MOST CREATURES' brains secrete a simple substance which acts as the brain's own painkiller. Why should such an effective self-installed mechanism exist in such a variety of animals? Lewis Thomas, chancellor of Memorial Sloan-Kettering Cancer Center in New York, can only wonder. "There it is, a biologically universal act of mercy," he writes. "I cannot explain it, except to say that I would have put it in had I been around at the very beginning, sitting as a member of planning committee."

In his new book, The Youngest Science, Thomas praises the many miracles of biology. But he concentrates even more on the tremendous advances made during the past 80 years in man's understanding of these miracles, especially in the treatment of human illnesses.

Thomas' previous books, The Lives of a Cell and The Medusa and the Snail, described many aspects of science, technology, computers, and their effects on human life. Writing in a graceful and easy prose, he showed a flair for bringing technical and erudite concepts within reach of a layman. In his latest book, he repeats this feat, writing smoothly and understandably of histocompatibility complexes, mycoplasmas, and endotoxins.

The dramatic changes in the practice of medicine during Thomas's lifetime are the focus of the book. When his father, also a doctor, practiced medicine in the early 1900s, few doctors profitted financially and most felt helpless because they couldn't cure patients. Medical schools focused on diagnosis, the art--and as Thomas describes it, it was an art--of determining an ailment from a few external symptoms. They learned how to recognize illnesses without being able to treat them.

Thomas was graduated from Harvard Medical School in 1937, there the realization dawned on him and his contemporaries that "we didn't know much that was really useful, that we could do nothing to change the course of the great majority of the diseases we were so busy analyzing, that medicine, for all its facade as a learned profession, was in real life a profoundly ignorant occupation."

But in the 1950s, Thomas observed that "medicine was turning into a science." The discovery and use of a host of drugs such as sulfanilamide and antibiotics meant that many otherwise fatal diseases were easily curable. A new optimism swept through the profession; doctors became "convinced, overnight, that nothing lay beyond reach for the future. Medicine was off and running."

Not only have the tools at the doctor's disposal changed, but so has the doctor-patient relationship. In the early 1900s, a doctor was primarily a comforter, someone who stayed at the side of the patient and tried to guide him through his illness. More than anything, the doctor's function was the "laying on of hands," the handling and touching of the patient in an effort to provide much needed attention.

This personal world of housecalls and stethoscopes in black bags is gone. "Instead of spending forty-five minutes listening to the chest and palpating the abdomen, the doctor can sign a slip which sends the patient off to the X-ray department for a CT scan," Thomas observes, continuing later that "the doctor can set himself, if he likes, at a distance, remote from the patient and the family, never touching anyone beyond a perfunctory handshake as the first and only contact."

Despite the loss of the personal touch. Thomas insists that with the use of modern drugs and techniques. "Many patients go home in good health, cured of their diseases. In my father's day this happened much less often, and when it did, it was a matter of good luck or a strong constitution. When it happens today, it is more frequently due to technology."

THE YOUNGEST SCIENCE IS not just a graceful account of the impact of 20th century scientific advances on the practice of medicine; the book is also Thomas' autobiography. Something of an impressionistic self-portrayal, it paints him at various points in his life: his youth with a doctor and a nurse as parents, his time at the Med School, his first internship in Boston, and his moves around the country as he took positions at different medical schools.

His accounts are valuable in portraying the development of medicine throughout his career. Treatments administered when he had just left school seem archaic compared to the techniques he now uses at the Sloan-Kettering Center. That Thomas can write a book that simultaneously traces both his own life and the come of medicine underscores his important role in medical research over the past 50 years.

His voice is a learned one and his words merit attention. He writes:

Human beings are getting themselves, and the rest of the world, into deeper and deeper trouble, and I would not lay heavy odds on our survival unless we begin maturing soon. The good thought I have about this is that we are to begin with most improbable of all the earth's creatures, and may be it is not beyond hope that we are also endowed with improbable luck:

If this luck holds, perhaps, as Thomas suggests people will someday see the medical profession become more humanely attend as well as technically advanced.