By idiosyncrasy is understood a peculiarity limited to a comparatively small number of individuals. Four varieties of idiosyncrasy may be described.
The first consists in an extreme susceptibility to the action of certain things, or an extreme lack of susceptibility. Thus most people at some time or other inhale the pollen of grasses, but only in a few cases does it produce that troublesome and distressing complaint—hay asthma. In certain persons a very minute dose of iodide of potassium produces distressing symptoms; in most cases these symptoms arise if the drug is taken for a prolonged period; but in a few cases it may be taken for an indefinite period without troublesome result. The case of a physician at Bath is very curious. The smell of hyacinths in bloom always made him faint away; so constant was this result, that before entering a room during the hyacinth season, he always asked the servant if there were any hyacinths in it.
The second form of idiosyncrasy consists in the production of poisonous results by common articles of diet. Thus some people cannot partake of shell-fish or lobsters without having severe nettlerash. In rare instances the smallest amount of egg, or in other cases mutton, or pepper, or some other substance will produce severe indigestion or nettlerash.
The third form consists in an inversion of the usual effects of certain substances, especially drugs. Thus opium in rare cases produces convulsions; while the aperient Epsom salts have been known to produce constipation.
A fourth form, that of mental idiosyncrasies, may be added, as where there is a strange preference or aversion for objects usually regarded as indifferent. Many cases of mental peculiarity, short of actual insanity, will come under this head; as will instances of depraved appetite for food, etc.
Age and Sex.—According to the period of life, danger arises from different sources. In infancy and old age extreme changes of temperature are especially dangerous, and additional protection is required (see also page [271]). Thousands of deaths occur in the first year of life, from substituting starchy foods for milk, the natural food for infancy and childhood (see page [303]). In childhood the danger from bad feeding is still present, and is evidenced by the frequency of rickets (page [28]); infectious diseases claim their thousands; and the disorders associated with dentition are common. In youth rapid growth is proceeding, and so the food must be abundant and nutritious. A proportionately larger amount is required than by an adult, as the functions of the body not only require to be carried on, but material is necessary to build up the growing tissues.
Manhood is the period of greatest stability of health. The health now depends on the use made of the previous periods of life, and on the habits acquired.
With the onset of old age come various degenerative diseases. The tendency is to death by gradual decay—a euthanasia or easy death, which is too seldom seen. Commonly, bronchitis or apoplexy or kidney diseases bring the scene to a somewhat premature end.
The mortality of man is greater than that of woman at all ages except 5—20.
Habits.—The immense power of habits in the formation of character is perhaps duly appreciated; but their influence on physical health is not so well appreciated; though it would be difficult to exaggerate it. The laws of health are as inexorable and unaltering as all other laws of nature; and whether broken through carelessness or ignorance, the Nemesis of disease inevitably follows. Whatever a man sows he reaps, in health as in other matters.