All alkaloids are of vegetable origin. They all contain nitrogen, and in some respects resemble ammonia. Many of the alkaloid compounds are used in medicine. They affect primarily the nervous system, and may cause freedom from pain, or that abnormal state of exhilaration of which the opium addict is a typical representative. Substances of this alkaloid group doubtless have useful functions in the plant in which they grew, but in the animal body they are disturbing factors. Among the most important alkaloids may be mentioned opium, co-cain, nux vomica, quinin, and acetanilid.