Groucho Marx said, “Outside of a dog, a book is a man's best friend. Inside of a dog it's too dark to read.” With this wisdom available for decades, the question arises: Why did author Mary Roach stick her arm inside a living cow's stomach, where it's too dark to write? Answer: Because that's the kind of thing Roach does when she's researching books that will be read outside of a dog. Also, the bovine was what's called a fistulated cow, meaning that you can get in through a hidey-hole, which is important because it's exceedingly messy to go in any other way.

I met with Roach in early April when she visited New York City to promote her latest book, Gulp: Adventures on the Alimentary Canal. I last wrote about her in 2010 when her book Packing for Mars came out, which dealt in part with the unfortunate realities of hitting the head in a weightless environment. “People who hadn't read the book, who only heard the publicity, believe that I wrote an entire book about going to the bathroom in zero gravity,” Roach told me. “It's not true—it was one chapter. But that chapter drew a tremendous amount of attention. Which also led me to think I'm not the only one with a 12-year-old's sensibility when it comes to these things.”

Gulp begins with a description of a taste test and ends with a first-person account of a colonoscopy. It thus covers “the whole food chute,” as Roach put it.

As do many great literary works, this book owes its existence to an editor who had a profound influence on a writer. “Many moons ago,” Roach recalled, “the assignment was ‘flatulence.’ And I went to [the originators of] Beano, where they have a research center and I was a subject. And my editor at the time did not have quite the same sense of fun that I do—she let the air out of the story—so the piece, I felt, was not able to capture the true joys and surprises of flatulence research. And I got the sense that there was just so much more fun to be had on the alimentary canal.”

Indeed, what could be more fun than wiggling your fingers, encased in a plastic sleeve though they may be, inside a cow's rumen (which, according to Gulp, is “the largest of its four stomach compartments” and “the size of a thirty-gallon trash can”)?

Fistulated cows are ag-school staples, Roach writes. Using just a topical anesthetic, veterinarians cut a hole the size of a coffee can cover in the hide and a similar opening in the rumen. They stitch the two holes together, put a plastic stopper on the surface and, voilà, a “holey cow,” as the ag students call it. (A fistula is an abnormal biological passageway, not a place to shove in your fist.) Roach's close encounter happened at the University of California, Davis, where the cows offer students instruction that is exceptionally vivid, if not quite disarming.

“Inside the rumen, it's fermenting,” Roach told me. “It's a composter, it's hot in there. The cow is breaking down its food by bacterial action. We're using gastric acid and enzymes; the cow has a fermenter going on. Plus, you put your arm in there and you feel these amazing contractions. I actually was worried that a finger would break,” she ruminated in multiple ways. “So it's a pretty instantaneous and unforgettable introduction to the digestion of animals other than us.”

Of course, many humans have had instantaneous and unforgettable introductions to the digestion of animals, for example, tigers or crocodiles. Such passages usually lead to tails rather than tales. Fortunately, Roach survived both probing and being probed and emerged from deep contemplation of beast bellies, like Jonah, with a fascinating story to tell. With Roach at the tiller, Gulp's canal cruise is tasty popular science writing: informative, entertaining and highly digestible.