Secondary Effects. The various diseases of the respiratory sys tem occasion modifications of respiration in absolute rate, ratio borne by inspiration to expiration and rhythm. Thus the absolute rate is increased in acute pneumonia owing to diminution of the available area for respiration and to the fever; in anaemia when extra exertion is called for, because the necessary oxygen is not provided at a normal rate of breathing owing to the deficient haemoglobin content of the red blood corpuscles. Similarly, ex piration is prolonged in emphysema because the elasticity of the lung tissue is impaired, and rhythm is modified in exhaustion of the respiratory centre in the medulla oblongata (Cheyne-Stokes breathing). They lead to the introduction of abnormal respiratory acts, e.g., cough, and to a whole series of constitutional changes, e.g., fever, delirium, altered secretions by virtue of the pathological conditions underlying them, and by the interference they impose upon oxygenation of the blood they induce secondary changes in other organs. The more complex those organs the sooner they show the strain ; heart, kidney, brain fail sooner than skin, muscle, bone. Nevertheless in time even the last show changes ; skin be comes obviously oedematous, muscle gains or loses water as evi denced by its specific gravity, and bone may undergo associated changes as in the so-called pulmonary osteoarthropathies.
Hitherto the relations considered have been those pertaining to the various parts of the body, but diseases of the respiratory system also are related to the composition of the air inhaled. Irri
tating gases as ammonia, chlorine, poison gases as used in the World War induce oedema of the glottis, bronchitis, oedema of the lung, pneumonia; even tobacco smoke in excess leads to granular pharyngitis, irritable larynx, excessive secretion of mucus in the bronchi. Air-borne bacteria as B. tuberculosis and possibly the organisms or viruses of influenza, pneumonia, common catarrh, pleuro-pneumonia in cattle induce inflammatory changes, more or less acute, in bronchi and pulmonary tissue. Dust particles in haled in the course of occupation occasion a whole group of indus trial diseases. These affect stone-masons, knife-grinders, miners, wool and cotton spinners, indeed, any trade wherein the air of the factory is heavily laden with dry particles. In all these instances the lung condition is chronic and therefore associated with great formation of fibrous tissue (fibroid pneumonia) and lung dam aged in this way is peculiarly liable to tuberculosis. Hence the industrial diseases under consideration consist in the main of fibroid tuberculosis (fibroid phthisis). There is reason to believe that silica particles when inhaled are particularly injurious per haps by the local formation of small quantities of silicic acid.