The thyro-hyoid membrane extends between the posterior aspect of the upper border of the hyoid bone and great cornu, and the upper border of the thyroid cartilage, and is rather over an inch in vertical depth. This arrangement permits of the ascent of the larynx behind the hyoid in deglutition. Between the under surface of the hyoid bone and the anterior aspect of the membrane the thyro-hyoid bursa is placed. This sometimes becomes enlarged, necessitating removal. The membrane is superficial in front, is covered laterally by the sterno-, thyro-, and omohyoid muscles, and is pierced by the superior laryngeal artery and internal laryngeal nerve. The deep surface is connected by a pad of fatty tissue with the epiglottis and aryteno-epiglottic folds. The membrane is not infrequently divided in suicidal wounds, as are likewise the epiglottis and anterior wall of the pharynx. The superior thyroid artery and internal laryngeal nerve frequently suffer, while the larger vessels, owing to their mobility, escape. Division of the internal laryngeal nerve causes anaesthesia of the laryngeal mucous membrane, and permits particles of food to enter the air passages and set up septic pneumonia. An incision made through the membrane affords access to the upper portion of pharynx and larynx, and is sometimes useful in removing foreign bodies or tumours from these parts.
The larynx moves up and down in certain movements of the head and neck, in deglutition, slightly in respiration, and laterally from passive movement. It is more highly placed in children and women than in men, the cricoid being at the level of the lower border of the fourth cervical in the infant, while by puberty it has reached the adult position, opposite the lower border of the sixth. It is lined with mucous membrane, which is continuous with that of the pharynx above and trachea below. In the middle line it is quite superficial, being covered only by skin, subcutaneous tissue, and deep fascia, but laterally is covered by the sterno-, thyro-, omohyoid, and sterno-thyroid muscles, and the lateral lobes of the thyroid gland. Posteriorly, the lumen of the larynx is maintained at the expense of that of the pharynx, which is generally flattened. Laterally, on each side, there is a recess, the pharyngo-laryngeal, the upper extremity of which (situated above and external to the aryteno-epiglottic folds, and in which foreign bodies may lodge) is known as the pyriform fossa. The upper aperture of the larynx, when at rest, looks almost directly backwards. It is bounded in front by the epiglottis, laterally by the aryteno-epiglottic folds, and posteriorly by the arytenoid cartilages and the notch between them. The interior of the larynx is lined by mucous membrane, which varies much in thickness and laxness, according to the amount of subcutaneous tissue. The thickest portions are at the aryteno-epiglottic folds, the ventricle, false cords, and laryngeal portion of the epiglottis, and these are particularly affected in laryngitis and oedema of the glottis. The latter affection is a very serious one, which specially affects the aryteno-epiglottic folds (not the vocal cords), and so may cause asphyxia. At the true vocal cords the mucous membrane is firmly adherent, and presents stratified epithelium, whereas at other parts it is lined with ciliated epithelium. Epithelioma is not infrequently found at the point of junction of the two forms of epithelium, and this position is also a favourite one for papilloma. The mucous membrane is rich in mucous glands, except at the vocal cords, and these glands are specially numerous in the regions of the arytenoids, ventricle, and base of the epiglottis. The glands become affected in chronic glandular laryngitis, clergyman's sore-throat, etc. The mucous membrane receives its chief nerve-supply from the two internal laryngeal nerves (from the superior laryngeal of the vagus), and is extremely sensitive to contact with a foreign body, calling forth spasm and reflex expulsive cough, by which entrance of such bodies to the trachea is prevented. When these nerves are divided, this sensation is lost, foreign matter may enter the lungs, and so set up a septic broncho-pneumonia. As these nerves are also inhibited to a considerable extent by chloroform, it is necessary for the administrator to see that no blood, mucus, or other foreign matter is inspired during narcosis. The other branch of the superior laryngeal is the external laryngeal, which supplies the crico-thyroid, while the recurrent laryngeal, also a branch of the vagus, supplies all the intrinsic muscles, except the crico-thyroid. The mucous membrane receives its blood - supply from three vessels on either side - superior laryngeal and crico-thyroid from the superior thyroid, and inferior laryngeal from the inferior thyroid-and is very vascular, save at the true vocal cords. The lymphatics from parts above the glottis pass upwards and outwards through the thyro-hyoid membrane to end in glands under the great cornu of the hyoid, or at the bifurcation of the common carotid ; while those from below the glottis terminate in glands on either side of the trachea. The mucous membrane is thrown into two antero-posterior folds on either side, forming the vocal cords, which divide the larynx into three compartments-an upper, or vestibule, which extends from the aryteno-epiglottic folds to the false cords; a middle, between the false and true cords ; and a lower, which extends from the true cords to the lower border of the cricoid. The upper pair of cords, or false cords, are not so markedly developed, do not closely approach one another, and do not produce the voice. The true vocal cords, or inferior thyro-arytenoid ligaments, are inserted together anteriorly in the angle of junction of the thyroid alae, midway between the median notch and the lower border, while posteriorly they diverge, and are inserted into the processus vocales of the arytenoid cartilages. They consist of white fibrous tissue, covered with stratified mucous membrane. The lateral wall of the larynx, in the middle compartment, presents a recess, or pocket, on either side-the ventricle, from which, anteriorly, is given off a small diverticulum, the laryngeal saccule, which extends upwards between the false vocal cord and the ala of the thyroid cartilage. The rima glottidis is the fissure formed anteriorly by the true vocal cords (glottis vocalis), and posteriorly by the bases and vocal processes of the arytenoids (glottis respira-toria). It is the narrowest part of the larynx, being about 1/3 inch at its widest part, and is nearly ι inch long antero-posteriorly. It is situated rather below the centre of the cavity, and forms the communication between the middle and lower compartments of the larynx. In making a laryngoscopy examination, the tongue is pulled forwards and downwards, and the warmed mirror is introduced, pushing the soft palate upwards and backwards. Light from the forehead mirror having been thrown on it, the base of the tongue, epiglottis, and glosso-epiglottic folds are seen with the vallecula on either side of the median glosso-epiglottic fold (in which may lodge a foreign body). Then the aryteno-epiglottic folds, presenting posteriorly the elevations due to the cuneiform cartilages externally, and those of Santorini internally, are seen. The false cords appear fairly wide apart, and of a pink colour, while the true cords project inwards toward one another, are of a pearly-white colour, and are wide apart when the patient says ' ah,' and close together when he says 1 ee.' Between the false and true cords a dark interval indicates the entrance to the ventricle. When the glottis is wide open, it is sometimes possible to see as far as the bifurcation of the trachea.