GULP: Adventures on the Alimentary Canal. By Mary Roach. Norton. 344 pages. $26.95.

When it comes to food and the systems by which we process it, science generally is banished to the least desirable table in the restaurant, while fancy, well-dressed “cuisine” and “connoisseurship” enjoy VIP seating.

It’s the yuck factor, you see.

Not in Mary Roach’s world.

The wonders of the alimentary canal, aka the human gastrointestinal tract or, more broadly, every structure from your mouth to the nether regions involved in digestion, are far more intriguing to this most amusing of science writers than the cultural niceties that obscure them.

In “Gulp,” she’s equally taken with the committed, sometimes eccentric people whose studies ply the canal.

Consider, if you will, the miracle of saliva.

Of all the players that keep our digestive systems humming, good old saliva may be the most fascinating. It does far more than prep food for ingestion; it is an anti-microbial superhero, protecting the enamel of the teeth and trapping and consigning some 40 species of bacteria inhabiting our mouths at any given time (100 million of the little critters per milliliter) to the lethal acidic bath of the stomach.

“Wounds that would take several weeks to heal on one’s skin disappear in a week inside the mouth,” Roach reminds us.

Beyond disinfecting, other components of saliva also duke it out with some especially nasty viruses.

Yet we cloak this trusty servant in a double standard. As long as it stays inside our bodies, hunky dory, “no more offensive than the water it tastes like,” the author says. Outside the confines of the mouth, it’s “vile and contemptible.”

The same applies to other “body products.” We are, notes Roach, “large, mobile vessels of the very substances we find most repulsive.”

We also learn: that the three main digestive enzymes used in laundry and dish detergents (amylase, protease and lipase) are the same ones we employ in our bodies, that our powerful jaw muscles evolved an automated braking system beyond our conscious control, that complexity and quality (as in wine) have little correlation, that the satisfactions of crispy/crunchy foods may have much to do with the appeal of destructive power, that we discern the freshness of foods as much with our ears as our mouth and nose, and that fire-breathing “dragons” may have a basis in fact.

More, that the HQ of the International Academy of Proctology is in Flushing, N.Y., that the human colon is a scaled-down version of a biowaste storage tank, that the Beano company sponsors hot air balloon races and sells souvenir windbreakers, and that in the lexicon of digestion, a transient lower esophageal sphincter relaxation is better known as a burp, and that the growling and gurgling of the stomach is called borborygmi.

Scintillating stuff, and Roach is a pip of a tour guide.

Frequently funny, always informative, “Gulp” bears witness to the magnificent complexity of the human body.

Though most of us spend our lives not once having a look at our most valuable possessions, our organs, they are “the most amazing thing we own.”

Reviewer Bill Thompson is a writer based in Charleston.