At the angle of the thyroid cartilage in front are attached the epiglottis at the top and just below that the _superior_ or _false vocal cords_, two folds of mucous membrane enclosing the _superior thyro-arytenoid ligaments_. Lower still are found the _inferior_ or _true vocal cords_, which are formed by the _inferior thyro-arytenoid ligaments_ covered with a thin, tightly fitting mucous membrane. Both sets of vocal cords as well as the epiglottis may be seen by means of a head and a throat mirror. Between the true vocal cords is a narrow triangular interval called the _glottis_. It is by means of the vibrations of these cords that sound is produced. The false vocal cords cannot produce sound, though they can modify it indirectly. Quality of voice, as treble, base, etc., depends upon the size of the larynx and the length and elasticity of the vocal cords. Modulation is produced by changing the form of the cavity of the mouth and nose. In whispering the lips take the place of the vocal cords and produce sound by the vibration of their muscular walls. Instead of tracheotomy _laryngotomy_ is sometimes done in the depression between the thyroid and the cricoid, which may be felt on the living. _Foreign bodies_ sometimes get into the larynx and have to be removed, or the mucous membrane may become inflamed, causing _laryngitis_. _Syphilis_ attacks the larynx, and _tuberculosis_ and _cancer_ of the larynx occur, these last two being generally fatal. _Edema of the glottis_ may also occur. [Illustration: FIG. 47.--The larynx, trachea and bronchi (After Sobotta.)] =The Trachea.=--The trachea is a membranous tube extending down from the larynx for about four and a half inches to the fourth or fifth dorsal vertebra, where it divides into the right and left bronchi. It is formed of sixteen to twenty imperfect cartilaginous rings, open behind, enclosed in a double elastic fibrous membrane, and is lined with ciliated mucous membrane. The rings are for strength and in the interval at the back where they are wanting there is one layer of longitudinal and another of transverse unstriped muscle fibers. The passage is kept clear by the action of the cilia, which sweep up and out any particles of dust that become entangled in the mucus. _Tracheotomy_ is generally done about one inch below the cricoid, just above the sternal notch, incision being made through the cartilage. [Illustration: FIG. 48.--The upper thorax of a child eight years old, showing the thyroid and thymus glands. (Sobotta.)]