There are surely many stark lessons in the sad case of Capt. Donal Billig, the Navy heart surgeon being tried for involuntary manslaughter on the operating table.

There are two lessons for all of us, not just the military.

*If you're told you need surgery, serious or seemingly minor, strongly consider getting another opinion. Sometimes this is not possible or necessary; very often it is. Last year, a second opinion became a requirement of Maryland law for 17 common operations.

*Second, find out all you can about your surgeon. Ask questions about the surgeon and the need for the surgery. A life can end by a slip of the knife or other mishap in the most innocent-seeming invasion of the human body.

It is hardly possible to ask questions in a dire emergency. President Reagan could not ask, "Is this operation necessary?" when he stumbled into an emergency room in danger of bleeding to death. But when there is a less immediate need for the operation, says Dr. Eugene Robin of Stanford University, "be as certain as you can that the proposed operation will contribute to your well-being . . . Unnecessary surgery is common. . . . Insist that the surgeon extensively define his reasons for recommending surgery and predict the result he expects. Even if this insistence makes him impatient or uncomfortable, persevere. If he remains uncomfortable, find another surgeon."

Dr. LaSalle Leffall, Howard University surgery chief and a recent president of the American Cancer Society, echoes this last recommendation: "If a physician shows any umbrage at your wanting a second opinion, I'd leave him immediately. That's your life. And it's your right."

If a doctor shows unwillingness to have you seek another opinion, Leffall adds, it may mean he or she is not all that confident in his or her opinion.

Leffall does not urge a second opinion in every "relatively minor" case, though "even then, if the patient wants it," the answer should be an ungrudging yes. Nor does he favor "mandatory" second opinions -- insurance programs that require one before paying. Not every patient wants another opinion.

Still, many plans now penalize patients who fail to seek one for some procedures. In some contracts, for example, Washington area Blue Cross-Blue Shield pays 20 per cent less if the patient has not sought a second opinion in such cases, though the patient need not abide by it and often does not.

The Maryland legislature last year ordered Maryland hospitals to require second opinions for 17 classes of procedures, except for surgery that does not require an overnight stay and Medicare-Medicaid patients (supposedly protected by other controls). The procedures:

Cataract extraction; gall bladder surgery; hysterectomy; tonsillectomy-adenoidectomy; transurethral prostate surgery; laminectomy, discectomy and spinal fusion (back operations); diagnostic arthroscopy (knee examination); hernia repair for children under two; radical and modified radical mastectomy (breast surgery); anal-rectal surgery; coronary artery (heart) bypass; appendectomy; athroplasty (joint repair); intestinal bypass or stomach stapling for obesity; blood vessel grafting to restore lower limb circulation.

Does this mean you need not be concerned if your scheduled operation is not on this list? Not at all. These are listed simply because of their frequency, expense or possible over-use.

Next week: What if you get two conflicting opinions? And other issues. Feb. 19: Improving your chances for good care in the military.