Part 5 (1/2)
It is situated below the thyroid cartilage, with which it is connected by a membrane, the crico-thyroid. The wider part of this signet-ring is situated behind, where it affords attachment to large muscles. It also furnishes a base of support for two very important structures, the _arytenoid_ (_arutaina_, a ladle) cartilages. As the vocal bands are attached behind to them, and as they have a large degree of mobility, they are from a physiological point of view the most important of all the solid structures of the larynx.
There are two pairs of small bodies, the _cartilages of Santorini_, or _cornicula laryngis_, surmounting the arytenoids, and the _cuneiform_, or _cartilages of Wrisberg_, situated in the folds of mucous membrane on each side of the arytenoids; but these structures are of little importance.
The whole of the inner surface of the larynx is lined with mucous membrane, though that covering over the true vocal bands is very thin, and so does not cause them to appear red like the false vocal bands, which are merely folds of the mucous membrane. However, the true vocal bands may become red and thickened when inflamed, because of this same mucous membrane, which, though ordinarily not visible to the eye, becomes so when the condition referred to is present; for inflammation is always attended by excess in the blood supply, with a prominence of the small blood-vessels resulting in a corresponding redness. The same thing happens, in fact, as in inflammation of the eyes or the nose, both of which are more open to observation. Bearing this in mind one can readily understand why in such a condition, which is often approached if not actually present in the case of ”a cold,” the voice becomes so changed. Such vocal bands are clumsy in movement, as the arms or any other part would be if thus swollen. The plain remedy is rest, cessation of function--no speaking, much less attempts at singing. Like the nose the larynx, and especially the vocal bands, may be catarrhal, and such a condition may call for medical treatment before the speaker or singer can do the most effective vocal work.
While the _false vocal bands_ have little or nothing to do with phonation directly, they do serve a good purpose as protectors to the more exalted true vocal bands. When coughing, swallowing, vomiting, holding the breath tightly, etc., these folds of mucous membrane close over the true bands, often completely, and thus shut up for the moment the whole of that s.p.a.ce between the bands known as the glottis, or glottic c.h.i.n.k, to which reference was made in a previous chapter as the s.p.a.ce through which the air finally gains access to the lungs.
The true vocal cords (which, because of having some breadth and being rather flat, are better termed vocal bands) are composed largely of _elastic tissue_. The reader may be familiar with this structure, which is often to be found in the portions of the neck of the ox that the butcher sells as soup beef. It is yellow in color, and stretching it has furnished many a boy with amus.e.m.e.nt. It is so unmanageable when raw that when it falls to the dog he usually bolts it, the case being otherwise hopeless. Such elastic tissue is, however, the very material for the construction of vocal bands, as they require to be firm yet elastic.
[Ill.u.s.tration: FIG. 25 (Spalteholz). A view of the larynx from behind.
Several of the muscles are well shown, of which the two indicated above are of the most importance. The arytenoideus proprius tends to bring the cartilages from which it is named, and therefore the vocal bands, toward each other; while the posterior crico-thyroid, from its attachments and line of pull, tends to separate these and lengthen the vocal bands.]
[Ill.u.s.tration: FIG. 26 (Spalteholz). Showing structures as indicated above. The mucous membrane, that naturally covers all parts within the vocal mechanism, has been dissected away to show the muscles.]
[Ill.u.s.tration: FIG. 27 (Spalteholz). Showing the parts indicated above; and of these the crico-thyroid muscle is to be especially observed. The oblique (especially so in the posterior part) direction of its fibres is evident, so that when it contracts, it must pull up the ring cartilage in front, and so tilt back its hinder portion and with it the arytenoid cartilages, and so lengthen and tense the vocal bands, as in the utterance of low tones.]
[Ill.u.s.tration: FIG. 28 (Spalteholz). A back (posterior) view of the larynx, etc. Note how the arytenoid cartilages rest on the cricoid; how the epiglottis overhangs, as its name implies, the glottis; and that the posterior part of the windpipe is closed in by soft structures, including (unstriped) muscle.]
It is important to remember the relative position of parts and to bear in mind that most of the laryngeal structures are in pairs. To this last statement the thyroid and cricoid cartilages and the epiglottis are exceptions, being single.
Of the _epiglottis_, a flexible cartilage, it is necessary to say little, as its function in voice-production, if it have any, has never been determined. It hangs as a flexible protective lid over the glottis, and food in being swallowed pa.s.ses over and about it. It no doubt acts to keep food and drink out of the larynx, yet in its absence, in some cases, owing to disease, no very great difficulty was experienced, probably because certain muscles acted more vigorously than usual and tended to close up the glottic c.h.i.n.k.
The following simple diagram will, it is hoped, make the relative position of parts plain so far as the anterior (front) attachments of parts to the thyroid cartilage are concerned. It will be understood that the inner anterior surface is meant, and that by ”middle line”
is intended the middle line of the body, the imaginary vertical diameter pa.s.sing like a plumb-line from the middle plane of the head, let us suppose, downward just in front of the larynx.
[Ill.u.s.tration: FIG. 29.]
The angle made above and in front where the two wings of the thyroid cartilage meet is termed _Adam's apple_ (_Pomum Adami_), and in some cases, mostly males, is very prominent. Adam's apple has in itself, however, no special significance in voice-production.
The little concavity between the false vocal bands above and the true vocal bands below is termed the _ventricle of the larynx_. It allows of more s.p.a.ce for the free movements of the bands, especially those more important in voice-production.
The vocal bands are attached behind to the projecting angle of the base of the arytenoid cartilage, which is itself somewhat triangular in shape, the base of the triangle being downward and resting on the upper and posterior (back) surface of the cricoid cartilage, with which it makes a free joint, so that it can move swivel-like in all directions. This is most important, because through it is explained the fact that the vocal bands may be either tensed and lengthened or relaxed and shortened.
_The muscles act on these movable cartilages, and nearly all the changes in the vocal bands are brought about through the alterations in position of the arytenoid cartilages, to which they are attached behind._
Before describing the muscles of the larynx, the reader is reminded of the order of structures from above downward, in front, which is as follows:
The hyoid bone.
The thyro-hyoid membrane.
The thyroid cartilage.
The crico-thyroid membrane.
The cricoid cartilage.
The trachea.
The latter is connected with the cricoid cartilage by its membrane.
All the above structures can be felt in one's own person, the more readily if he be thin and have a long neck. The hyoid bone, or tongue-bone, is that hard structure just above the cricoid cartilage, and which one may easily demonstrate to be much more movable than the larynx itself. The tongue muscles are attached to it above, and from it, below, the larynx is suspended, as already explained.