toms of all the diseases point to nervous derangement; why there are varying periods of incubation; why there are hereditary tendencies for and against the particular diseases; why a certain measure of protection is afforded by an attack of one of these diseases; why a purely mental or nervous act excites these affections; and why there is so close a correspondence, running even with season, between physical and moral outbreaks of spreading and contagious affections.

Lastly, I maintain the hypothesis of nervous origin because of the practical usefulness of it in relation to prevention of disease. If the view were true, that the air around us is charged with invisible germs which come from whence we know not, which have unlimited power to fertilize, which need never cease to fertilize and multiply, what hope is there for the skill of man to overcome these hiddden foes? Why on some occasion may not a plague spread over the whole world, and destroy life universally?

The nervous hypothesis presents an altogether different aspect. It says to living men and women, it is you who are the producers of the communicable diseases, or if it be not you yourselves it is one of your lower earthmates in creation, some domestic animal that shares with you the power of producing a poisonous secretion and of giving an hereditary stainp of production to such poisonous product. It looks on the man or animal affected with a contagious disease as one precisely, for the time, in the position of the cobra or other animal that is naturally secreting a poison; and, recognizing this fact, it suggests at once that the danger is all but limited to the person affected.

Isolate that person from the rest of mankind, take care that his secretions, volatile, fluid, or solid, do not come in contact with the secretions of susceptible healthy persons, and the danger is over. With the recovery of that person, that is to say, with restoration in him of a natural secretive process, the poison is destroyed; or should he, unfortunately, die, then with the failure of his power to produce further secretion the danger is over, except some of the poison formed before the death be actually carried away from the dead body. In a word, if the hypothesis be true, we sanitarians have complete mastery over the diffusion of the poisons of all the communicable diseases. We have but to keep steadily in view that the producing and reproducing power

is in the affected body itself, and we can then limit the action to the propagating power of that body,-its power, I mean, of secreting and diffusing secretion,-even with our present knowledge, all but completely.

Beyond this, if the hypothesis be true, we must expect, as we reduce the communicable diseases of one generation to reduce the tendency to them in the next generation, so that in time the heredity to particular spreading disease shall be thoroughly wiped

out.

CHAPTER IX.

INDUSTRIAL AND ACCIDENTAL CAUSES AND ORIGINS OF

DISEASE.

In the last chapter I brought to a close the study of the principal causes of the natural diseases affecting mankind. The causes of the diseases which are induced by our own acts and deeds were exposed as the diseases were, one by one, described in the chapters of the Second Book on Acquired Diseases. I need, therefore, do little more in this present chapter than condense the chief of these acquired causes, as they affect the industrial populations, into the following classified forms.