Abnormal functional action as a cause of morbid results is seen when the eyes are injured by reading, writing, or doing any delicate work in a bad light; for instance, late twilight. Also, in a secondary or accessory manner, when a near-sighted person, having the action of the muscles of convergence in excess of his accommodation, or a long-sighted (hyperopic) person, whose accommodation is in excess of convergence, suffers from asthenopia, perhaps with headache, distress, nausea, etc. Another example of abnormal functional exercise and its effects is that of self-abuse, where the unnatural mechanical imitation of the physiological act of sexual coition induces disturbances of the nervous and circulatory systems, besides debility from excess.
Ingestive causation is a sufficiently fit designation for all errors of diet, as well as misuse of medicines, and poisoning. Starvation or inanition belongs to the same category by negation. Gluttony and intemperance are major members in the ingestive series, while haste in taking food, without mastication, and the use of heavy bread, unripe fruit, and other indigestible articles, account for many cases of dyspepsia and some of colic, cholera morbus, diarrhoea, etc. With young children, especially, no more frequently acting cause of disorder exists than dietetic mismanagement, most of all during the period of dentition, and earlier, when, from absence or insufficiency of mother's milk, they have to be artificially fed. Then the supply of good fresh cow's, goat's, or ass's milk may carry them well through infancy, while a regimen of arrowroot or gum-arabic and water, or stale, half sour milk, may either starve or sicken them to death. On the subject of poisons and of misuse of medicines we have no occasion here to make special remark. Only it may be mentioned that the possibility of either is always to be remembered by the physician in making up his mind in regard to the origin of symptoms observed.
Enthetic causation is a large subject, including all origination of disease by the introduction of morbid materials from without the body.6 Medical opinion has generally accepted, and facts fully sustain, the recognition of three groups of enthetic disorders, viz.: those which are personally contagious; such as are locally epidemic; and epidemic diseases. Of the first group it will suffice to mention, as an example, syphilis; of the second, intermittent fever; of the third, influenza.
6 Simon has proposed the term exopathic to indicate the origin of such maladies; autopathic disorders being those which originate within the body itself.
Were all maladies whose causation is evidently of external origin capable of the same clear discrimination as these, we should have no difficulty with the present topic. But, in fact, no subject connected with the history of disease has become surrounded by more intricate controversy. Many times the same facts are, or appear to be, explicable in two or three different ways. What some hold to be proofs of contagion from person to person, others are ready to account for by the subjection of a number of persons or of a whole community to either a common local or a widespread migrating (epidemic) influence. It is sometimes impossible, in the nature of things, to obtain an absolute demonstration of the truth of one or another of these theories without such experiments upon human beings as are impracticable.
While endeavoring to ascertain the limits of our present knowledge upon these questions, let us first notice what are the most positive facts concerning them, some of which are common to the whole group or class of what have been, since Liebig, often called zymotic,7 but latterly more often enthetic, diseases.
7 The term zymotic has, with many authors, fallen into disrepute, chiefly because Liebig's hypothesis concerning the chemico-physical action of ferments, as well as of contagia, has lost ground in comparison with the vital or disease-germ theory. Yet the analogy between fermentation, putrefaction, and the action of a virus on an animal organism persists; whatever may be the theory of their explanation, something appears to be common or similar in all these processes.
These diseases may be enumerated as follows:
1. Only produced by contact or inoculation.
Primary Syphilis,
Gonorrhoea,
Vaccinia,
Hydrophobia.