TONE NOT THE EXTENT OF VIBRATIONS. 171

strength, a sound is emitted; but, as before observed, the sound ceases before such a string returns entirely to rest. To produce an impression on the ear, but a moderate velocity in the vibrations is necessary. It appears that a sound may remain audible with a velocity of no more than oth part of an inch in a second perhaps it may be heard even with a much smaller velocity than this. Nevertheless, in such a case it is probable that the initial velocity must be considerably greater than that here described.

What is termed molecular undulation is exemplified in the alternate condensation and rarefaction of air. When we blow into a

FIG. 82.

tube open at both ends, the air contained first becomes condensed in the middle, and rarefied at the two extremities, as seen by the

FIG. 83.

lighter shade in the cut; but after a while the case is reversed, the rarefaction being in the middle, the condensation at the extremities. What is termed difference of tone depends upon the number of vibrations in a given period of time. When the number of vibrations is small, a grave sound is produced; when the vibrations are numerous in the same time, an acute sound is heard. Their thickness being equal, a long string produces fewer vibrations than a short string in a given time. Thus, by lengthening a string, the tone of a musical sound produced by its vibration passes from the acute to the grave. When the quality of a sound is spoken of, it has no reference to the number of vibrations in a given time, quality being dependent on the molecular constitution of the sounding body. From what has been already said, it must be seen that the mere extent of vibrations does not affect the tone. It appears, however, that loudness of sound is dependent chiefly on the extent of the vibrations.

Musical sounds, then, are produced either by the vibrations of solid bodies, of liquid bodies, or of aëriform bodies, or by a combination of the vibrations of two or more of these.

There are even musical instruments, or musical combinations pro

172 PRINCIPLES OF MUSICAL INSTRUMENTS.

duced by solid bodies, independently of any musical tension. For example, melody may be extracted from cylinders of glass, or of metal, struck either directly or by means of keys - the tuning-fork, the gong, the cymbal, the bell, are examples of the same kind. The harmonica consists of a series of glass vessels made to yield musical sounds by the friction of the fingers. But the most important case of this kind, as bearing on the explanation of the phenomena of the voice, is the vibration of an elastic plate produced by a current of air which it continually emits and excludes. Such a plate is employed in those forms of the organ-pipe which have been termed the vox humana pipe, and regal pipe. The vibrations of water are hardly employed to produce musical sounds; nevertheless, the purling rill and the distant cataract plainly fall within the description of musical sounds.

In simple wind-instruments of music, we have examples of musical sounds produced by molecular undulations of air: in the flute, the flageolet, the diapason organ-pipe, the humming-top, the cavity of the mouth in whistling, and playing on the Jew's-harp, the molecular undulations of the air are the sources of the musical sounds. In the flute and flageolet the length of the tube is altered at pleasure by opening or shutting the holes. When a hole is opened it is the same thing as if the pipe were cut off a little beyond the place of the hole.

In many musical instruments the vibrations of solid bodies cooperate with the undulation of the air to produce the musical sounds. This is the case in the trumpet and in the various kinds of horn. In these instruments, the force of the inflation produces what are termed harmonic divisions. The trombone is so contrived that the length of the tube may be altered at pleasure. In what are termed the reed pipes of an organ, there is an elastic plate which vibrates in unison with the column of air which they contain. The vibrations of animal membranes, when put on the stretch, as a source of musical sounds, are usually considered separately, both from the sounds produced by solid bodies and those produced by the molecular undulation of air. There are examples of this effect of those membranes in the use of such instruments as the drum and tambourine. These instruments are chiefly prized for their loudness. Under the same head, however, falls the membranous tongue, which bears closely on the illustration of the human voice. What is here referred to is not, indeed, a musical instrument, but a contrivance employed to exhibit the effects of sound under circumstances analogous to those of the human larynx. The most remarkable experiment of this kind is made by placing two thin plates of India rubber over the end of a tube, so as to leave a very slender fissure between their margins in the middle line, and fixing these by a liga

ORGANS OF VOICE AND SPEECH IN MAN.

ture. Two pieces of leather employed in the same manner produce a similar result. When two such tongues or membranes are placed over the orifice of an organ-tube, and the current of air made to rise through it, vibratory motion is maintained by this current, and a considerable range of musical sounds is produced. The two tongues or membranes in this experiment should be in the same place, and the space between them should be very minute: the edges of the tongue should not be farther apart than from th tooth of an inch.

173

FIG. 84.

The experiment succeeds even better when the edges actually touch. To this experiment reference will be made hereafter, when we come to speak of what occurs in the human larynx during the exercise of voice.

Organs of Voice and Speech in Man. The organs concerned in voice and speech may be described as the chest and lungs, the windpipe, the larynx, the posterior cavity of the mouth, the nostrils, which communicate with that posterior cavity, the palate, the tongue, the teeth, and the lips. The sounds which constitute voice belong to the order of musical sounds, independently altogether of the singing voice. All that is properly termed voice takes place in the larynx, which is properly the instrument of voice. But even independently of the modifications by which voice is changed into articulate speech, the voice is variously affected by the other parts which have been enumerated: by the chest, as regulating the force of the air; by the windpipe, as susceptible of several degrees of length and tension; by the posterior cavity of the mouth, as offering an expanded vault; by the nostril, as affording a double passage of exit for the breath; and by the various conditions of the tongue, the palate, the teeth, the lips, according to the position in which they happen to be at the moment.

The chest and lungs together constitute, in reference to the voice, a musical bellows, capable of supplying air with more or less force to the organs of voice The peculiarity of these bellows consists in the fact that the air must be renewed, at short intervals, by entering from without by the same passage by which it is expelled when the voice is exercised. It can, however, supply air without interruption, in a continued stream, for about fifteen seconds. The lung consists of two large bags of air, and does not materially differ from the windbox of an organ, or rather from the bag of a bagpipe. No air can enter the lung, or escape from it, except through the windpipe. The walls of the chest are everywhere in contact with the outer surface of the lung, and close in around the point at which the windpipe rises upwards to the larynx. The chest is capable of expansion in every direction; that is to say, by means of muscular action its walls

174

STRUCTURE OF WINDPIPE.

récede from the surface of the lung, so that the cavity in which this air-bag is contained, is augmented in every direction,-in length, in breadth, in depth. Whenever this enlargement commences the air begins to enter from without. By this process, in two or three seconds, many cubic inches of air can be drawn into the lungs. So nice is the action of the muscles, by which the chest is again contracted in size, and the lung is compressed, that the stream of air which shall issue in a given period through the larynx, by the influence of the will, is under the most complete control. The prominence of the larynx on the fore part of the neck is popularly known by the name of Adam's apple, by which, probably, its remarkably greater prominence in the male than in the female is referred to. The long succession of minute tubes, by the gradual union of which the other trunks, and finally the windpipe, are formed, has this peculiarity, that the aggregate of the areas of the smaller tubes greatly exceeds the area of the trunks which they combine to form. From the windpipe throughout, almost to their origin in the minute cells,

the tubes are provided with tense walls, by means of the cartilaginous appendages before referred to; in the windpipe itself these cartilages assuming a more definite form. They are in complete rings of cartilage, being deficient posteriorly, that is, each ring of

STRUCTURE OF WINDPIPE.

175

the windpipe traverses about two-thirds of its circumference, leaving the remaining one-third, on its posterior aspect, destitute of this support. The number of rings in the windpipe is from fifteen to twenty; in other respects the tube is chiefly membranous, yet provided with muscular fibres capable of diminishing its calibre, by drawing together the extremities of the rings. It has been proved, by sufficient experiments, that when the larynx is raised by the powerful muscles attached to it, the windpipe is drawn up from the chest in a corresponding extent, and that at the same time its diameter is diminished by about one-third.

The base, or lowest part of the larynx, rests on the upper part of the windpipe, and this base consists of a ring, somewhat more developed than any of the rings of the windpipe, yet not so different from these but that it might be regarded as the summit of that tube. This ring differs from the rings of the windpipe in being complete all round; it is not, however, of a uniform breadth in the direction from below upwards, being broader at the posterior part. It may be likened, then, to a ring, with a stone or a seal, the expansion behind corresponding to the stone or seal. On the upper edge of the expanded portion of this ring, at the base of the larynx, are set two slender bodies of a pyramidal form, which bear the most important part in the mechanism of the larynx as an organ of voice. These two bodies are exactly alike, and are placed almost close together, like two miniature obelisks set on end. The connection of their inferior extremities with the basement ring of the larynx, is by articulation, viz., by a true joint, like the shoulder-joint; that is to say, they are movable on the cartilaginous ring which supports them. From the one to the other, on their posterior aspect, muscular fibres extend, by the contraction of which these two minute pyramids are made to approximate together. From the fore part of each, near their bases, an elastic substance proceeds forwards, converging, to interlace with its fellow at the anterior part of the larynx; that is to say, a minute somewhat triangular space is formed by two portions of elastic tissue, which cross the basement ring of the larynx from behind forwards, the base of this triangle being the space between the two pyramidal bodies just spoken of and its apex behind, a portion of the larynx to be presently alluded to. This triangular space between these two portions of tissue, vocal ligaments, as they are called, is the aperture by which the breath enters and issues in respiration, and by which, when contracted to a narrow chink, the air is forced through in the exercise of voice. These, then, are the most essential parts of the larynx; the two pyramidal bodies each resting on the posterior part of the basement ring, while the two ligaments proceed forwards, each from the base of one of these pyramids, to form a triangle, the apex of which is so directed as to