Brilliantly written by the same agency that just weeks ago gave details on how fingernails grow, an item in the Nov. 7 Federal Register (page 73955) describes how and why hair grows.

The reason for the presentation from the Food and Drug Administration, was the well-publicized FDA-proposed rule that would remove from the over-the-counter market preparations that claim to promote hair growth or prevent its loss but are "not generally recognized as effective."

But the real news in the register notice came within the "general discussion" section. It led off with a statement that: "The common type of baldness, male-pattern alopecia , is inherited, as are hair color, texture, and curliness."

The question of how hair grows was tackled next. First, a hair follicle -- or gland -- produces one strand of hair. Second, the hair's growth is "cyclical," according to the FDA experts, and passes through three phases.

The first, or anagen , phase is the period of growth. For a scalp hair it can last from three to five years; for an eyebrow hair, about six months. About 85 percent of the hair on your head is in the anagen phase.

The second, or catagen , phase is a transitional one, and quickly leads to the final phase, called telegen. During this period, which lasts for several months, the full-grown hair rests in the follicle before it is shed.

Then a new hair begins to grow out of that follicle, and the cycle starts again.

Hormones affect hair growth, a situation best illustrated by beard hair. Before puberty, beard hair is very fine, has no color and grows only to a short length. After puberty, with the hormone androgen dihydrotestosterone introduced into the system, the beard hair turns coarse, and gains color and grows longer. i

The FDA found, however, that taking hormones will not prevent a person who inherited male-baldness traits from losing his hair. To prove the opposite point, the FDA authors briskly referred to a scientific study that found that "baldness did not occur in the absence of male hormones, such as in men casterated before puberty . . . ."