On either side of it lie the great blood vessels of the neck, behind it forms a part of the boundary of the pharynx, and is covered by the mucous membrane lining that cavity. Its vertical extent corresponds to the fourth, fifth, and sixth cervical vertebrae. It is placed somewhat higher in the female than in the male. The movements of the head affect the position of the larynx. When the head is drawn back, the larynx is lifted, and when the chin approaches the chest the larynx is depressed. During swallowing the larynx moves distinctly; during singing it moves slightly. Until puberty there is no marked difference between the larynx of the male and that of the female. In the male after puberty all the cartilages increase in size, and the larynx becomes prominent as the Adam's apple in the middle line of the neck. In the female after puberty the increase of size is only slight. The larynx is broad above, where it presents a triangular appearance, flattened behind and at the sides. Below it is narrow and cylindrical. It is composed of cartilages which are connected together by ligaments and moved by numerous muscles. It is lined by a mucous membrane which is continuous above with the lining of the pharynx and below with that of the trachea. The arteries that supply the larynx are the laryngeal arteries, branches of the superior and inferior thyroid arteries. The superior laryngeal vein runs into the superior thyroid vein and then into the internal jugular vein, while the inferior laryngeal vein runs into the inferior thyroid vein and then into the innominate vein. =The Trachea.=—The trachea or windpipe is a cartilaginous elastic, cylindrical tube, flattened posteriorly. It extends from the lower part of the larynx, on a level with the sixth cervical vertebra to opposite the body of the fourth dorsal, where it divides into two bronchi, one for each lung. [Illustration: FIG. 17—The cartilages of the larynx; the trachea and bronchi. (Gray)] The trachea is in the median line of the body. It measures about four and one-half inches in length. The diameter is from three quarters to one inch, being always greater in the male than in the female. The trachea is composed of imperfect cartilage rings, not coming quite together in the back. The artery that supplies the trachea is the inferior thyroid artery. The vein that withdraws the blood is the inferior thyroid vein. _The Right Bronchus._—The right bronchus is shorter, and wider than the left bronchus. It is about one inch in length. It enters the lung opposite the fifth dorsal vertebra. _The Left Bronchus._—The left bronchus is smaller and longer than the right. It is two inches in length and enters the lung at a point opposite the body of the sixth dorsal vertebra. Each bronchus divides into smaller divisions called bronchial tubes. Each bronchial tube divides into still smaller divisions called bronchioles. Each bronchiole ends in the air cell.