situations in which they are heard, in order that he may be able, in abnormal conditions, to recognize that something is wrong. The chief departures from the normal may be grouped under the following heads: (1) The pulmonary murmur may lose its soft, smooth, sighing character owing to inflammation of the alveoli and infundibula. It may be as loud in expiration as in inspiration. Only a practised ear can estimate the significance of these changes. (2) The laryngeal murmur may be reinforced by “râles”—a convenient term for supplementary sounds. The source of such râles may be a cold in the chest, laryngitis, or bronchitis of various degrees. (3) The laryngeal murmur may be heard in situations in which lung intervenes between the ear and the larger bronchial tubes. This can be due only to the lung being in an abnormal condition as a conductor of sound. Instead of being as spongy as well-made Vienna bread, its air-spaces are filled with solid or fluid deposit. It is as firm as dough. To such a condition it attains at the height of pneumonia—a stage termed “hepatization” because in section it looks like liver rather than lung. Breathing is the enlargement and diminution of the chest, which causes air to be drawn into and expressed from the lungs. The windpipe being open, the air inside the lungs is, of course, at the same pressure as the atmosphere. Expansion of the chest results in the equal expansion of the lungs. Since there is no air-space between the outer surface of the lungs and the inner surface of the chest-wall, the lungs cannot separate from the chest-wall when it expands. But the lungs contain elastic tissue always slightly on the stretch. If the chest be punctured, and air admitted between the chest-wall and the lungs, the lungs collapse. The expiratory movement, the contraction of the chest, is due to the elasticity of the lungs. This tendency on the part of the lungs to contract is sufficient in quiet respiration to restore the chest to its usual size after inspiration, and thus to expel air. The lungs are held open owing to the negative pressure in the space which separates them from the chest-wall. This negative pressure has a most important relation to their permeability by air. Imagine the condition reversed. Picture a lung into which air is forced by a muscular pump. After each stroke of the pump the lung would collapse. Its finest tubes and their dilated terminations could be maintained as open spaces, between the strokes of the pump, only by giving a considerable thickness and firmness to their walls. Such a substantial structure would be unfavourable to an interchange of gases between the blood and the air. The reverse of this condition is found in Nature. The lung is stretched from without. Its tissue, delicate as crêpe, cannot collapse even at the end of the deepest expiration.