One of the lesser-known benefits of keeping a diary is that it allows the writer to hijack another man’s biography. In science writer Bill Hayes’s new book, “The Anatomist: A True Story of Gray’s Anatomy” (Ballantine, 250 pages, $24.95), “Anatomy” illustrator Henry Vandyke Carter, an engaging diarist, nearly walks off with a story that was meant to follow the textbook’s author, Henry Gray. But the ultimate star of the disappointing new book, unfortunately, is Mr. Hayes himself.

Ostensibly, “The Anatomist” marks the 150th anniversary of the publication of the anatomy guide that has sold about 5 million copies and is likely the world’s most famous textbook. (There’s no television show called “Cocker’s Arithmetic.”) Gray’s “Anatomy, Descriptive and Surgical” was published in 1858, became an immediate best seller, and was published in America the following year. The contemporary review in the Lancet declared, “There is not a treatise in any language, in which the relations of anatomy and surgery are so clearly and fully shown.” Despite the success of his book, little is known about London doctor Henry Gray. He died of smallpox at 34, and Mr. Hayes speculates that his papers and possessions were burned in the name of public health.

Despite carrying his name, “Gray’s Anatomy” does not owe everything to Gray’s text. Luckily for Gray’s readers (and Mr. Hayes’s), the text is accompanied by beautiful, precise illustrations by Carter, several of which are reproduced in “The Anatomist.” Carter’s chief innovation was to place anatomical names directly on the parts they described — not peppering the images with numbers and arrows — making the book especially useful for students. Not incidentally, the drawings are also strikingly elegant and precise. Indeed, the first edition’s spine read, in equal-sized type:

GRAY

ANATOMY

CARTER

Gray and Carter met at St. George’s Hospital in London. The two men worked closely together for a year and a half on the project, which Gray initiated. Carter had to produce an average of two drawings every three days, while Gray churned out 10 pages of detailed text a week, according to Mr. Hayes’s calculations. At the time, Carter was also a tutor and an unpaid demonstrator of anatomy at the hospital, where Gray was holding down three separate paid positions.

In his diary, Carter is likeable: melancholy, lusty, ambitious, dutiful, adventurous, and sincerely religious. He took notes on Sunday sermons that rivaled his notes in the anatomy classroom. And his drawing for Gray’s book was just the beginning of his career. After moving to India, he worked as a medical researcher, wrote one of the first scientific treatises on leprosy, and became the principal of Grant Medical College in Bombay, head physician of a large hospital, and a surgeon-major in the Indian Medical Service. Late in life, he married respectably, his second marriage, and had two children, before dying in his hometown of Scarborough at 65.

The strongest sections of “The Anatomist” come when Mr. Hayes allows Carter’s diary to speak for itself, especially in a long, fascinating passage about a scandal surrounding his first marriage in colonial Bombay. Unfortunately, Mr. Hayes cannot resist psychoanalyzing him in 21st-century terms. When Carter chastises himself for being “idle,” and writes “must work better” in his diary after a busy day, Mr. Hayes wonders whether he had “a kind of personality dysmorphic disorder.” When Carter orders calling cards for himself, a perfectly humdrum professional move, Mr. Hayes sees him as making a grand statement: “It’s as though he were underscoring his new identity: I Am My Own Man.” He is condescendingly “amused” by Carter’s struggles with faith.

Henry Gray remains essentially a cipher despite his mentions in Carter’s diary. In absence of real insights, Mr. Hayes fleshes out the book’s narrative with tales of his own anatomical education, enrolling for a year as an anatomy student at the University of California–San Francisco School of Pharmacy. These sections are simply dull, a pedantic record of each lab exercise, each incision and, always, Mr. Hayes’s emotions, upon performing each one.

As a diarist, Mr. Hayes is no Henry Carter. Throughout the book, Mr. Hayes names every lab partner, librarian, and instructor he encounters, and takes special pride to note when any of them “insist” that Mr. Hayes call them by their first name. He catalogs each trip to each research library he visits, and in far too many cases explains their checkout processes and locations of their microfiche machines. Even a future biographer of Mr. Hayes could barely care to know this much about his life.

The success of “Gray’s Anatomy” lies in the scope of the project Gray set out for himself: to craft the definitive guide to the human body — the only book a 19th-century surgery student would ever need. He and Carter wanted it to be affordable, readable, and comprehensive. It was contained in one volume, unlike Jonas Quain’s popular three-volume “Elements of Anatomy,” whose illustrations Carter often worked from. Gray’s pages were also larger, making it easier to read than Quain and the other major anatomical textbook of the day, Erasmus Wilson’s “The Anatomist’s Vade Mecum.”

These were not trivial innovations: In making the textbook more accessible, Gray and Carter were participating centrally in the development of modern medicine, and in its transformation from ignorant hucksterism to empirical practice. Published just 26 years after the Anatomy Act legalized the dissection of unclaimed bodies from workhouse morgues, “Gray’s Anatomy” arrived at a time when anatomy was emerging as a reputable science, leaving behind its centuries-long reputation as the domain of disreputable body snatchers. The book is now in its 39th edition. While its author’s life remains a mystery, his work ensured that the human body need not remain one as well.

Ms. Graham is an editor at Domino magazine.