BROCA'S BRAIN is just that: the preserved gray matter of one Paul Broca, the 19th-century French physiologist who first idenitified, in the cerebral cortex, what is now called "Broca's" area." He left, as well, an elaborate collection of brains, ranging from those of madmen and assassins to his own. The last specimen-"in formalin and in fragments"-is what according to Carl Sagan, launched these "Reflections on the Romance of Science." And while that seems a suspiciously simple genesis for so wide-ranging a collection of essays, it is nonetheless appropriate. "Broca's area" is that portion of the brain that controls articulate speech, and in matters scientific, Sagan continues to develop as an articulate and engaging popularizer.

Borca's Brain is Sagan's least structured book thus far, a fact the author himself acknowledges in the introduction. The essays extend from meditations on pseudoscience to sharply-etched portraits of the earth's planetary neighbors, with detours through religion, cosmology and the birth experience. For many, Sagan's most enjoyable chapters will no doubt be his tactile evocations of the solar system and the implications of space exploration. One piece, on "comparative planetary climatology," is a perfect, compressed example of Sagan's ability to capture the imagination through imagery and example-and then, at the last minute, to deftly shift into a subtle advertisement for the importance of a continuing space program.

Sagan's least popular section will certainly by "The Paradoxers"-an exercise in the debunking of popular pseudoscience. Starting with an analysis of some classic examples-like the Cardiff Gaint, or Clever Hans (the German horse said to be a gifted mathematician)- Sagan goes on to a lengthy consideration of the elaborate notions of Immanuel Veli-kovsky. In recent years Sagan has expended much energy on the thankless task of evaluating Veli-kovsky's unaccountably popular theories of plantetary catastrophism. The distillation in Broca's Brain is high-class debunking indeed: closely reasoned, impeccably research, gently humorous, utterly devastating. Debunking being what it is, it will probably also do little good. It will be savored by other dedicated debunkers, used as evidence by true believers that the scientific establishment is axing another Copernicus, and it will probably bore the rest of us.

Sagan probably knows all that already, and so his devotion to the task is admirable. More admirable is the fact that he has not fallen into the bitterness that often afflicts weary debunkers. In fact, he charitably attributes the public's insatiable hunger for pseudoscience to the scientific impulse itself. And that notion returrns the focus to the unifying themes of Broca's Brain: the importance of scientific literacy in the general public, and the inherent appeal and drama of scientific thought.

The display of those last qualities is Sagan's real strength-and also a source of some peer criticims. "In exchange for freedom of inquiry," he writes, "scientists are obliged to explain their work." Certainly few modern scientists have met that obligation with more zeal-and success-than Sagan. Yet in turn, between best sellers and regular television appearances, Sagan attracts a certain amount of quiet enmity from many scientists.

Scientists, of course, often mistrust (and perhaps envy) colleagues who take some part of their thinking straight to the public. Sagan once recalled in an interview that the first time he garnered some newspaper coverage, as a graduate student, he was greeted by one of his professors who observed disapprovingly, "I've been following your career in The New York Times."

Past that, however, Sagan generates additional resentment with his inclination to stray from his own academic pasture. While portions of Broca's Brain are closer to astronomy, Sagan's own specialty, than was The Dragons of Eden, it is nonetheless clear that the author has no intention of limiting his curiosity to his own doctoral subject.

And that, in a way, is Sagan's unique contribution. It is reminiscent of the kind of verbal recreation in which scientists engage during off-hours. Hypotheses are offered, shot down, reconstructed, and demolished once again, in an atmosphere where speculation outside one's own purview is totally acceptable. One young scientist I know calls it scientific "brainstorming"-an earnest yet playful creativity that the public is rarely privy to.

Sagan's more speculative essays might be seen as a form of public brainstorming. The final piece in Broca's Brain, for example, travels in a few thousand words from the birth experience to cosmology, by way of psychedelics and organized religion. The skeptical reader may suspect that an expert in any of those categories would be quick to offer some specific objections. And that may well be the case.

Yet Sagan is not presenting a scholarly paper, but rather the example of a sharp and verbal scientific thinker, having fun with his material and not terribly concerned about the rigid protocols of peer approval. In that, Sagan transcends rote education and instead-as promised-reflects just where the romance of science indeed begins. CAPTION: Illustration, no caption; Picture, no caption; Copyright (c) , Carl Sagan John Metzger