Whoever knew Jonathan Miller was a doctor?

You remember Jonathan Miller, that amiable lunatic among lunatics (Peter Cook, Dudley Moore, Alan Bennett) from "Beyond the Fringe?"

But a doctor? Well, actually he doesn't have much time for practicing medicine these days what with producing operas for the English National Opera, 12 Shakespeare plays for the BBC and writing and lecturing on almost anything under the sun. But MD he is -- specialties in neurology and pathology -- and a lucky thing it is for us too.

Because Jonathan Miller, as amiable as ever and even occasionally as lunatic, is host for public television's "The Body in Question," a 13-week series on man and his innards that debuts tonight on WETA (Channel 26) at 9.

Dr. Miller manages to take something as potentially off-putting to TV viewers as physiology and endow it with, heaven help us, a sense of humor.

(Where, he asks the audience, do most of us get most of our concepts about what goes on inside us? "School science, medical advertisements and butcher's offal.")

And he proves it, of course, with his opening shots of man-in-the-street opinions as to size and placement of such human portions as heart, liver, kidneys, lungs and pancreas. (Do you know where your giblets are? Never mind, almost nobody did.)

He spices a discussion of liver's myriad duties with lines like, "But let me stop this hepatic rhapsody . . ."

Not, however, before he discusses the "French national obsession" with that much-abused organ.

It's a given in France that any medical problem between the neck and the knees is the liver, unless it's a pregnancy. But in Germany, reports Dr. Miller,"It is circulation," which is regarded as an amorphous organ of its own, a kind of "central heating system," for whatever ails you.

"As yet," he says, "there is no American national organ . . . but the British have this national fantasy about the last four feet of the intestine . . ."

For the next 30 seconds or so, Miller engages in a mugging display of epic and irresistible proportions. (This is a doctor?)

In any case, this first, only slightly grisly, tour of some of the things that go on inside that most of us don't think about much, is the kind of modern medical physiology lesson that ought to be required watching for everybody from about the age of 8 on up, especially, perhaps, for those do-it-yourself-health folk among us.

Tonight's opener is called "Naming the Parts," and is, in fact, a preview of the dozen programs to come, each of which will focus on what modern medicine is finding out about our remarkable feedback systems operating just under the surface -- and what ancient medicine thought about them as well. j

These programs ought not to be missed. Alas, WETA has, as yet, scheduled no repeats for this sugar-coated medical cram course and its zany, brilliant, eloquent host.