It is not possible to make anything like an accurate estimate of the number of alveoli in the lungs; 725,000,000 is a figure arrived at by measuring the average cubic capacity of an alveolus, and comparing it with the total cubic capacity of the lungs. Each alveolus supports some forty or fifty capillary vessels. The superficial area of vascular membrane exposed is placed at 90 square metres, or about 100 times the area of the skin. Figures such as these convey very little meaning, but they help one to realize the magnitude of the provision made for the aeration of the blood. Pneumonia is a condition in which the lining of the air-chambers is inflamed, usually, possibly always, owing to the entrance of bacteria. Lymph exudes through the walls of the alveoli. Epithelial scales flake off. Pus cells (dead leucocytes) accumulate in the air-chambers. Respiration is curtailed, and dyspnœa results. After a time, if the case progresses favourably, “resolution,” as it is technically termed, begins to occur. The exuded substances are either expectorated or absorbed, and the lung-tissue returns to a normal condition. Here a few words may be devoted to respiratory sounds. _Spirare_ means to sigh. Breathing received the name by which it is known in physiology from the sound which accompanies the exit of air from the nostrils. Since the introduction of auscultation as a means of ascertaining the condition of the lungs, other sounds, not heard until the ear or a stethoscope is placed against the chest, have acquired great importance. These sounds, termed “murmurs,” may be divided into two classes. (_a_) When the ear is placed against the windpipe, or in the middle of the back between the shoulder-blades, a murmur is audible, due to the movement of air through the larynx. If the larynx, the trachea, or the bronchi contain mucus, it is a harsh, rough, bubbling, or crackling sound. It accompanies both inspiration and expiration. (_b_) A softer, more delicate murmur is heard when the ear is placed against the front or the side of the chest. This is the vesicular or pulmonary murmur. It is heard during inspiration, and is due to the passage of air out of the smallest bronchi into the more spacious infundibula in which they end. These two kinds of murmur must be rigidly distinguished—the laryngeal murmur, heard in situations in which no lung-tissue intervenes between the ear and the great tracheal or bronchial tubes; and the pulmonary murmur, heard over all regions where the bronchi are buried in lung. Healthy lung is as bad a conductor of sound as a sponge or a wad of cotton-wool. The laryngeal murmur is inaudible in regions in which lung lies beneath the chest-wall. It would be far beyond the scope of this book to attempt to describe the very varied alterations in the chest-sounds which may be produced by disease. The student would do well to familiarize himself with the nature of the sounds which are heard in health, and the