the excreted nitrogen approximates more and more closely to the ingested nitrogen until at last the amounts are equal in both cases. This is called nitrogenous equilibrium. There may, however, be an increase of weight which is due to the putting on of fat. If this is the case it must be apparent that the protoplasm of the tissues is able to form fat out of proteid material and to split it up into urea and fat. If fat be given in small quantities with the meat, for a time the carbon of the egesta and ingesta are equal, but if the fat be increased beyond a certain point the body weight increases from a deposition of fat; not, however, by a mere mechanical deposition or filtration from the blood, but by an actual act of secretion by the protoplasm whereby the fat globules are stored up within itself: similarly as regards carbohydrates, if they are in small quantity, the carbon appears in the excreta, but beyond a certain amount a considerable portion of it is retained in fat, having been by the protoplasm stored up within itself in that material. The amount of proteid material required to produce nitrogenous equilibrium is considerable, but it may be materially diminished by the addition of carbo-hydrate or fatty food or of gelatine to the exclusively meat diet.

It is of much interest to consider how the protoplasm acts in converting food into energy and decomposition products, since the substance itself does not undergo much change in the process except a slight amount of wear and tear. We may assume that it is the property of protoplasm to separate from the blood the materials which it may require to produce secretions, in the case of the protoplasm of secreting glands, or to enable it to evolve heat and energy, as in the case of the protoplasm of muscle. The substances are very possibly different for each process, and the decomposition products, too, may be different in quality or quantity. Proteid materials appear to be specially needed, as is shown by the invariable presence of urea in the urine even during starvation; and as in the latter case, there has been no food from which these materials could have been derived, the urea is considered to be derived from the disintegration of the nitrogenous tissues themselves. The removal of all fat from the body in a starvation period, as the first apparent change, would lead to the supposition that fat is also a specially necessary pabulum for the production of protoplasmic energy; and the fact that, as mentioned above, with a diet of lean meat an enormous amount appears to be required, suggests that in that case protoplasm obtains the fat

it needs from the proteid food, which process must be evidently a source of much waste of nitrogen. The idea that proteid food has two destinations in the economy, viz., to form organ or tissue proteid which builds up organs and tissues, and circulating proteid, from which the organs and tissues derive the materials of their secretions or for producing their energy, is a convenient one, and it is unlikely that protoplasm would go to the expense of construction simply for the sake of immediate destruction.

CHAPTER XVI.

THE VOICE AND SPEECH.

The Larynx.-In nearly all air-breathing vertebrate animals there are arrangements for the production of sound, or voice, in some parts of the respiratory apparatus. In many animals, the

Fig. 302.-The Larynx, as seen from the front, showing the cartilages and ligaments. The muscles, with the exception of one crico-thyroid, are cut off short. (Stoerk.)

sound admits of being variously modified and altered during and after its production; and, in man, one such modification occurring in obedience to dictates of the cerebrum, is speech.

It has been proved by observations on living subjects, by means of the laryngoscope (p. 500), as well as by experiments on the larynx taken from the dead body, that the sound of the human voice is the result of the vibration of the inferior laryngeal ligaments, or true vocal cords (A, cv, fig. 307) which bound the glottis, caused by currents of expired air impelled over their edges. If a free opening exists in the trachea, the sound of the voice ceases, but it returns if the opening is closed. An opening into the air-passages above the glottis, on the contrary, does not prevent the voice being produced. By forcing a current of air through the larynx in the dead subject, clear vocal sounds are elicited, though the epiglottis, the upper ligaments of the larynx or false vocal cords, the ventricles between them and the inferior ligaments or true vocal cords, and the upper part of the arytenoid cartilages, be all removed; provided the true vocal cords remain entire, with their points of attachment, and be kept tense and so approximated that the fissure of the glottis may be narrow.

Fig. 303.-The larynx as seen from behind after removal of the muscles. The cartilages and ligaments only remain. (Stoerk.)

The vocal ligaments or cords, therefore, are regarded as the proper organs for the production of vocal sounds: the modifications of these sounds being effected, as will be presently explained, by other parts-tongue, teeth, lips, etc., as well as by them. The

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structure of the vocal cords is adapted to enable them to vibrate like tense membranes, for they are essentially composed of elastic tissue; and they are so attached to the cartilaginous parts of the larynx that their position and tension can be variously altered by the contraction of the muscles which act on these parts.

Thus it will be seen that the larynx is the organ of voice. It may be said to consist essentially of the two vocal cords and the various cartilaginous, muscular, and other apparatus by means of which not only can the aperture of the larynx (rima glottidis), of which they are the lateral boundaries, be closed against the entrance and exit of air to or from the lungs, but also by means of which the cords themselves can be stretched or relaxed, shortened or lengthened, in accordance with the conditions that may be necessary for the air in passing over them, to set them vibrating and produce various sounds. Their action in respiration has been already referred to.

Anatomy of the Larynx.-The principal parts entering into the formation of the larynx (figs. 302 and 303) are the thyroid cartilage; the cricoid cartilage; the two arytenoid cartilages; and the two true vocal cords (fig. 307). The epiglottis (fig. 303), has but little to do with the voice, and is chiefly useful in protecting the upper part of the larynx from the entrance of food and drink in deglutition. It also probably guides mucus or other fluids in small amount from the mouth around the sides of the upper opening of the glottis into the pharynx and œsophagus: thus preventing them from entering the larynx. The false vocal cords (ers, fig. 307), and the ventricle of the larynx, which is a space between the false and the true cord of either side, need be here only referred to.

Cartilages. (a) The thyroid cartilage (fig. 304, 1 to 4) does not form a complete ring around the larynx, but only covers the front portion. (b) The cricoid cartilage (fig. 304, 5, 6), on the other hand, is a complete ring; the back part of the ring being much broader than the front. On the top of this broad portion of the cricoid are (c) the arytenoid cartilages (fig. 304, 7), the connection between the cricoid below and arytenoid cartilages above being a joint with synovial membrane and ligaments, the latter permitting tolerably free motion between them. But although the arytenoid cartilages can move on the cricoid, they of course accompany the latter in all its movements, just as the head may nod or turn on the top of the spinal column, but must accompany it in all its movements as a whole.

Joints and Ligaments.-The thyroid cartilage is also connected with the cricoid, not only by ligaments, but also by joints with synovial membranes ; the lower cornua of the thyroid clasping, or nipping, as it were, the cricoid between them, but not so tightly but that the thyroid can revolve, within a certain range, around an axis passing transversely through the two joints at which the cricoid is clasped. The vocal cords are attached (behind) to the front portion of the base of the arytenoid cartilages, and (in front) to the re-entering angle at the back part of the thyroid; it is evident,

therefore, that all movements of either of these cartilages must produce an effect on them of some kind or other. Inasmuch, too, as the arytenoid cartilages rest on the top of the back portion of the cricoid cartilage, and are connected with it by capsular and other ligaments, all movements of the cricoid cartilage must move the arytenoid

cartilages, and also produce an effect on the vocal cords.

Intrinsic Muscles. The so-called intrinsic muscles of the larynx, or those which, in their action, have a direct action on the vocal cords, are nine in number-four pairs, and a single muscle; namely, two crico-thyroid muscles, two thyro-arytenoid, two posterior ericoarytenoid, two lateral crico-arytenoid, and one arytenoid muscle. Their actions are as follows:-When the crico-thyroid muscles (10, fig. 306) contract, they rotate the cricoid on the thyroid cartilage in such a manner, that the upper and back part of the former, and of necessity the arytenoid cartilages on the top of it, are tipped backwards, while the thyroid is inclined forward; and thus, of course, the vocal cords being attached in front to one, and behind to the other, are "put on the stretch."

The thyro-arytenoid muscles on the other hand, have an opposite actionpulling the thyroid backwards, and the arytenoid and upper and back part of the cricoid cartilages forwards, and thus relaxing the vocal cords.

Fig. 304.-Cartilages of the larynx seen from the front. 1 to 4, thyroid cartilage; 1, vertical ridge or pomum Adami; 2, right ala; 3. superior, and 4, inferior cornu of the right side; 5, 6, cricoid cartilage; 5, inside of the posterior part; 6, anterior narrow part of the ring; 7, arytenoid cartilages. x 1.

The crico-arytenoidei postici muscles (fig. 303) dilate the glottis, and separate the vocal cords, the one from the other, by an action on the arytenoid cartilage. By their contraction they tend to pull together the outer angles of the arytenoid cartilages in such a fashion as to rotate the latter at their joint with the cricoid, and of course to throw asunder their anterior angles to which the vocal cords are attached.

These posterior crico-arytenoid muscles are opposed by the crico-arytenoidei laterales, which, pulling in the opposite direction from the other side of the axis of rotation, have of course exactly the opposite effect, and close the glottis.

The aperture of the glottis can be also contracted by the arytenoid muscle (tig. 305), which, in its contraction, pulls together the upper parts of the arytenoid cartilages between which it extends.

Nerve Supply.-In the performance of the functions of the larynx the sensory filaments of the superior laryngeal branch of the vagi supply that acute sensibility by which the glottis is guarded against the ingress of foreign bodies, or of irrespirable gases. The contact of these stimulates the nerve filaments; and the impression conveyed to the medulla oblongata,