Again—the next surface is that involution of the skin which covers the eye, and which lines the mouth, throat, and the whole of the interior surface of the respiratory tubes and the digestive organs. Here again we find the _next_ seat of greatest frequency, and the conservative tendency, to coincide. We need only refer to the comparative frequency of what are called colds, ordinary sore throat, and so forth; as contrasted with those more serious diseases which occur in the corresponding surfaces of the respiratory organs and alimentary canal. In tracing diseases onwards in the order of their number, we never lose sight of this conservative tendency. When _organs_ become involved in disease, we find that, for once that the _substance_ of the organ is so _affected_, the _membrane covering_ it is affected a hundred, perhaps a thousand times. This is equally observable with respect to the brain, heart, lungs, digestive organs, and some other parts; and it is of great importance practically to know how readily affections are transferred from the _lining_ of the alimentary canal and other parts to the membrane _covering_ it, rather than to the intermediate texture of the organ; again impressing, though now in a dangerous type truly, the conservative _tendency_ of the law. Finally, then, we arrive at diseases of Organs; and here we see this conservative tendency still typed in the site first chosen, which is almost always (where we can distinguish the two structures) not so much in the _actual tissue_ of the organ as in that which connects it together—what we term the _cellular tissue_. This is remarkable in the lungs; where tubercular deposits are first seated; not in the essential structures of the organs, but in those by which they are joined together. All those various depositions also which are called tumours, generally begin in, and are frequently confined to, the _cellular tissue_; and even though there is, in certain malignant forms of tumour, a disposition to locate themselves in organs, there is a very curious tendency towards such, as may have _already fulfilled_ their purposes in the animal economy.