air leads to such marked contraction of the bronchi that respiration becomes very difficult. Such an exaggerated tendency to reflex action constitutes the neurosis, asthma. In this malady the mechanism is unduly sensitive. Very slight stimulation leads to a maximum discharge of impulses to the muscular tissue of the bronchi. The trachea has a length of about 4 inches. It extends from the lower edge of the cricoid cartilage, which is easily felt in the neck beneath the thyroid cartilage (Adam’s apple), to the under side of the arch of the aorta, where it divides into the right and left bronchi. The epithelium which lines the trachea and bronchi is ciliated. The cilia propel the secretion which accumulates on its surface upwards towards the larynx. The wall of the windpipe is kept open by rings of cartilage which are incomplete behind, where the trachea and œsophagus are in contact. Rings and plates of cartilage also support the bronchi. The bronchi divide and subdivide until their diameter is reduced to about 0·2 millimetre. Each bronchiole then breaks up into a bunch of very thin-walled, elongated infundibula, club-shaped, and with a diameter about five times that of the bronchiole with which they are connected. They may be three or four times as long as they are broad. The wall of an infundibulum is pitted like a piece of honeycomb into shallow chambers—the air-cells or alveoli. The walls of the air-chambers, or alveoli, are formed of a membrane upon which is spread a network of capillary bloodvessels. The air-chambers are so closely packed together that a common wall separates one chamber from the next adjoining. Minute bloodvessels pierce the partitions which separate the chambers, appearing now on one side of the wall, now on the other. The air-chambers are lined by thin epithelial scales or tiles. The blood in the capillary vessels is separated from the air in the air-chambers by the wall of the capillary; by a lymph-space, probably rather potential than actual; and by the epithelial tiles. This covering suffices to prevent the escape both of red corpuscles and of plasma, yet offers very little resistance to the passage of gases from the blood into the air, and from the air into the blood. Leucocytes make their way between the tiles, and creep over their internal surfaces, searching for cell débris or foreign matter. Anything that they find they carry to the clumps of lymphoid tissue which occur in the outer wall of the bronchi. In a town-dweller, leucocytes are found in these lymph-thickets, charged with particles of soot. They show droplets of fat and other evidences of degeneration. At other spots are to be seen little collections of soot which have been left behind after the dissolution of the leucocytes which brought them there.