From what has gone before it will appear that the condition of the tissues forms frequently a strong predisposing element in the causation of disease. It will also appear, however, that there are certain conditions of the tissues which in themselves are counted diseases, and which arise without any apparent external cause having acted: they are in this sense spontaneous, although this expression, as well as the synonymous one idiopathic, merely indicates that the cause is obscure.

Looking at the tissues as living, we recognize that in their activity they are capable of morbid change in the direction of defect on the one hand and excess on the other. External causes may produce either of these morbid changes, damaging the tissues on the one hand- or stimulating them to undue activity on the other. But the tissues in themselves may be primarily defective or excessive, and may be so either in actual structure or in their tendencies and susceptibilities. It will be proper to consider several different kinds of influence.