Abstract
The problem of selective degeneration of single organs or parts of organs is increasingly occupying the attention of physicians. To an ever greater extent these lesions are being traced to the activity of chemical substances or to deficiencies of specific chemical substances (vitamins). Wherever they occur, whether in the liver, the bone marrow, the adrenal gland or the nervous system, they show certain features in common. Although the substance thought to be responsible may be in common use, only very few persons react unfavorably to it, and when a person does react in this way it may be to relatively small doses. The same individuality is seen in reactions to deficiencies of specific chemical substances. A constitutional factor is often further manifest in the form of familial or racial distribution of cases. The repeated use of the drug over a relatively long period seems to enhance the activity. Substances chemically