The mucous membrane of the larynx is continuous with that of the pharynx at the aryteno-epiglottidean folds which run from the sides of the epiglottis to the top of the arytenoid cartilages (see fig. 3). To the outer side of each fold is the sinus pyriformis (see PHARYNX). From the middle of the junction of the alae of the thyroid cartilage to the vocal processes of the arytenoids the mucous membrane is reflected over, and closely bound to, the true vocal cords which contain elastic tissue and, as has been mentioned, are the upper free edges of the lateral parts of the crico-thyroid membrane. The chink between the two true vocal cords is the glottis or rima glottidis. lust above the true vocal cords is the opening into a recess on each side which runs upward and backward and is known as the laryngeal saccule; its opening 1 is the laryngeal sinus. The upper lip of this slit-like opening is called the false vocal cord.
The mucous membrane is closely bound down to the epiglottis and to the true vocal cords, elsewhere there is plenty of sub mucous tissue. In the upper part of the front and sides of the larynx and over the true vocal cords the mucous membrane is lined by squamous epithelium, but elsewhere the epithelium is columnar and ciliated : it is supplied by the superior laryngeal of these long tubes or stem bronchi is outside the lung and in the middle mediastinum of the thorax, the lower part embedded in the substance of the lung. The structure of the bronchi is practically identical with that of the trachea. (See G. S. Hunting ton's "Eparterial Bronchial System of the Mammalia," Am. Journ. Med. Sci. [Phila. 1898]. See also Quain's Anatomy, Lon don, last edition.) Lungs.—The Lungs are two pyramidal, spongy, very vascular organs in which the blood is oxygenated. They are pink normally, but, often in city dwellers are slate-coloured from local deposition of soot particles. Each lies in its own side of the thorax and is surrounded by its own pleural cavity (see COELOM AND SEROUS MEMBRANES), and has an apex which projects into the side of the root of the neck, a base which is hollowed for the convexity of the diaphragm, an outer surface which is convex and lies against the ribs, an inner surface concave for the heart, peri branch of the vagus nerve and above the glottis is peculiarly sensitive.
Trachea.—The Trachea or windpipe (see fig. 5) is the tube which carries the air between the larynx and the bronchi; it is from four to f our and a half inches long and lies partly in the neck and partly in the thorax. It begins where the larynx ends
at the lower border of the sixth cervical, and divides into its two bronchi opposite the fifth thoracic vertebra. The tube is kept always open by rings of cartilage, which, however, are wanting behind, and, as it passes down, it comes to lie farther and farther from the ventral surface of the body, following the concavity of the thoracic region of the spinal column. In the whole of its downward course it has the oesophagus close behind it, while in front are the isthmus of the thyroid, the left innominate vein, the innominate artery and the arch of the aorta. On each side of it and touching it is the vagus nerve.
The cervical part of the tube is not much more than an inch in length, but it can be lengthened by throwing back the head. This is the region in which tracheotomy is performed, and it should be remembered that in children, and sometimes in adults, the great left innominate vein lies above the level of the top of the sternum.
The trachea is made up of an external fibro-elastic membrane in which the cartilaginous rings lie, while behind, where these rings are wanting, is a layer of unstriped muscle which, when it contracts, draws the hind ends of the rings together and so diminishes the calibre of the tube. Inside these is plentiful sub mucous tissue containing mucous glands and quantities of lym phoid tissue, while the whole is lined internally by columnar ciliated epithelium.
Bronchi.—The Bronchi (see fig. 5) are the two tubes into which the trachea divides, but the branches, which these tubes give off later, are also called bronchi. Put shortly, they are two long tapering tubes which run from the bifurcation of the trachea to the lower and back part of each lung, and give off a series of large ventral and small dorsal branches. The upper part of each cardium and great vessels, a sharp anterior border which overlaps the pericardium and a broad, rounded posterior border which lies at the side of the spinal column. Each lung is nearly divided into two by a primary fissure which runs obliquely downward and forward, while the right lung has a secondary fissure which runs horizontally forward from near the middle of the primary fissure. The left lung has therefore an upper and lower or basal lobe, while the right has upper, middle and lower lobes. On the inner surface of each lung is the root or hilum at which alone its vessels, nerves and ducts (bronchi) can enter and leave it.