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Rh draws the hind ends of the rings together and so diminishes the calibre of the tube. Inside these is plentiful submucous tissue

After D. J. Cunningham, from Cunningham's Text-Book of Anatomy.

. 5.—The Trachea and Bronchi. The thyroid body is indicated by a dotted line.

containing mucous glands and quantities of lymphoid tissue, while the whole is lined internally by columnar ciliated epithelium. The Bronchi (see fig. 5) are the two tubes into which the trachea divides, but, since the branches, which these tubes give off later, are also called bronchi, it may be clearer to speak of primary, secondary and tertiary bronchi. Each primary bronchus runs downward and outward, but the right one is more in a line with the direction of the trachea than the left. The right primary bronchus has also a greater calibre than the left because the right lung is the larger, and for these two reasons when a foreign body enters the trachea it usually enters the right bronchus.

The first secondary bronchus comes off about an inch from the bifurcation of the trachea on the right side and, as it lies above the level of the pulmonary artery, it is known as the eparterial bronchus. On the left side the first branch is about two inches from the bifurcation and, like all the remaining secondary bronchi, is hyparterial: the left primary bronchus is therefore twice as long as the right. After the eparterial secondary bronchus is given off the direction of the right primary bronchus is carried on by the hyparterial secondary bronchus, and this, just before reaching the hilum of the lung, divides into upper and lower tertiary bronchi, while the left lower secondary hyparterial bronchus does not divide before reaching the hilum of its lung. Into the hilum or root of the right lung, therefore, three bronchial tubes enter, while on the left side there are only two. The firmly rooted habit of associating the term bronchi with those parts of the main tubes which lie between the bifurcation of the trachea and the point where the first branch comes off makes it very difficult to suggest a nomenclature which calls up any picture of the actual state of things to the mind. Certainly the classification into primary, secondary and tertiary bronchi only goes a very little way toward this, and it should be realized that, call them what we may, there 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 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. Huntington's " Eparterial Bronchial System of the Mammalia, " Am. Journ. Med. Sci. (Phila. 1898). See also Quain's Anatomy, London, last edition.)

The Lungs are two pyramidal, spongy, slate-coloured, very vascular organs in which the blood is oxygenated. Each lies in its own side of the thorax and is surrounded by its own pleural cavity (see COELOM and Smiocs 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, pericardium 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 Jissure 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. The structures contained in the root of each lung arerthe branches and tributaries of (1) the pulmonary artery, (2) the pulmonary veins, (3) the bronchi, (4) the bronchial arteries, (5) the bronchial veins, (6) the bronchial lymphatic vessels and glands, (7) the pulmonary plexuses of nerves. Of these the first three are the largest and, in dividing the root from in front, the veins are first cut, then the arteries and last the bronchi. As has been pointed out already, the eparterial bronchus on the right side is above the level of the artery, but all the others -(hyparterial) are on a lower level.

The bronchial arteries supply the substance of the lung; there are usually two on each side, and they lie behind the bronchi. The blood which they carry is chiefly returned by the pulmonary veins bringing oxidized blood back to the heart, so that here there is a normal and harmless mixture of arterial and venous blood. If there are any bronchial veins (their presence is doubted by some, and the writer has himself carefully but unsuccessfully searched for them several times), they open into the azygos veins of their own side. The bronchial lymphatic vessels lie behind the pulmonary vessels and open into several large glands which are black from straining off the carbon left in the lungs from the atmosphere.

There is an anterior and posterior pulmonary plexus of nerves on each side, the fibres of which are derived from the vagus and the upper thoracic ganglia of the sympathetic. Structure of the Lungs.—As the bronchi become smaller and smaller by repeated division, the cartilage completely surrounds them and tends to form irregular plates instead of rings -they are therefore cylindrical, but when the terminal branches (lobular bronchi) are reached, the cartilage disappears and hemispherical bulgings called alveoli occur (fig. 6 A). At the very end of