There remains for consideration a large class of what may be termed _constitutional diseases_, in which the local ailment is the outcome of a previous disorder of the whole system. These diseases are either acute or chronic. The acute constitutional diseases belong to the class of fevers. These are marked by certain local characteristics, as the swelling of the joints in acute rheumatism, the sore-throat in scarlatina, or the eruption on the skin in smallpox, and their course is more or less strictly limited by distinct periods of increase, acme, and decline. No such rule obtains in the case of consumption, scrofula, and rickets, which are instances of chronic constitutional diseases. In them too the local manifestations of the general disease vary also: the lungs being affected in one case of consumption, the bowels in another; while scrofula may show itself by affection of the glands in one case, by the formation of abscesses in a second, or by disease of the bones in a third. =Chronic Constitutional Diseases.=--It may perhaps be convenient to study first the chronic constitutional diseases; and afterwards to make a few, and they will be but few, remarks on fevers. =Consumption= and Scrofula, though similar, are not the same disease. Both, however, depend on some defect in the blood, as the result of which certain materials, incapable of being converted into the natural constituents of the body, are deposited in the substance of different external parts or internal organs. If deposited in small quantities, these materials may be absorbed, as it is termed, that is to say, got rid of, by natural processes, which even now we understand but imperfectly. If deposited more abundantly, they press upon and gradually spoil the healthy parts in which they are seated, and thereby interfere with the proper performance of their duties. Thus, the deposit of consumption encroaches on the proper substance of the lungs, and so lessens the area in which the blood is exposed to the air and purified: the deposit of scrofula around and in a joint interferes with its powers of movement. Nor is this all; but wherever any deposit has once taken place, it tends especially to increase in that very spot, guided as it were by a certain affinity; and the substance of the previously healthy part is removed as fresh deposit comes to occupy its place. Further, the matter deposited has no power of being changed into healthy substance of lung, or of bone, or of any other part.