ANATOMY AND PHYSIOLOGY OF THE LARYNX 147 columnar ciliated over the rest of the larynx. This latter type comrnonK undergoes squamous metaplasia in response particularly to atmospheric pollution and smoking. The supraglottis is rich in mucous glands and lymphatics and so metastases are commoner in supraglottic tumours than others. There are no mucous glands in the glottis and very few in the subglottis. This latter compartment has a much poorer lymph drainage than the supraglottis accounting for the fact that only 1 in 5 patients with subglottic cancer have metastatic lymph nodes. The mucosa of the glottis and supraglottis is firmly bound down to the underlying tissue, but not so in the subglottic region. Here, the laxity of the tissue allows a dangerous degree of oedema, especially in children, where the diameter of the area is relatively smaller than in the adult. In this situation, a degree of oedema, which would not cause too much trouble to an adult, could be fatal in childhood. LARYNGEAL COMPARTMENTS The larynx is divided into three different compartments, namely, the glottis, supraglottis and subglottis. This division is not only of academic anatomical interest but is of prime importance in the staging and treatment of cancer of the larynx. The glottis is basically composed of the vocal cords from the extreme lateral edge of the cord to the inferior border of the medial surface of the cord. Anteriorly it extends from the anterior commissure (which is where the vocal cords meet) to the posterior commissure (where the arytenoids meet). Approximately half the length of the glottis is formed by the arytenoid body and vocal process (i.e. cartilage) and half by the membranous vocal cord. The subglottis extends from the lower border of the glottis to the inferior border of the cricoid. Below this is the trachea. The subglottis is dome shaped and is relatively small and restricted. The supraglottis extends from the upper border of the glottis inferiorly (the ventricle of the larynx) to the hyoid bone superiorly being bounded laterally by the ary-epiglottic folds; the pyriform fossa, which is part of the pharynx, begins at the edge of the ary-epiglottic folds. The suprahyoid epiglottis and vallecula are considered part of the oropharynx. PHYSIOLOGY The functions of the larynx are: (i) to protect the lung, (ii) to control air-flow and (iii) to initiate phonation. The protective function is well known to all who have choked on a crumb and have experienced the laryngeal spasm which follows. It is provided by three sphincter-like actions: (a) the approximation of the ary-epiglottic folds assisted by the epiglottis which tilts posteriorly, (b) the apposition of the false cords (ventricular folds) and (c) the closure of the vocal cords (folds) to prevent the entry of foreign material. During this phase respiration is arrested but once the foreign particle is ejected into the hypopharynx a long inspiration occurs.