When a teen-age boy with a numb chin came to Atlanta internist Dr. William M. McClatchey for help in late 1984, the youth's single weird symptom baffled the physician.

Just before examining the patient, McClatchey took 30 seconds to tap into the computerized index of the world's medical literature at the National Library of Medicine in Bethesda. His computer search of the library's 3,000 journals discovered a single reference. Of 19 patients who reported numbness on both sides of their chins as the only symptom, nine were later discovered to have malignant tumors, the journal said.

McClatchey immediately ordered a CAT scan, which revealed a "rapidly proliferating mass -- a lymphoma" behind the eye -- an area where tumors cannot be detected with routine exams. Forty-eight hours later the boy was getting aggressive chemotherapy.

The computerized search made the difference between a quick diagnosis and sending an otherwise healthy-looking adolescent home with a rapidly growing cancer, McClatchey said. Ultimately, however, the therapy failed and the boy died.

McClatchey is one of thousands of national and international health practitioners in 14 countries who use their home computers or terminals in operating rooms, hospitals, medical schools, clinics and libraries to dial up the library's cornucopia of more than 20 data bases to get medical information quickly, 24 hours a day.

But the library is more than just a dial-up information center. "It is a national treasure of medical history and research," says director Dr. Donald A.B. Lindberg.

With more than 3.5 million books, journals and technical reports in 70 languages, the library contains the world's most complete collection of medical manuscripts and audiovisuals covering everything from botany to veterinary medicine to artificial hearts, all tucked into relatively obscure corner of the sprawling campus of the National Institutes of Health in Bethesda.

This year, the library turns 150 years old. On Friday, it will celebrate the occasion with an open house.

The road to greatness began when Civil War surgeon Dr. John Shaw Billings -- surgeon general of the U.S. Army, a planner of Johns Hopkins Hospital and a prominent statistician -- took over and enlarged the Army's small medical library from 1865 to 1895.

Billings was "a packrat . . . He collected everything in his effort to document the history of western medicine," said John Parascandola, chief of library's the history of medicine division. Billings' voracious appetite for books was so well known that Oliver Wendell Holmes once wrote that he regarded Billings as "a positive danger to the owner of a library, if he ever be let loose in it."

A century and a half later, the National Library of Medicine's historical division alone contains close to 500,000 printed pieces -- 70,000 of which were published before 1801. The oldest text in the library is a closely guarded illustrated Arabic manuscript by al-Razi dealing with gastrointestinal diseases dating from 1094.

Although Parascandola says "Western medicine from the Renaissance on is the real strength of the library," the history division also owns about 2,000 Chinese and Japanese medical texts, about 530 pre-1500 western and Arabic transcripts -- many printed on parchment or vellum -- and 70,000 historical prints, photographs and medical illustrations, many by such famous artists as Daumier.

History buffs can read hand-written notes from Florence Nightingale, Louis Pasteur and Benjamin Rush or trace evolving medical treatments from the Middle Ages forward.

Woody Allen used the historical photo and print collection for his movie "Zelig," as have producers of the public television series "Nova" and "The Brain," said Lucinda Keister, head of the prints and photo collection.

A portrait of Sigmund Freud is the most requested historical item, says Keister, who fields about 130 reproduction queries a month.

In 1942, fearful of East Coast bombing raids, library officials shipped the entire historical collection to Cleveland in 952 boxes for safekeeping. During the war, more than 10,000 volumes were repaired by master bookbinder Jean Eschmann in anticipation of their return to Washington. But the library's building was too crowded, and the historical collection remained in Ohio for 20 years before being reunited with the main library at its new headquarters.

In 1962, a four-van convoy, guarded by Pinkerton detectives, moved the materials to the just-completed Bethesda site after Lloyd's of London insured the book caravan for $6 million.

Established in 1836 as the Library of the Army Surgeon General's Office, the medical library moved to Riggs Bank one block from the White House in 1862 and then again to Ford's Theatre in 1866 after President Lincoln's assassination. From 1887 through the early 1960s, the collection was housed in a brick building on the Mall where the Hirshhorn Museum now stands.

The current building, designed in the Cuban missile crisis era, has two-foot thick concrete and granite walls sunk underground. The windows on a domed roof are designed to blow out, rather than in, in the event of a blast, and the stacks are underground to protect the collection. One apocryphal story, so widely believed that even a library official thought it was true, held that the roof would fall down to protect the stacks in the event of a nuclear explosion.

Today the library is heavily involved in computer technology and electronic information retreival systems.

"First comes the collecting, and then a system to reach out and get the information to people in the most usable form," said Lindberg.

Billings began with the notion that he wanted an index of all known medical literature and created Index Medicus, a monthly publication that now indexes 315,000 journal article listings a year.

Every one of the 26,000 journal entries that Thelma Charen and her crew of 72 indexers abstract each month for Index Medicus is now automatically fed into Medline -- the library's main computerized bibliographic data base. Charen, 69, has been with the library since 1944 and says, "we still have a commitment to the world to list in a way that a researcher or physician can see how detailed an article is and whether it will be helpful."

Improving the speed and accuracy of medical decision-making remains one of the library's primary goals, Lindberg said.

"When you use the computerized indexes , you want facts, not just journal citations . . . Physicians are after answers, not just a finger pointing them in the right direction. I think this is a harbinger of things to come," he said.

For David Rollason, 35, a physician's assistant at the only community clinic to serve 2,000 residents in a 20,000-square-mile area in rural South Dakota, Medline keeps him in touch with the rest of the world's medical community.

Rollason used the library's computerized bibliographic files to persuade local women that they could not get cancer by having mammograms done.

"It was great to be able to access specific literature studying possible links between mammography and the development of new cancers -- and find none."

His ability to tap into the library's vast data bank to seek answers to chronic medical problems he sees is critical because the closest medical library is 100 miles away in Bismark, N.D. Although he can confer by phone with staff at a regional hospital, Rollason is essentially on his own until a physician visits the isolated area once a week.

In Hawaii, on the remote island of Kaui, Dr. David J. Elpern said the library's computerized files give him "the same access to medical literature as professionals at Johns Hopkins, Georgetown or NIH. I feel connected, not like a dermatologist stuck out here in the middle of the Pacific."

Elpern said recently he was sitting with a patient who had had an adverse sun reaction to a drug and was able to use his office computer to call up an abstract to the only journal article implicating the drug with that kind of reaction.

"Such information is extremely crucial in rural areas," said Elpern, who frequently runs five searches a week for himself and three or four more for other physicians at a cost of between $1 and $3 a search. "I use it to browse and to get quick help. I could use private data base vendors, but the cost would be much higher."

Before a U.S. team of physicians and hazardous waste specialists took off for Bhopal, India, in the wake of the world's worst industrial accident at the Union Carbide plant, members used the library's toxicological and hazardous substances data bases -- which profile more than 4,300 chemicals and have 1.7 million references to their effects on humans and the environment -- to compile information on what specific health problems to expect and how best to deal with them, said Tim Baker, emergency response coordinator for the federal government's Agency for Toxic Substances and Disease Registry.

"This information has continued to be useful to Indian medical authorities as more health problems arise," Baker said.

Time often means lives in emergency response, said Kathryn Deck of CDC's center for environmental health. Instant access to the library's toxicological banks makes it possible to determine whether and when to evacuate a community, what kind of protective clothing workers should wear, what specific organs are most acutely at risk, whether local drinking water is safe and what kinds of explosions, fumes, gases or other reactions to expect if the spilled substance interacts with another chemical, she said.

Field supervisors for the Environmental Protection Agency are already using portable computers at spills and accidents to "get good sound information on what we're dealing with before we go in," said Stephen D. Jarvela, EPA federal on-site coordinator. Much of this data comes from library sources.

Dr. Michael Rooney, an oncologist in Lewiston, Idaho, uses a data base compiled by the National Cancer Institute called PDQ (Physicians Data Query) to get his cancer patients into experimental treatment programs at other hospitals in the northwest. PDQ allows physicians to find the most current cancer treatments for more than 80 tumor types and provides a national directory of 10,000 oncologists as well as institutions doing investigative studies.

Lindberg believes medical schools must be pushed into the computer age, too, reluctant or not. Current medical textbooks aren't written with a computer in mind, Lindberg said, but in fact computers can now model the body's physical systems in very lifelike ways, allowing students to examine, diagnose and treat a wide array of "patients" and ailments, and have their performance evaluated, without leaving the classroom.

Lindberg would like to see more medical students using computers to attack and explain medical problems and to do medical research. The library currenlty funds the country's only training programs for computers in medicine to the tune of $14 million in 1985, a scale and amount insufficient, Lindberg said, if medical practice is to keep pace with technology.

"Insulin was discovered by Frederick G. Banting and Charles H.Best, and Best was a medical student trying to make diabetes amenable to treatment . . . By the same token we need our students to be experimenting with computers." Public Tours

The National Library of Medicine will host an open house and public tours on Friday, from 9 a.m. to 5 p.m., Building 38, National Institutes of Health campus on Rockville Pike at Jones Bridge Road. By Metro, the entrance is 200 yards from the Red Line's Medical Center stop. For more information, call 496-6308.