For centuries humans have observed that animals lick their wounds. Now a group of scientists have discovered why: they are applying nature's healing balm, a substance made in large amounts by the salivary glands.

The discovery hints at the possible development of a wound-healing drug and it opens new avenues for research into the mysterious way the body regulates cell growth -- the central riddle in cancer, a disease of unregulated growth.

Mammals apparently have "known" instinctively about the wound-healing substance for millennia. Since human saliva also contains the restorative chemical in high concentrations, it's possible that we obey some ancient self-medicating impulse when we automatically put a finger in our mouths after it has been injured.

The self-medication turns out to be a substance called nerve growth factor (NGF), a mysterious protein discovered nearly 30 years ago in cancer cells and later found to be secreted by nearly every type of body cell. NGF was one of the earliest known of a rapidly expanding class of cell proteins that regulate the growth of specific types of target cells.

In recent experiments, scientists found that when NGF is applied to an open wound -- either experimentally by the scientist or by a hurt animal or its companions, through licking -- it accelerates wound healing four-to five-fold.

"We believe it is the first time anyone has found a naturally occuring product that can drastically increase the healing of superficial wounds," said Dr. Michael Young of the University of Florida Medical School, leader of a scientific team that includes researchers at the Massachusetts General Hospital and the Shriners Burn Institute in Boston.

The discovery was made in experiments with mice. However, many other mammals, including humans, are known to have high concentrations of NGF in their saliva. Young cautiously suggests that it may eventualy be possible to use NGF to make a wound-healing medication that would be especially useful following surgery and in cases of severe burns and other trauma.

Such an elixir would be especially valuable for patients whose wound-healing capacity is compromised by illness or age.

Another hope, derived from NFG's originally recognized nerve-stimulating properties, is that the natural substance or an artificially engineered cousin might one day be used to reverse the devastating effects of spinal cord injury, muscular dystrophy and other diseases that destroy nerve cells.

There now is no way to make the cells of the central nervous system -- the brain and spinal cord -- regrow after they have been severed. Peripheral nerves (those outside the spinal cord that enervate muscles of the body) can often be induced to regenerate, but only if the ends of the severed nerve are sutured together.