An eye is irritated when a spark from a locomotive, or a bit of sand, or an inverted eyelash, get into it. A mustard-plaster first stimulates the circulation of the skin where it is applied; this may be quite within the bounds of healthy action, if the mustard be soon withdrawn. If it remain longer, irritation is shown by pain and soreness; next, if still allowed to act, it will produce inflammation. Irritation of the stomach may be caused by indigestible food, or, more serious in degree, by certain poisonous substances; as strong acids, alkalies, arsenic, or corrosive sublimate.

Congestion (or Hyperæmia).

This may be an active flowing of more blood than common through a part, or a passive collection of blood in the part. Stimulation produces the former; when it passes beyond the line of health into irritation, passive congestion occurs at the centre of the irritation, active congestion in the parts around it. Determination of blood towards any portion of the body may be, when very decided, called local hyperæmia. A bloodless condition of an organ is called a local anaemia. This first simply means excess of blood; the second, deficiency of blood.