In examining the lungs after death we seldom meet with them in a perfectly normal condition. They may be abnormally adherent to the wall of the thorax, or unduly pigmented, or there may be cicatrices in them, or oedema, and so on. The explanation of this is that the lungs are peculiarly exposed to deleterious influences in two directions. The air passing into them is apt to carry irritating materials with it, and the blood circulating so richly through their tissue is liable to variations in its constitution and degree of pressure. We have already seen, for instance, that organic disease of the heart has serious effects on the pulmonary circulation, but apart from that, simple weakness of the heart may, as we shall afterwards see, have important effects on the lung. It must not be forgotten also that there is no organ of the body whose tissue is so intimately related to its blood-vessels as the lungs. These organs are little more than a congeries of blood-vessels with a sufficient supporting stroma. Any deleterious substance circulating in the blood, therefore, is very prone to affect the lungs, especially if there be any special weakness in this direction. We have abundant illustration of this in the frequency of lung complications in the acute fevers.

There are one or two points in the anatomical relations of the lungs which should be kept in mind. They are supplied with two different sets of blood-vessels, those of the Pulmonary artery on the one hand, and of the Bronchial artery on the other. We should remember the distribution of these, and not confuse effects due to obstruction of the one with those due to obstruction of the other. Then we speak of diseases affecting the Respiratory surface on the one hand, and the Supporting structures on the other; that is to say, there are some diseases which affect the surfaces of the finer bronchi and of the alveoli, while others involve the walls of the alveoli and of the bronchi and their supporting connective tissue. It is true that these two are generally involved together, but in different cases the one or the other is primarily concerned, and usually retains the lead. It is obvious that to a certain extent the determining cause of the disease will have something to do with this. An agent which acts by being carried into the lungs with the air will mostly affect the surface of the alveoli and bronchi in the first place, whereas an agent arriving by the blood will be more apt to attack the walls. We shall see, however, that to this there are important exceptions, because, on the one hand, the capillary vessels have very close relations with the surface of the alveoli, and, on the other hand, substances arriving from without very readily penetrate into the substance of the lung. A more important consideration arising from the anatomical relations has reference to the distribution of a lesion in larger or smaller districts of the lung. When a disease is one primarily of the parenchyma of the lung, then we should expect it to be distributed over a large extent of lung tissue or over the whole; the disease will be Lobar. But if the disease is one primarily of the bronchi and affects the alveoli secondarily, then we should expect it to extend to the proper lung tissue more irregularly, here and there an extension corresponding with a particular minute bronchus"; the disease will be Lobular.

The Lymphatics of the lung are important as they often convey different kinds of solid particles and are the means of dissemination of these. The lymphatics are present wherever connective tissue exists. They exist in the walls of the alveoli and in the interlobular tissue, in the subpleural tissue, and in the connective tissue around the bronchi and vessels, where they form the peribronchial and perivascular lymphatics. The internal surface of the alveoli is closely related to the lymphatics, so that solid particles readily pass from the alveoli into the lymphatics and may be carried thence throughout the lymphatic-system of the lung, and onwards to the bronchial lymphatic glands. (See under Inhalation of Foreign Substances).