering the character of the chamber through which the column of air from the lungs is forced, thus changing the sounds produced. This can be easily demonstrated by pronouncing the vowels, A, E, I, O, U, in succession and mentally noting the changes that meanwhile result in the position of the lips, tongue, and cheeks, and the consequent alteration in the contour of the mouth cavity while executing these distinctive sounds.
In this connection it is a suggestive fact that the evolution of voice has a close dependence upon emergence from water to life in air. It is obvious that the traditional episode of the Tower of Babel could only have been staged very late in the evolutionary story, after the human larynx had come into its own.
There is no true larynx in fishes, but in voiceless salamanders the larynx consists of two tiny triangular guardian cartilages, lateral cartilages (Fig. 352), embedded one on either side of the glottis. In some cases when there is enough trachea to permit, as for example in Siren, there are additional fragmentary cartilaginous rings below the laterals. These prophetic cartilages of urodeles become developed into elongated laryngeal cartilages in the musical frogs, toads, and hylas. The “brek-ek-ek-kex, ko-ax, ko-ax” of Aristophanes’ famous frogs, imitated in one of the modern college yells, is a characteristic bit of virile vocalization familiar to everyone whose experiences include a frog pond in springtime. The mechanism by which these haunting nocturnes are produced consists of a pair of arytenoid cartilages, and in addition, of a new cartilage, the cricoid, which is an elaboration of the first tracheal ring or rings (Fig. 353). Dilator and adductor muscles operate these skeletal elements.
Two folds on the inner walls of the laryngotracheal chamber, the vocal cords, lie parallel with the slitlike glottis. The Anura, especially the males of the species, have besides, internal vocal sacs in the throat region which are apparent externally when inflated. These serve as chambers of resonance for increasing the carrying quality of their vocalizations. A single median vocal sac is common to all hylas (Fig. 354), as well as the toad, Bujo, while two lateral sacs show at the shoulders of frogs, being especially pronounced in the male bullfrog, Rana catesbiana, when its twanging "jug-o’-rum” note is being broadcast.
As a matter of fact the lungs of frogs and toads are largely organs for producing sound after the manner of bagpipes, the respiratory function being taken care of mostly by the skin, as shown by the fact that the cutaneous arteries exceed the pulmonary arteries in size.
Reptiles and Birds
Reptiles and birds have less larynx and more trachea than Anura. Compensating this deficiency, birds have evolved an additional secondary “larynx,” called the syrinx, which is located at the lower end of the trachea at its junction with the bronchi, instead of near the glottis (Fig. 355). The position of this unique voicebox is in line with the extreme structural modifications of birds whereby all possible weight is centralized for purposes of equilibrium in flight.
There are no vocal cords in the reduced larynx of birds. Instead sounds are due to vibrations of membranes in the syrinx. The median wall of the beginning of each bronchus is a thin membrana tympaniformis interna (Fig. 356). As there is a projection of the interclavicular air sac which lies between the bases of the bronchi, the tympaniform membranes can vibrate as air is expelled past them. In singing birds there is also an unpaired vibratory membrana semilunaris, extending dorso-ventrally near the junction of the bronchi and trachea. A bony ridge, the pessulus, supports the semilunar membrane. The cartilages of the syrinx, which are modifications of tracheal and bronchial rings, are combined with intercartilages, membranous walls, and a variety of muscles, into an efficient vocal mechanism. By means of this apparatus the bird is enabled to change the shape of the tracheobronchial chamber, thus producing a variety of different sounds.
The cartilages of the mammalian larynx include, in addition to a pair of triangular arytenoids around the glottis, and a bandlike cricoid just below, a relatively large quadrilateral shieldshaped ventral thyroid cartilage, that in man forms the movable prominence in the neck known in academic circles as the pomum Adami, and elsewhere as “Adam’s apple.”
The thyroid cartilage is originally paired, as shown by the fact that in monotremes it is made up of two separate lateral plates instead of a single piece, and in all mammals it is derived embryonically from the remains of paired branchial arches.
Still another structure developed in mammals is a cartilaginous lid above the glottis, called the epiglottis. This aids in closing off the tracheal tube from food that is passing down to the esophagus, a process accomplished not so much by the closing down of the epiglottis as by the elevation of the tracheal tube to fit against the overhanging lid. That the larynx is temporarily elevated during the act of swallowing is easily demonstrated by placing the thumb and finger lightly against the Adam’s apple, at the same time imitating the transit of food by swallowing.
Furthermore, in mammals the horseshoe-shaped hyoid bone, situated above the glottis region, is made a part of the laryngeal complex by connecting ligaments which suspend the larynx. In addition certain minor laryngeal cartilages are described in human anatomy, for example, two minute rodlike cuneiform cartilages of Wrisberg, in the fold between the epiglottis and the arytenoids; two small conical nodules, the cartilages of Santorini, surmounting the apices of the arytenoids; and the triticeous cartilages, embedded in the ligaments connecting the hyoid bone on 'either side with the thyroid cartilage, so called because of a resemblance to grains of wrheat (Triticum). The location and relationships of the most important cartilages that make up the laryngeal voicebox of man are indicated in Figure 357.
The vocal cords, which reach their greatest differentiation in mammals, are two pairs of bandlike folds on the inner wall of the larynx, one above the other, extending between the arytenoids and the thyroid cartilage (Fig. 358). Of these the upper pair are called “false,” and the lower pair “true” vocal cords. Made up of dense bands of elastic fibers covered over by mucous membrane, their position and tension may be altered by means of accompanying muscles. During ordinary breathing they do not vibrate sufficiently to produce sound, but when desired the column of air that is forced over their surface is modified through their activity into audible vibrations.
In most cases sound is produced by expiration of air over the vocal cords, although in exceptional instances, a familiar example of which is the “heehaw” of the donkey, the air does vocal scrvice both going in and coming out.
Between the true and false vocal cords a groove or concavity, known as the ventriculus laryngis Morgagni, which is particularly well-developed in certain howling monkeys and vociferous apes, has the capacity of swelling out on either side into air-filled resonance sacs that function much as do similar structures in frogs, by adding intensity to the sounds produced. Curiously elephants are without false vocal cords, while hippopotami have no others.
In young marsupials that remain attached to the nipples of the mother for protracted periods by means of an automatic sphincter muscle around the mouth, the larynx becomes so elongated that it extends up into the nasopharynx behind the soft palate, enabling these young animals to breathe and take in milk at the same time. This device eliminates complications usually attendant upon the double traffic of air and food in the pharyngeal crossing of the ways.
Whales have an elongated larynx which extends into the nasopharynx so far that the glottis can be enwrapped by the soft palate, an arrangement that mitigates some of the difficulties to which these aberrant mammals are subjected by their marine existence (Fig. 359).