A diagram showing a vertical section through the middle of the larynx at right angles to the vocal cords shows some important facts in connection with the mechanism of this portion of the vocal instrument (_vide_ fig. 8). It will be observed that the sound-pipe just beneath the membranous reed assumes the form of a cone, thus the expired air is driven like a wedge against the closed glottis. Another fact of importance may be observed, that above the vocal cords on either side is a pouch called a ventricle, and the upper surfaces of the vocal cords slope somewhat upwards from without inwards, so that the pressure of the air from above tends to press the edges together. The force of the expiratory blast of air from below overcomes the forces which approximate the edges of the cords and throws them into vibration. With each vibration of the membranous reeds the valve is opened, and as in the case of the siren a little puff of air escapes; thus successive rhythmical undulations of the air are produced, constituting the sound waves. The pitch of the note depends upon the number of waves per second, and the _register_ of the voice therefore depends upon two factors: (1) the size of the voice-box, or larynx, and the length of the cords, and (2) the action of the neuro-muscular mechanism whereby the length, approximation, and tension of the vocal cords can be modified when singing from the lowest note to the highest note of the register. Thus the compass of the-- Bass voice is D to f 75- 354 vibs. per sec. Tenor " c " c'' 133- 562 " " Contralto " e " g'' 167- 795 " " Soprano " b " f''' 239-1417 " " The complete compass of the human voice therefore ranges from about D 75 to f''' 1417 vibrations per second, but the quality of the same notes varies in different individuals. [Illustration: Fig. 9] [Description: Fig. 9.--_Description of the laryngoscope and its mode of use_.--The laryngoscope consists of a concave mirror which is fixed on the forehead with a band in such a way that the right eye looks through the hole in the middle. This mirror reflects the light from a lamp placed behind the right side of the patient, who is told to open the mouth and put out the tongue. The observer holds the tongue out gently with a napkin and reflects the light from the mirror on his forehead on to the back of the throat. The small mirror, set at an angle of 45° with the shaft, is of varying size, from half an inch to one inch in diameter, and may be fixed in a handle according to the size required. The mirror is warmed to prevent the moisture of the breath obscuring the image, and it is introduced into the back of the throat in such a manner that the glottis appears reflected in it. The light from the lamp is reflected by the concave mirror on to the small mirror, which, owing to its angle of 45°, illuminates the glottis and reflects the image of the glottis with the vocal cords.]