“On Ugliness” (Rizzoli, 456 pages, $45), the new anthology of grotesque images and texts edited by Umberto Eco, is a book to give children nightmares. Page after page features the kind of bizarre, uncanny, and repulsive sights that, if you were to come upon them unprepared and at a susceptible age, you might never be able to forget.

It is not just the overtly, deliberately terrifying pictures that have the power to breed monsters under the bed — though the classic examples of pictorial horror are all here. Mr. Eco does not stint on details from Hieronymus Bosch’s “The Temptations of St. Anthony” (1505–06) with its hunched birdmen and huge crawling fish, or the greenish, pockmarked bodies of Matthias Grünewald’s Isenheim Altarpiece (1515). Next to these visionary pictures, with their morose fusion of piety and misanthropy, the provocations of our century — like Frida Kahlo’s “The Broken Column” (1944), in which the artist’s body is shown split in half and studded with nails — seem merely flippant.

More disturbing than all these, however, are the straightforward, naturalistic representations of human deformity and decay. There is Ghirlandaio’s “Portrait of an Old Man with his grandson” (c. 1490), in which the grandfather gazes thoughtfully at a boy, who is distracted by his pustulant, cauliflower-like nose. There is Rembrandt’s “Anatomy Lesson” (1632) with its respectable burghers gaping at the flayed arm of a corpse. There is the illustration from an early-19th-century medical text showing a man’s face destroyed by syphilis, reduced to lumps of blue-gray matter. The matter-of-factness with which these images treat ugliness — their assumption that ugliness is not devilish or demonic, but simply, unavoidably there — makes them shocking in a way that the most fevered productions of the medieval imagination can’t match. For the modern world, addicted to youth and ashamed of death, it is sickness, not sin, that is the real scandal.

Mr. Eco divides his compendium into 15 themed chapters, each representing a different period or style in the history of ugliness. That history, as Mr. Eco notes in his introduction, is rather harder to unearth than the history of beauty, which he treated in an earlier anthology. “In every century, philosophers and artists have supplied definitions of beauty,” he points out, “and thanks to their works it is possible to reconstruct a history of aesthetic ideas over time. But this did not happen with ugliness. Most of the time it was defined as the opposite of beauty, but almost no one ever devoted a treatise of any length to ugliness.”

Yet Mr. Eco, with his famously catholic erudition, has managed to put together an impressive number of specimens. Alongside the images, he includes hundreds of brief texts on the subject of ugliness, ranging from philosophical analyses (Plato, Aristotle, Hegel) to horrifying celebrations (Sade, Tzara, Wilde). By joining word and image, Mr. Eco makes “On Ugliness” something more than just a perverse coffee-table book: It becomes a thought-provoking study of the attraction ugliness has always held for human beings.

“On Ugliness” begins by looking at ugliness in the classical world, where it was largely the province of mythological creatures like sirens and centaurs. Significantly, there are few genuinely ugly works that survive from ancient Greece and Rome; when Mr. Eco wants to find an image of a mythic beast, he must turn to much later pictures, like Moreau’s “The Chimera” (1867) or Rubens’s “Saturn Devouring his Son” (1636). This artistic aversion to ugliness began to change with the rise of Christianity, as the sanctification of Jesus’s Passion brought a new legitimacy to the depiction of torture and death. Notoriously, there are no more sadistic images in art history than the pious depictions of Jesus on the cross. (In a typically clever juxtaposition, Mr. Eco shows how Mel Gibson’s “The Passion of the Christ” — whose Jesus doesn’t so much bleed as drip with gore — borrows its iconography from paintings such as Hans Memling’s “Christ at the Column,” from 1485–90.)

Then there is the ugliness of obscenity, and the ugliness of freaks and monsters. For both these types, too, the Middle Ages was a high point. The classical world was strained by its aesthetic idealism, and the modern world is constrained by its humanitarian pity, from approaching these varieties of ugliness with the comic gusto available to the medieval or Renaissance artist. Indeed, when approached with the eagerness of a François Rabelais, even the grotesque can become lovable. It is hard to resist a creation like Pantagruel, the giant whose “fetid” fart “engendered more than fifty-three thousand little men, misshapen dwarves, and with a poop which he made, he engendered as many little bowed women.” Such ugliness is literally life giving, and Mr. Eco provides the inevitable gloss from Mikhail Bakhtin, the Russian literary theorist who celebrated the “carnivalesque” spirit of the Middle Ages. The grotesque, according to Bakhtin, “takes an interest in all that sprouts, protrudes, or emerges from the body, everything that tries to flee the confines of the body” — a demotic version of liberation.

Finally, in the book’s last chapters, Mr. Eco shows how Romanticism and Modernism effected what he calls “the Redemption of Ugliness.” Now ugliness became sympathetic, thanks to characters such as Victor Hugo’s Quasimodo and Mary Shelley’s Frankenstein, or titillating, as in the voluptuous grotesques of Klimt, Böcklin and Schiele. In today’s nihilistic art world, it is almost senseless to distinguish between beauty and ugliness: All that matters is novelty. As Mr. Eco dryly observes, “contemporary art also deals with and celebrates ugliness, but no longer in the provocative way of the avant-garde movements of the early twentieth century.” The same gallery-goers who “take aesthetic pleasure in a fine landscape [or] a handsome child,” he points out, will happily attend a performance where an artist like Marina Abramovich mutilates her body with a knife.

Yet Mr. Eco’s moralistic conclusion is called into question, in a way that he seems unwilling to acknowledge, by his own book. For “On Ugliness,” with its hundreds of repulsive images, is also, of course, meant to be an object of desire. It is a handsomely produced, fairly expensive art book, offered to the public on the assumption that people will want to own it. (Handsome but imperfect: The text formatting, the translations from Italian, and the index all have small but noticeable flaws.) It exists, that is, on the premise that ugliness is not ugly, at least not always. What is it about art that allows it to neutralize the repulsion of ugliness? Why do we flock to look at representations of things that, if we encountered them in real life, would send us fleeing?

This is the issue that most of the texts in “On Ugliness” try to resolve. Aristotle was the first to recognize that “Things that we normally view with disgust we instead view with pleasure when images of them are portrayed with accuracy, such as repugnant beasts and dead bodies.” In his view, the reason for this peculiar taste was man’s love of reason. It is possible to confront loathsome images where we would simply shun the loathsome realities: The bloody sockets of eyeless Oedipus move us on the stage, but would simply nauseate us in life. Such representations are opportunities to think and learn, and we embrace them because “learning gives great pleasure, not only to philosophers but also to others.” Thus, our taste for ugliness, which is potentially a scandal for human nature, becomes a badge of honor. Art represents the taming of chaos, the triumph of the organizing intelligence over formless reality. In the last few centuries, however, it has been harder for writers to maintain this flattering view. Since the Enlightenment, there has been an ever more painful divergence between what might be called the official view of human nature — according to which we are originally sinless, instinctively generous, and capable of infinite improvement — and the reality of modern history, which shows us to be as murderous and hateful as ever. In the gap between man as he is supposed to be and man as he is, a new understanding of ugliness has grown up, which holds that our taste for ugliness is a rebuke and an exposure of our true selves.

That is why modern representations of ugliness always seem to have a polemical, even political edge. You can see it in Duchamp’s “Torture-Morte” (1959), a picture of the sole of a foot crawling with flies, or in Buñuel’s film “Un chien andalou” (1929), with its famous image of a woman’s eye being sliced open. Such violent ugliness demonstrates what Adorno, in a passage quoted by Mr. Eco, called modern art’s ambition: “to denounce, through ugliness, the world that creates it and reproduces it in its own likeness. … Art accuses power and bears witness to those things repressed and denied by that same power.”

Yet this art of denunciation does not always follow the imperatives of justice, as Adorno implies; not all artistic vandalism is a cry for redress. This is made by clear by the ugliest specimen in “On Ugliness,” the one that does most to outrage the feelings. Salvador Dalí’s picture “Atavism at Twilight” (1933–34) is a Surrealist parody of Millet’s “The Angelus,” (1858–59) which portrays a peasant couple pausing at their work in the fields to pray. Dalí has worked his usual deformations on the image: He gives Millet’s man a skull-face, and a long red cable grows out of the woman’s back. But this visual assault is not as shocking as Dalí’s accompanying text, in which he viciously rewrites the scenario of Millet’s picture. He suggests, with a straight face, that Millet’s peasant has lowered his hat not because he is praying, but to conceal his erection. He writes that the pitchfork is really a scalpel, “stuck into the living, substantial flesh that cultivated land has always represented for mankind,” and that, therefore, farming is a form of cannibalism.

This compulsion to desecrate the holy quietness of Millet’s image, this restless shamelessness, is evidence of an unhappiness far uglier than any of Dalí’s own paintings. The frightening thing about modernity, Mr. Eco leaves us to surmise, is the way it makes artists such as Dalí its celebrities and representatives — as though ugliness were no longer beauty’s necessary negative, but the only true mirror of our age.

akirsch@nysun.com