WE WALK AROUND all our lives with these bodies of ours, and you would think by the time we has spent 20 or 30 years with them, we would get to know them pretty well.

Fat chance. Most of us probably meet our maker without ever having known, or cared, that it takes 12 muscles or pairs of them just to swallow a harsh word, or that there are nerve cells in our bodies that stretch all the way from a stubbed toe to inside the skull.

At least, however, we know what we see. Or do we? The landmarks are familiar: freckles, dimples, wrinkled and scars, ther marks of time and chance. but we do not even accurately perceive what stares back at us from the unpretending mirror.

As for what's under the skin, most of us have so foggy a notion of what goes on there that it is as if we were populated by some entirely other physical being to which, paradoxically, we do not have access, despite the fact that it is us. Most of us are not given to pondering the miracle of our own functioning, though miraculous it is.By the time we're old enough to trace in our skin the veins that hint of the underlying structure, we are either preoccupied with ignoring or disspating or preserving whatever has survived adolescence, or at best are trying to keep ouselves in good mechanical order.

But the idea of what makes for good order has certainly become more popular since Our Bodies, Ourselves , the women's self - help handbook, drew attention to what people could do outside doctors' offices. That first book was in some ways the best, though it was as much a political document as an information source. Then in 1975, among a spate of similar books, Paddington Press published the Diagram Group's Man's Body: An Owner's Manual (now available from Bantam, $2.25), followed a year later by Woman's Body (coming in April from Bantam, $2.95) and now here is Child's Body: A Parent's Manual .

These books claim to be nonpolitical - "not emotive or angry, just factual and descriptive" - though in truth they are intersting as much for what the choice of material says about our cultural images of ourselves as for the actual data they present.Woman's Body , for instance, is graced with two pages on compulsive overeating with a dismal drawing of an obese female, drooping breasts and bulging stomach neatly labeled; there is no such section in Man's Body , although both contain several pages on obesity. Yet the average man in American is even more over - weight than the average woman (he weighs 20 to 30 pounds too much; she, 15 to 30), and compulsive eating is by no means relegated to one gender.

And one of the more charming drawings in Man's Body outlines male sexual organs, erect, with labels: "small average" "average," and "large average" - reassurance, it would seem, that no matter what size your organ is, you need not be concerned that you are not average.

To be fair, Man's Body was published first, so much of the material common to the two books has benefited from revision during the intervening year. But they are nonetheless frequently repetitive and it's unlikely anyone would want to own both, though they're fun to browse, generally well-intentioned and contain some useful material.

Their shortcomings, unfortunately, extend to the newest member of the family, Child's Body , which like its predecessors fails to include direct references, often leaving the reader wondering frustratedly about an information source, particularly when the material is controversial or in doubt. (At one point the book says that amniotic fluid in the newborn's ears causes a few hour's deafness after birth; at another point, it mention Leboyer's reccomendation, among others, that noise in the delivery room be kept at a minimum to help make the birth process less traumatic. If the first were correct - and we are not told the source, so how are we to evaluate it? - the second would surely fail to make a difference.)

And the diagrams which appear on every page, while usefull on the whole, are at times simplistic and at times incomprehensible to anyone not previously familiar with the concepts presented; they tend to be copious to the point of absurdity. Let anyone but a physician or a physiologist try to make sencse out of the diagrams on the child's circulatory system; they leave you wondering and wondering, and wondering.

Sections on behavioral difficulties and childhood anxieties tend to be sketchy, poor substitutes for the fine, longer commentaries of writers like Dr. Spock or Selma Fraiberg. The practical information - the sections on growth, reflexes, and what to expect of an infant and child at different stages of development - are adequate for most, easy to understand, and interesting. The section describing the anatomy of the child's body may be all right for the reader who has studied some anatomy, but terms used wothout clear definitions and the attempt at brevity probably leave too much to the imagination.

The first-aid information is less specific than what you would get from your pediatrician (who would tell you how old a child must be to be trusted with an oral thermometer, or how much aspirin to give, or what soothing lotion to rub on) but it is well orgainzed and reasonable.The table of infectious diseases, on the other hand, is worrisome; its varied entries include bacterial meningitis, coughs and colds, polio and so on, but the treatment section does not specify which of these require the doctor - fast - and which can be treated adequately at home.

A few other quarrels: some information does not reflect the latest thinking. Blue eyes, for instance, are described as resulting from simple dominant/recessive inheritance. But as anyone who reads Ann Landers regularly can tell you, three sets of genes are now considered to be involved in the inheritance of eye color, not one. And in the section of Rh factor, the commentary fails to mention that women who are Rh negative and have had abortions may also have become sensitized and should be treated with RhoGAM following the procedure.

Probably the best thing you can say for these books is that they are an attempt, if inadequate, at filling a real need: that people really would, if given the chance, like to know a bit more about how their bodies work; and that the information, if presented properly, might help us to deal with problems like high blood pressure, sexual inadequacy, the experience of aging. But a comprehensive, accurate, easy-to-understand sourcebook, individually geared to the problems of the man, or the woman or the child, has not yet appeared. These Diagram Group books - if revised with references, careful editing and simplification of the diagrams - might go some way toward filling that need.