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bral column, with some tendency to fall forward. There are no ligamentous structures which can lock the joints so as to keep the head in the erect position ; therefore, without the aid of muscular force, the head will fall forward or backward, according to the position it may be in when the muscles suddenly relax, as happens in falling asleep in an upright posture.
From the foregoing facts it will be seen that there exists a kind of coördinated antagonism at work in ordinary easy standing which keeps the elastic, pliable body upright, without the rigidity adopted when standing "at attention." The muscular action is more exercised when we are not on steady ground, and varied coördination becomes necessary; for instance, when we go on board ship for the first time. Standing then takes some little time to become easy, and requires new associations of movement. The gastrocnemius and soleus relax the ankle in a degree just proportionate to the amount of flexion of the knee permitted by the quadriceps extensor cruris, while, simultaneously, the great gluteal muscle allows the body to incline forward so as to keep its centre of gravity in the proper relation to the basis of support.
WALKING AND RUNNING. Walking is accomplished by poising the weight on one foot and then tilting the body forward with the other, which is then swung in front and placed on the ground to prevent falling. These acts are performed alternately by each leg, so that the "swinging limb” becomes the “ supporting limb " of the next step. The swinging leg is described as having two phases, (1) active, while pushing off from the ground, and (2) passive, while swinging forward like a pendulum. In starting, one foot is placed behind the other, so that the line of gravity lies between the two, the hindmost limb having the ankle and knee a little bent. By suddenly straightening these joints it gives a “push off" with the toes and propels the body forward, so as to move it around the axis of motion of the fixed, or supporting ankle joint. At the end of the swing, the pendulous leg comes to the ground, and leaves the other limb in the attitude ready for the push off. Thus, on level ground walking is carried on with but small mus
cular exercise ; but in ascending an incline or going up stairs, the supporting limb has to elevate the body at each step by extending the knee and ankle joints by the thigh extensors and the calf muscles.
Running is distinguished from walking by the fact that, while in the latter both feet rest on the ground for the greater part of each pace, in the former the time that either foot rests on the ground is reduced to a minimum, and the body can never be said to be balanced on either leg, so that, in fact, there is no longer
support limb.” The legs are kept in a semiflexed position, ready for the push off or spring, which is so forcibly carried out that the body is propelled through the air without any support between each step, and has a constant tendency to fall forward. Thus, an interval of greater or less duration, according to the pace, exists during which both the feet are off the ground, because, the moment either foot comes to the ground, it at once executes a new spring without waiting for the swing of the other.
VOICE AND SPEECH.
The human voice is produced by an expiratory blast of air being forced through the narrow opening at the top of the windpipe, called the glottis. This glottis, which lies in the lower part of the larynx, is bounded on each side by the edges of thin, elastic, membranous folds that project into the air passages. These membranous folds, called the vocal cords, are set vibrating by the current of air from below, and in turn communicate their vibrations to the air in the air passages situated above them.
ANATOMICAL SKETCH. The vocal apparatus produces sound in the same manner as a musical instrument of the reed-pipe variety. If we compare it
. with the pipe of an organ, we find all the parts of the latter represented. The lungs within the moving thorax act as the bellows. The bronchi and trachea are the supply pipes and air box. The vocal cords are the vibrating tongues ; while the larynx, pharynx, mouth and nose act as the accessory or resonating pipes. The blast of air is produced and regulated by the respiratory muscles; and special intrinsic muscles of the larynx change the conditions of the vocal cords so as to alter the pitch of the notes produced. Other sets of muscles, by altering the conditions of the resonating pipes, give rise to many modifications in the vocal tones, and thus produce what is called speech.
The larynx, which may be regarded as the special organ of voice, is made up of four cartilages, viz., the cricoid, thyroid and two arytenoids, jointed together so as to allow of considerable motion. Of these the inferior, the cricoid, is attached to the trachea, which it joins to the others. It forms a ring, which is thin in front, but deep and thick behind, owing to a peculiar projection upward of its posterior part. The thyroid consists of two side wings so bent as to form the greater part of the anterior
and lateral boundaries of the voice box, and can be felt easily in
FIG. 196. upper part of the cricoid cartilage can move backward and forward. The arytenoid cartilages are little three-sided pyra- he midal masses placed on the upper surface of the posteriori part of the cricoid, to which they are attached by a loose joint. They are so placed that one surface looks inward, the second backward, and the third forward and outward, while the inferior surface rides on the cricoid. One point looks forward, and to it is attached the vocal cord on each side, hence it has been called the vocal process. The apex, which looks, outward and backward, gives attachment to some of the in- Anterior half of a transverse vertical section
through the larynx near its middle, seen trinsic muscles, and hence has
More is cut away on the been called the muscular pro
upper part of the right side. 1. Úpper division of the laryngeal cavity ; 2. Central portion; 3. Lower portion continued into
4, trachea; e, epiglottis; d', its cushion ; The thyroid cartilage is con
t, thyroid cartilage seen in section.
ventricle of larynx ; s', saccule. (A. Thomnected with the cricoid below,
son.) and with the hyoid bone above by ligaments and tough membranes, which hold the parts together, fill in the intervals, and complete the skeleton of the larynx.
The vocal cords are composed of small strands of elastic tissue, which are stretched between the anterior processes of the arytenoid cartilages and the inferior part of the thyroid, where they are attached side by side to the posterior surface of the angle
formed by the junction of the two lateral parts or alæ of the thyroid. The mucous membrane which lines the larynx is thin,
and closely adherent over the vocal cords. The surface of the laryngeal cavity is smooth and even, the lining membrane passing over the cartilages and muscles so as to obliterate all ridges except the vocal cords and two others, less sharply defined, called the false vocal cords, which lie parallel to and above the true vibrating cords.
Between these is the cavity known as the ventricle of the larynx. B'
MECHANISM OF VOCALIZATION. Shape of the Opening of the Glottis. – Taking the thyroid cartilage as the fixed base, the cricoid and arytenoid cartilages undergo movements which bring about two distinct sets of
changes in the glottis and its elastic edges, c'
namely, (1) widening and narrowing the opening ; (2) stretching and relaxing of the vocal cords. During ordinary respiration the glottis remains about half open, being slightly widened during inspiration (B'). During forced
inspiration the glottis is widely dilated by musDiagrams taken from cular action (C). If an irritating gas be
laryngoscopic inspired, the glottis is tightly closed by a spasview of the larynx, showing trans- modic action of certain muscles, so that the true verse section the position in which the vocal cords act as a kind of valve. vocal cords and the arytenoid cartilages During vocalization the glottis is formed into are supposed to be during different ac- a narrow chink with parallel sides (A'), while tions of the larynx. A'. Vocal chink, as in the cords are made more or less tense, accord
singing B”. In casy, quiet inha. ing to the pitch of the note to be produced; both C'. In forced inspira- these changes are brought about by muscular
action. The opening of the chink of the glottis is accomplished chiefly by a muscle called the posterior crico-arytenoid, which passes from the posterior surface of the cricoid cartilage to the outer
lation of air.