In September 1862, the future Supreme Court Justice Oliver Wendell Holmes Jr. was one of 22,717 men who fell during the Battle of Antietam. His father, Oliver Sr., set out on an epic journey to find him and, a couple of months later, wrote about it for The Atlantic.

“My Hunt After the Captain” is an incredible firsthand account of what Maryland looked and felt like just after the bloodiest battle of the Civil War. Holmes describes what he saw on the streets of Frederick: “Delicate boys, with more spirit than strength, flushed with fever or pale with exhaustion or haggard with suffering, dragged their weary limbs along as if each step would exhaust their slender store of strength.” He notes what the ground looked like after the battle, with “dark red patches where a pool of blood had curdled and caked, as some poor fellow poured his life out on the sod.”

But there’s one especially memorable sentence that has nothing to do with the war. It comes near the beginning, as Holmes is recalling his train ride down from New England:

Many times, when I have got upon the cars, expecting to be magnetized into an hour or two of blissful reverie, my thoughts shaken up by the vibrations into all sorts of new and pleasing patterns, arranging themselves in curves and nodal points, like the grains of sand in Chladni's famous experiment,—fresh ideas coming up to the surface, as the kernels do when a measure of corn is jolted in a farmer's wagon,—all this without volition, the mechanical impulse alone keeping the thoughts in motion, as the mere act of carrying certain watches in the pocket keeps them wound up,—many times, I say, just as my brain was beginning to creep and hum with this delicious locomotive intoxication, some dear detestable friend, cordial, intelligent, social, radiant, has come up and sat down by me and opened a conversation which has broken my day-dream, unharnessed the flying horses that were whirling along my fancies and hitched on the old weary omnibus-team of every-day associations, fatigued my hearing and attention, exhausted my voice, and milked the breasts of my thought dry during the hour when they should have been filling themselves full of fresh juices.

This sentence (and it is one single sentence!) is amazing for all kinds of reasons. First, there’s the sheer length—it’s 198 words long. Then there are the metaphors. Holmes’s thoughts are “magnetized,” then “shaken up by vibrations.” He casually alludes to “Chladni’s famous experiment” (you can read about it on Wikipedia if you don’t own a copy of the 1787 classic Entdeckungen über die Theorie des Klanges). Then he compares his thoughts to kernels of corn, cogs in a self-winding watch, and carriages being pulled by flying horses. By the end, his thoughts are breasts, which his chatty friend has milked dry.

It’s not just Holmes’s writing that’s remarkable. It’s also the actual experience he’s describing. In this age of smartphones, it’s hard to remember a time when people actively sought out opportunities to daydream. But you can see it in just about every Atlantic article from the 19th century—our writers were in no hurry. They were enjoying the process of thinking on paper, letting their associations carry them along without worrying about where they might end up (or when they might need to use a period). Emerson wrote that way: James Russell Lowell once described the Concord sage’s prose as “a chaos full of shooting-stars, a jumble of creative forces.” But I never really understood the mindset behind this kind of writing until I read that sentence by Oliver Wendell Holmes.

To find out more, I called up David S. Reynolds, a distinguished professor at the CUNY Graduate Center who specializes in 19th century literature and history. As someone who studies that era, Reynolds chuckled affectionately at Holmes’s sentence. He said it reminded him of Herman Melville: All throughout Moby Dick and Bartleby, the Scrivener, “there are a lot of these longer sentences that go on and on, and yet they hang together and are filled with metaphors that are just wonderful.”

Reynolds pointed out that the 19th century was the Romantic age, a time when writers wanted to “luxuriate in language” and explore their inner worlds. A classic example, he said, was Walt Whitman’s 1855 “Song of Myself.” Reynolds’s students often have trouble understanding what Whitman meant when he wrote, “I lean and loafe at my ease ... observing a spear of summer grass.” “Some of them say, ‘What is this guy, a space cadet or something?’ But that’s the way Whitman was. He was able to really slow down and enjoy his environment.”

What made writers stop loafing in the grass? Mark Twain, another Atlantic contributor, had a lot to do with it. According to Reynolds, Ernest Hemingway was right when he observed, “All modern American literature comes from one book by Mark Twain called Huckleberry Finn.”

“I mean, the Holmes sentence was really designed for an educated reading public,” Reynolds said. “It doesn’t pretend to be at all vernacular. What Mark Twain did was to try and register the voices of people who weren’t necessarily educated, barely literate kids.” And Twain didn’t hide his disdain for those who wrote 200-word sentences. According to Reynolds, “He once stood up before a literary crowd at a formal banquet and went on and on about the windy, excessive language of writers like James Fenimore Cooper.”

American literature didn’t change all at once; Reynolds points out that Henry James went right on doing his thing even as Twain was writing his down-to-earth dialogue. But history was on Twain’s side. The spread of mass media, the rise of motion pictures, and the popularity of Strunk and White all helped shape the sensibilities we have now. Today’s Atlantic editors would never let some of the metaphors Holmes used into a finished story, let alone all of them in one endless sentence.

But that’s part of what makes Holmes’s writing so mischievously appealing. It breaks all our modern rules, but somehow, it works. He manages to capture the motion of the mind, the almost physical ways it floats and vibrates and whirs. Writers may write differently now, but our words and ideas still come from somewhere, and the process of bringing them to the surface is as wonderful and mysterious as it ever was. Sometimes it takes a 198-word sentence by a masterful writer to remind us of that.