Delicate, hair-like filaments, not unlike the pile on velvet, called cilia, spring from the epithelial lining of the air tubes.  Their constant wavy movement is always upwards and outwards, towards the mouth.  Thus any excessive secretion, as of bronchitis or catarrh, is carried upwards, and finally expelled by coughing.  In this way, the lungs are kept quite free from particles of foreign matter derived from the air.  Otherwise we should suffer, and often be in danger from the accumulation of mucus and dust in the air passages.  Thus these tiny cilia act as dusters which Nature uses to keep the air tubes free and clean (Fig. 5).

[Illustration:  Fig. 87.—­Bronchial tube, with its Divisions and Subdivisions. (Showing groups of air cells at the termination of minute bronchial tubes.)]

206.  The Lungs.  The lungs, the organs of respiration, are two pinkish gray structures of a light, spongy appearance, that fill the chest cavity, except the space taken up by the heart and large vessels.  Between the lungs are situated the large bronchi, the oesophagus, the heart in its pericardium, and the great blood-vessels.  The base of the lungs rests on the dome-like diaphragm, which separates the chest from the abdomen.  This partly muscular and partly tendinous partition is a most important factor in breathing.

Each lung is covered, except at one point, with an elastic serous membrane in a double layer, called the pleura.  One layer closely envelops the lung, at the apex of which it is reflected to the wall of the chest cavity of its own side, which it lines.  The two layers thus form between them a Closed Sac a serous cavity (see Fig. 69, also note, p. 176).

[Illustration:  Fig. 88.—­The Lungs with the Trachea, Bronchi, and Larger Bronchial Tubes exposed. (Posterior view.)

  A, division of left bronchus to upper lobe;
  B, left branch of the Pulmonary artery;
  C, left bronchus;
  D, left superior pulmonary vein;
  E, left inferior pulmonary vein;
  F, left auricle;
  K, inferior vena cava;
  L, division of right bronchus to lower lobe;
  M, right inferior pulmonary vein;
  N, right superior pulmonary vein;
  O, right branch of the pulmonary artery;
  P, division of right bronchus to upper lobe;
  R, left ventricle;
  S, right ventricle.
]

In health the two pleural surfaces of the lungs are always in contact, and they secrete just enough serous fluid to allow the surfaces to glide smoothly upon each other.  Inflammation of this membrane is called pleurisy.  In this disease the breathing becomes very painful, as the secretion of glairy serum is suspended, and the dry and inflamed surfaces rub harshly upon each other.

The root of the lung, as it is called, is formed by the bronchi, two pulmonary arteries, and two pulmonary veins.  The nerves and lymphatic vessels of the lung also enter at the root.  If we only remember that all the bronchial tubes, great and small, are hollow, we may compare the whole system to a short bush or tree growing upside down in the chest, of which the trachea is the trunk, and the bronchial tubes the branches of various sizes.