332 Tones of the Voice produced in the Larynx.

consists of a mouth-piece, the aperture of which is capable of being expanded or dilated, and of a tube which admits of being lengthened and shortened.

This organ is termed the larynx; the tube, which composes it, is placed upon the upper part of the trachea, so that as the air issues during expiration, it may cause the edges of the aperture through which it re-enters the larynx to vibrate.

If the upper part of the trachea be divided, on looking into the larynx from below, the tube from being cylindrical is seen to assume abruptly a triangular form. The two long sides of this triangle, extend horizontally inwards and forwards to meet at the front of the larynx. The base of the triangular opening is short, and has a transverse direction. The opening is termed the rima glottidis. The two long edges which meet at the fore part are termed the chordæ vocales.

When we look into the larynx from above, we notice, the epiglottis, a thin flap of fibrous cartilage, held vertically by its elastic connexions against the root of the tongue, but capable of being thrown down to cover the opening of the glottis; the lips of the glottis, or the reflexion of the mucous membrane from the edges of the epiglottis to the posterior margin of the larynx; and the ventriculus laryngis, as the shallow fossa is termed, situated immediately above and to the outside of the chordæ vocales, which allows these parts to vibrate freely,

If an incision be made in a living dog immediately below the cornu of the os hyoïdes, so as to expose the cavity of the larynx, the following phenomena are observable.

At each expiration, the rima glottidis is narrowed,

Instruments, which the Larynx resembles.

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and the chordæ vocales are brought nearer to each other, so as to come in contact for part of their length.

When the animal cries, the chordæ vocales appear to vibrate.

When the tone uttered is grave, the chordæ vocales seem to vibrate for their whole length, and the rima glottidis is proportionately larger.

When the animal utters a shrill cry, the rima glottidis is observed to become much narrower; and the chordæ vocales being in contact at their fore part, a portion only of each appears to vibrate.

The rima glottidis is the mouth-piece of the larynx, and corresponds with the reed in the clarinette, or with the lips of one playing upon the flute;-if indeed either of these comparisons be strictly just.

In pursuing the same similitude we look for a contrivance analogous to the stops in the flute or clarinette, by means of which the tube may be shortened or lengthened; and we find the effect we anticipate produced by the alternate rising and falling of the larynx. When the larynx is raised the vocal tube is shortened, when it is depressed the vocal tube is lengthened. Accordingly, when we utter an acute note, the larynx suddenly rises; and sinks, when the voice falls to a grave tone.

In either of the three cases, the flute, the clarinette, the larynx, the force with which the air is impelled into the instrument consists in the action of the muscles of the chest, that are employed in expiration. But as in playing upon wind instruments, the force of the air may be increased by the action of the muscles of the cheeks, which straighten the channel through which the air passes, so possibly in modifying the tones of the human

334

Use of the Epiglottis.

voice, a similar effect may be produced by the contraction of the transverse muscular fibres of the trachea.

The use of the epiglottis according to Magendie is to perfect the larynx as a musical instrument. It seems that in the clarinette a note swelled beyond a certain degree of loudness is liable to break into a higher note; now M. Grenié discovered that by placing a tongue of elastic substance to break the current of air, this imperfection is remedied. But the epiglottis is just such a contrivance in the vocal organ; the use of which was unknown, till accident thus discovered it.

We have now to raise the curtain, and to examine the mechanism by which the changes are produced, in the place of the larynx and in the size of the rima glottidis, which have been described.

The same muscles, that are employed to raise the pharynx in deglutition, are used to elevate the larynx in modifying the tone of the voice. This action for either afterwards we repeat at

purpose is primarily instinctive; pleasure an effort, which we recollect was attended with a result which pleased us.

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Other smaller muscles, which extend from point to point of the cartilages of the larynx, alter the dimensions of the rima glottidis.

The principal piece in the structure of the larynx is the cricoïd cartilage, a thick ring rising behind to the height of an inch: it is received between the two flat plates of which the thyreoïd cartilage consists: and upon its raised posterior margin, two little pyramids of fibrous cartilage, called the arytenoïd cartilages, are loosely articulated so as to move freely.

The edge of the chordæ vocales appears formed of a

Effects of dividing the recurrent Nerve.

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peculiar elastic substance, extending from the front of each arytenoid cartilage to the thyreoïd, so that any movement given to the former immediately affects the dimensions of the rima glottidis.

Muscles termed crico-arytenoïdei postici and laterales extend from the back and outer part of the cricoïd cartilage to the arytenoid of each side, and in their action draw the two apart from each other, and enlarge the rima glottidis.

Another broad but thin muscle termed the thyreoarytænoïdeus extends from the arytenoid cartilage to the thyreoïd. This muscle is parallel to the chorda vocalis of the same side, and enters into its composition.

The three preceding muscles are supplied by the recurrent nerve, a branch of the nervus vagus: upon its division animals lose their voice.

It is easy to account for this phenomenon by reference to the anatomical facts, which have been mentioned: when the muscles, which the recurrent nerve supplies, act together, the chordæ vocales are thrown into a state of tension; if the crico-arytanoïdei are stimulated to contract more forcibly than the thyreo-arytenoidei, the aperture of the rima glottidis is capacious and fitted for the production of grave notes: if the thyreo-arytænoïdei on the other hand act the most forcibly, the chordæ vocales must be drawn near to each other, and coming into contact at their fore part through the swelling of the shortening muscles which enter into their composition, are at liberty to vibrate in part only of their length.

Another set of small muscles is found at the upper part of the larynx; the arytenoïdeus transversus and

836

Agency of the Larynx in Deglutition.

the arytenoïdei obliqui extend across from one arytænoïd cartilage to another, and in their action draw these parts together, and entirely close the aperture of the glottis; these muscles, with the mucous membrane, which invests them and clothes the adjoining surface of the larynx, are supplied by separate branches of the nervus vagus, termed the superior laryngeal nerves: and though it is probable that their action in some degree influences the voice, yet they are principally concerned in other functions of the larynx, which have been already alluded to and may on the present occasion be fully explained.

The larynx is the guard of the respiratory apparatus during deglutition: when the food passes over its aperture, the muscles last described instinctively close it. When the nerve which supplies them is divided on both sides, deglutition can no longer take place perfectly, but each attempt at swallowing is attended with the entrance of some of the food into the trachea, which is immediately expelled by violent coughing, the sudden action of the expiratory muscles, which drives out the offending substance before the torrent of air, that is expelled.

The larynx again is intended to prevent the entrance of noxious substances into the lungs: for this purpose the mucous surface of the larynx is endowed with acute sensibility, and the instinctive operation of its muscles is so prompt and powerful as to oppose successfully every effort at inspiration, when an animal is immersed in fluids, the inhalation of which would be prejudicial: when an animal is placed in a vessel containing carbonic acid, its attempts to inspire are useless.