ceeds to walk, beginning with the right leg, the body is inclined so as to throw the centre of gravity forward; and, the right foot being raised, the right leg is advanced for the length of a step, and the foot is put down again. In the meanwhile, the left heel is raised, but the toes of the left foot have not left the ground when the right foot has reached it, so that there is no moment at which both

FIG. 55. THE COURSE OF THE DIGASTRIC MUSCLE.

D, its posterior belly; D', its anterior belly; between the two is the tendon passing through its pulley connected with Hy. the hyoid bone.

feet are off the ground. For an instant, the legs form two sides of an equilateral triangle, and the centre of the body is consequently lower than it was when the legs were parallel and close together.

The left foot, however, has not been merely dragged away from its first position, but the muscles of the calf, having come into play, act upon the foot as a lever of the second order, and thrust the body, the weight of which rests largely on the left astragalus, upwards, forwards, and to the right side. The momentum thus communicated to the body causes it, with the whole right leg, to describe an arc over the right astragalus, on which that leg rests below. The centre of the body consequently rises to its former height as the right leg becomes vertical, and descends again as the right leg, in its turn, inclines forward.

When the left foot has left the ground, the body is supported on the right leg, and is well in advance of the left foot; so that, without any further muscular exertion,

the left foot swings forward like a pendulum, and is carried by its own momentum beyond the right foot, to the position in which it completes the second step.

When the intervals of the steps are so timed that each swinging leg comes forward into position for a new step without any exertion on the part of the walker, walking is effected with the greatest possible economy of force. And, as the swinging leg is a true pendulum,-the time of vibration of which depends, other things being alike, upon its length (short pendulums vibrating more quickly than long ones), it follows that, on the average, the natural step of short-legged people is quicker than that of longlegged ones.

In running, there is a period when both legs are off the ground. The legs are advanced by muscular contraction, and the lever action of each foot is swift and violent. Indeed, the action of each leg resembles, in violent running, that which, when both legs act together, constistute a jump, the sudden extension of the legs adding to the impetus, which, in slow walking, is given only by the feet.

21. Perhaps the most singular motor apparatus in the body is the larynx, by the agency of which voice is produced.

The essential conditions of the production of the human voice are:

a. The existence of the so-called vocal chords.

b. The parallelism of the edges of these chords, without which they will not vibrate in such a manner as to give out sound.

c. A certain degree of tightness of the vocal chords, without which they will not vibrate quickly enough to produce sound.

d. The passage of a current of air between the parallel edges of the vocal chords of sufficient power to set the chords vibrating.

22. The larynx is a short tubular box opening above into the bottom of the pharynx and below into the top of the trachea. Its framework is supplied by certain cartilages more or less moveable on each other, and these are connected together by joints, membranes and muscles. Across the middle of the larynx is a transverse partition,

N

formed by two folds of the lining mucous membrane, stretching from either side, but not quite meeting in the middle line. They thus leave, in the middle line, a chink or slit, running from the front to the back, called the glottis. The two edges of this slit are not round and flabby, but sharp and, so to speak, clean cut; they are also strengthened by a quantity of elastic tissue, the fibres of which are disposed lengthways in them. These sharp

Diagram of the larynx, the thyroid cartilage (Th.) being supposed to be transparent, and allowing the right arytenoid cartilage (Ar.), vocal chords (V.) and thyro-arytenoid muscle (ThA.), the upper part of the cricoid cartilage (Cr), and the attachment of the epiglottis (Ep.), to be seen. the right crica-thyroid muscle; Tr. the trachea; Hy. the hyoid bone.

C.th.

free edges of the glottis are the so-called vocal chords or vocal ligaments.

23. The thyroid cartilage (Fig. 56, Th.) is a broad plate of gristle bent upon itself into a V shape, and so disposed that the point of the V is turned forwards, and constitutes what is commonly called "Adam's apple." Above, the thyroid cartilage is attached by ligament and membrane to the hyoid bone (Fig. 56, Hy.). Below and behind, its broad sides are produced into little elongations or horns, which are articulated by ligaments with the outside of a

great ring of cartilage, the cricoid (Fig. 56, Cr.), which forms, as it were, the top of the windpipe.

The cricoid ring is much higher behind than in front, and a gap, filled up by membrane only, is left between its

FIG. 57.-A VERTICAL AND TRANSVERSE SECTION THROUGH THE LARYNX, THE HINDER HALF OF WHICH IS REMOVED.

Ep. Epiglottis; Th. thyroid cartilage; a, cavities called the ventricles of larynx above the vocal ligaments (V); x the right thyro-arytenoid muscle cut across; Cr. the cricoid cartilage.

upper edge and the lower edge of the front part of the thyroid, when the latter is horizontal. Consequently, the thyroid cartilage, turning upon the articulations of its horns with the hinder part of the cricoid, as upon hinges, can be moved up and down through the space occupied by this membrane. When it moves downwards, the distance between the front part of the thyroid cartilage and the back of the cricoid is necessarily increased; and when it moves back again to the horizontal position, diminished. There is, on each side, a large muscle, the crico-thyroid, which passes from the outer side of the cricoid cartilage

obliquely upwards and backwards to the thyroid, and pulls the latter down (Fig. 56, C.th).

24. Perched side by side, upon the upper edge of the back part of the cricoid cartilage are two small irregularlyshaped but, roughly speaking, pyramidal cartilages, the arytenoid cartilages (Fig. 58, Ary.). Each of these is articulated by its base with the cricoid cartilage by means of a shallow joint which permits of very varied movements,

AND VIEWED FROM ABOVE

Th The thyroid cartilage; Cr. the cricoid cartilage; V. the edges of the vocal ligaments bounding the glottis; Ary. the arytenoid cartilages; Th. A. thyro-arytenoid; C.a.. lateral crico-arytenoid; C.a.p. posterior crico-arytenoid; Ar.p. posterior arytenoid muscles.

and especially allows the front portions of the two arytenoid cartilages to approach, or to recede from, each other.

It is to the fore part of one of these arytenoid cartilages that the hinder end of each of the two vocal ligaments is fastened; and they stretch from these points horizontally across the cavity of the larynx, to be attached, close together, in the re-entering angle of the thyroid cartilage rather lower than half-way between its top and bottom.

Now when the arytenoid cartilages diverge, as they do when the larynx is in a state of rest, it is evident that the