It is as true as it is trite that the cause of disease is never a single factor. Two elements always enter into etiology. One of these is inherent in the germ plasm of the individual; the other brought to bear upon the organism from beyond the confines of the germinal elements from which it has developed. Better than internal and external, endogenous and exogenous, or constitutional and environmental, to designate these two groups of factors, are the terms intrinsic and extrinsic.