"It's not. This is what's important," he said, motioning at a wall. It was a long, uniform wall of one of the ubiquitous branch banks in the city. Kent frowned.

"People walk faster by banks: there's nothing to do in a bank. Banks used to have a ballroom on the second floor in small towns, so that people would come and get familiar with a bank, and a.s.sociate it with pleasure. But a bank has become more like a place you just go to the bathroom."

I reflected on the last time I a.s.sociated visiting a bank with pleasure. A ballroom featuring, say, a pianist playing Gershwin would surely improve my impression of the mushrooming banks in my neighborhood.

Kent and I (quickly) walked by. We headed west on a long street that used to be industrial, then artists' residences, and now was full of the commerce of handbags, designer clothes, and technology. My gaze stayed down. Something sparkled underfoot: the sidewalk was studded with small gla.s.s domes, lit up from underneath. These, Kent explained, were remnants of the street's industrial past. The only natural light workers in the bas.e.m.e.nt of these buildings got was through these small sidewalk domes. With the advent of incandescent light, the roles were reversed, and they brought artificial light from the bas.e.m.e.nt aboveground.

These domes are rare now, the sidewalk equivalent of finding an early-twentieth-century copper penny in your pocket-book.3 We only noticed them because we were looking down at the ground. Funny, that: we were out to take a walk and see what was around us; what was underfoot should not have reached our consciousness. It turns out, though, that walkers of all ages spend a lot of time looking at the ground a step ahead of their toes. A recent study reported just how much time. The researchers sent pedestrians out to take a walk while fitted with a device tracking their eye movements as they walked along a level, unremarkable sidewalk. The walkers spent nearly one-third of the walk fixated at the near or far path-just as much time as they spent looking at the objects around them.

If the research is accurate, we might expect we would know a whole lot about our sidewalks. But I would guess that if I stopped a random pedestrian and queried him about the sidewalk, I would get a description along the lines of, It looks like a sidewalk- gray concrete, poured and leveled, divided into squares, hmm, that's about it. The sidewalk seems uninteresting and ahistorical, but this is borne of perceived familiarity. Sometimes we see least the things we see most.

Our use of the sidewalk-as a walking path-is now so entrenched that I cannot imagine it any other way, but un.o.bstructed mobility was not always the point of sidewalks. They were public s.p.a.ces. There has always been panhandling, ware-selling, soliciting, and loitering going on among the walking. Once automobiles began encroaching, the mixed use of public s.p.a.ce-horses following egg sellers abreast newspaperboys and people idling, convening, and chatting-was sorted into its const.i.tuent parts. The street belonged to cars. The sidewalks were for the people, but "Street Departments" were formed to inspect them and regulate their use.

Though sidewalks date back four thousand years, their popularity waxed and waned. In the nineteenth century, the sidewalk began its latest climb: 876 linear feet of it in Paris in 1822 swelled to 161 miles by 1847. At the same time, sidewalks began to be intentionally separated from the road by posts or by egg-shaped stones placed beside the route. These markers actually created a new location: the curb, or the gutter. It was not just a politically charged area (destination for unsavory loiterers) but also where dogs could be led to pee (and, in the wee hours, one might find more upright urinaters). Until last century, many sidewalks were still made of wood or gravel. Adjacent property owners were responsible for paying for and tending the paths. But there was little restriction on the position of their buildings or doorways, so the sidewalks in front of them were often irregular enough to require stairs to connect neighboring lots.

Now we walk on concrete, for the most part, set in slabs and drawn into squares or rectangles as it sets to dry. It is layered over a waterproof membrane and compacted sands. There are other options: stone slabs, cobblestones, bricks, even monolithic asphalt laid in a continuous sheet. But concrete is cheap, reflects light instead of getting warm with the sun, and it feels solid-not settling awkwardly like bricks or becoming slippery like stone. It does not, I am sorry to say, go down to the center of the earth: to stand on a sidewalk is not necessarily as secure as we imagine. City sidewalks are often hollow underneath, just covers for bas.e.m.e.nt s.p.a.ces that extend out from the building. Concrete also lasts for many decades, during which time dropped gum and splattered soda collect the dust of the city and darken.

I admired a particularly well darkened corner of the sidewalk, in front of a corner tobacco and magazine shop. Those blotches may go back eighty years, I thought, mildly disgusted.

At this point in our walk, nearly midday, the street was increasingly crowded with the sort of pedestrian activity that brought a smile to Kent's face: loitering, chatting, socializing. General liveliness. I navigated around another vendor's cart that was reducing the walkable sidewalk s.p.a.ce by half. This is the sort of thing that the early sidewalk ordinances regulated against: in New York City at the turn of the twentieth century, there was even a munic.i.p.al Bureau of Inc.u.mbrances tasked to remove the barrels of dead fish, bales of merchandise, pots of flowers, and squatters or loiterers in order to provide smooth thorough-faring. I suspected that Kent would be pleased that the Bureau was eventually dismantled. But "inc.u.mbrances" turn out to be facilitators in some ways. On another day, in a public lecture hall not far from where we were walking, I asked Iain Couzin, a mathematical biologist who studies "collective behavior" in animals at Princeton University, how the crowd of people in the auditorium should proceed smoothly out the exit doors, or proceed down a busy sidewalk so as to minimize congestion. To answer, he invoked fluid dynamics, describing the motion of fluids and gases, as well as the motion of people: "It's counterintuitive, but putting in a barrier can actually increase flow," he said. As long as it is visible, a bar or a pole in an exit door, or an obstruction in the path-just a little off of center-makes movement through the door or down the path run more smoothly. This paradox is related to the "packing problem": "if you're trying to pack all these things in [to a small s.p.a.ce], putting this thing off center and people having to avoid that breaks the symmetry, and then you get oscillating flows [through the s.p.a.ce], which is much more effective."

If a corridor is narrow, the crowd (or fluid or gas) oscillates going to and fro. It self-organizes. At some level, it does what these walks were doing: making me notice something new. In that way, an obstacle can aid movement, instead of stopping it altogether.

Kent himself suddenly stopped. We had worked our way around the block in a labyrinth path that led us back to his office before I had expected it to. Despite all our slowing down, we finished the walk quickly. Back up in his offices, he saw me admiring the open feeling of the s.p.a.ce, dotted with colorful chairs, and slyly quoted Whyte, "Please, just a nice place to sit." A person's experience in the urban environment, Whyte thought, had a lot to do with whether there was "a little something" they could control-like a chair not just to sit in, but that could be adjusted to their liking. His videotapes of people sitting in chairs in public s.p.a.ces showed lunchtime workers reliably making small adjustments to their chairs before sitting.4 One of Whyte's realizations was that people could feel very comfortable in a fundamentally noisy, public environment, if they just had a nice place to sit.

With that, Kent sat down and wished me well. Out I walked, slowly, to a totally changed, crowded, social street.

1 By contrast, there are now thousands of surveillance cameras in Manhattan set to constantly monitor the behavior of persons in their view. In some areas of downtown, there are nearly as many cameras per acre as there are residents per acre in Denver, the city nearest to where I grew up. But it is an open question whether anyone watches the video they record. While many have bemoaned the lack of privacy that cameras seem to impose, I find myself tickled that all of my walks undertaken for this project were recorded and registered in the aggregate of superintendents', police, and store owners' tapes.

2 Six and one half feet per second is the high end of comfortable fast walking: it is around 4.5 miles an hour, or, in New York City, ninety blocks an hour, an incredibly brisk pace. More people walk around five feet per second. This holds for the average healthy adult, though, not for the very old or very young. (On my walk with my son, we might have managed five feet per minute.) Until recently, the recommended time allotted for pedestrian travel across a street on the walk sign was based on the a.s.sumption that people can cover four feet every second. Even this conservative estimation almost certainly excludes most disabled and elderly walkers, whose pace leaves them barely halfway across the street by the time the light turns. With the latest walk-signal revision, they have been granted an extra couple of seconds.

3 Until the 1980s, the "copper" penny was around 95 percent copper and 5 percent other metals; not since the nineteenth century has the penny been fully copper. Pennies currently produced are mostly zinc, a less expensive metal, though the cost of production of one penny is, as of this writing, 2.41 pennies.

4 More than one thousand lightweight chairs now dot the block-long Bryant Park in Midtown Manhattan, and nearly all of them are regularly warmed by Manhattan behinds. Kent's PPS contributed to the park's renovation in the early nineties; it is a wildly successful public s.p.a.ce.

"What is life but a form of motion and a journey through

a foreign world? Moreover locomotion-the privilege of animals-is perhaps the key to intelligence."

(George Santayana).

The Suggestiveness of Thumb-nails.

"An older gentleman was resting in the median. . . . As he resumed, he teetered, and I swung widely around him so as to not knock him off course."

Before concocting the character of supersleuth Sherlock Holmes, Sir Arthur Conan Doyle was a doctor in training. Fans of his writing will see many allusions to his medical interest in the pages of his books, but the biggest allusion comes in the form of the behatted Holmes himself, modeled on one of his medical-school professors, Dr. Joseph Bell. Bell was a member of an increasingly disappearing cla.s.s: doctors who are able to make diagnoses simply by looking carefully at the patient in front of them-before taking a single blood test, ordering an X-ray, or even placing a hand on the patient. For the cast of one's skin, the smell of one's breath, one's posture and step are all diagnostic, in the language of medicine, of the condition which brings the patient to the doctor's gaze. To his students, Bell would exhort that they learn "the features of a disease or injury . . . as precisely as you know the features, the gait, the tricks of manner of your most intimate friend." One's friend can be identified in an instant amid a crowd, but so, he suggested, can a disease, if you know what you are looking for.

Doyle was stunned when Bell was able to identify a man's profession after a glance-by noticing a hobble and a worn area on the inside knee of his trousers (where a cobbler typically held a smooth, heavy lapstone for hammering leather). Sherlock Holmes was bequeathed this genius for minutiae-a genius for observation more than one of learning. I wondered if this practice would persist among doctors today, well schooled in hospitals typically well equipped. I have been a patient in a few hospital rooms where the heart-rate monitor by my bed was given more studious attention than my own heart in my chest. Are there still practicing doctors who attempt to interpret their patients' conditions through observation-of the seeming trifles that reveal things about ourselves-as much as through instruments? According to Doyle, Bell often spoke of the particular importance of the "infinitely little" detail: the visible traces on ourselves and others that we don't bother to notice. I set out to find a Sherlock Holmes of my own.

I found Dr. Bennett Lorber. He agreed to walk with me in his hometown of Philadelphia, a city I knew but had quit after college. Lorber is a professor at Temple University School of Medicine and, at the time, was the president-elect of the College of Physicians of Philadelphia, the country's oldest medical organization; we were to meet in its dark-paneled lobby. The college is home to the Mtter Museum, an astonishing repository of medical history ephemera. As I waited, I browsed through the museum display cases, one of which held the recently arrived slices of Einstein's brain. I considered whether being donated to a museum of medical grotesques was enviable or dreadful.

Lorber is also a practicing doctor. On our walk, I was asking him to, essentially, diagnose on the fly. Simply by being outside on the street, people are inadvertently revealing their life histories in their bodies, in their steps, in the hunch of their shoulders or set of their jaw.

Waiting for Lorber, I couldn't help but feel a little trepidation: What characteristic tic would I display? What would the flush of my cheeks disclose? What was I revealing with my pupils, my teeth, or the grip of my handshake that I may not know myself? I thought of Sherlock Holmes gently chiding his a.s.sistant Watson for his pitiful skill at noticing obvious details: "I can never bring you to realize the importance of sleeves, the suggestiveness of thumb-nails, or the great issues that may hang from a boot-lace." I tugged at my sleeve, straightening it, and peered down at my shoes: laceless. Whew. Glancing at my own thumbnail, I noticed it was ridged somewhat. Each thumb's nail had a tiny downturn, a kind of keratin pothole. My brow furrowed: What could this mean? Low iron? Liver disease? Imminent collapse? With my thumbs, I Googled "nail abnormalities" on my iPhone. Thousands of hits. Among the first, this news: "Superficial nail problems are caused by proximal matrix disruption, while deeper nail abnormalities are caused by distal matrix disruption." Uh-oh-a disrupted matrix? I felt wary of any medical news that used not one but two words I did not understand.

The appearance of a slender, be-suited man at the front desk in the lobby distracted me: that must be Lorber. Feeling proud of my detective work (he was the best-dressed man in the room, and it was the precise time we were to meet), I quickly forgot about my b.u.mpy thumbnails and approached him.

Lorber greeted me with a gentle smile and a look of calm exhaustion. He seemed to exhale deeply as he turned toward me and shook my hand (without commenting on my grip). He had just been lecturing on microbiology and art, the convergence of his professional and personal interests. We sat down on one of the dark wooden benches in the lobby for a few moments of repose before we began our walk. Lorber specializes in diagnosing and studying anaerobic infections, but I had come to walk with him because of his side interest in the physical exam. Like many professors, Lorber serves as preceptor to medical students who are learning clinical techniques. He demonstrates how to take a history and how to do a physical exam-and he clearly delights in it, for it gives him the opportunity to undo some of the damage done by years of pre-med memorization, and teach the students to see the patient in front of them again.

He described being influenced by his father, a metalworker who was also an artisan and draftsman, and who bequeathed to his children the visual awareness he had: "We'd go someplace, and when we would leave, [his father] would ask us to draw a floor plan-where was the piano, where was the window. He knew. Once we did that a few times we started really paying attention."

Years later, as an adult, Lorber turned these early visual lessons into a similar test he devised for his medical students. Leading a group of students into a patient's room, he allows them introductions and a look, and then he says, Everybody turn around and look at the wall.

"And then I'll say to one of them, 'Tell me one thing about Mrs. Johnson that you learned. A single thing. Anything.' And they usually say, 'She has an IV.' That's the most consistent answer. And I'll say 'Right, she does. Where is it? Is it in her arm? Her right arm? Her left arm? Is it in her neck?' And they very often can't tell you that. And then I turn to the second person-and they always say, 'I was going to say she has an IV.' "

Then Lorber proceeds to rattle off the details he has noted: the patient has a bible on her nightstand and one on her lap; there are photographs on the wall and chair and notes beginning "Dear Granny," and so forth. So he knows she is a religious woman, has lots of adoring grandchildren . . . and suddenly a picture of this patient starts emerging from behind the IV. The next time Lorber's students enter a patient's room, he reported, they look intently at the patient and all around her-and they start seeing things they otherwise would have missed.

We had seated ourselves to give him a breather, but Lorber was clearly not a man who rests for long. Before I knew it we were up and he was giving me a tour behind the velvet ropes of the college building. Between the grand cla.s.sroom s.p.a.ces, ancient medical libraries, artwork, and medical paraphernalia, there were various examples of a kind of art reality. For here was a Thomas Eakins portrait of an ophthalmologist, one Dr. William Thomson-and in front of the oil painting lay the ophthalmoscope that the subject holds in the portrait. Up a grand staircase, between us and a large photograph of a late-nineteenth-century surgeon performing a dissection before a large audience of eager students, was his very dissection table, a huge marble slab with a large webbed drain in its belly.

We, too, were about to be artwork embodied, springing forth out of this building, which stands as a kind of museum to medical investigation, and doing our own medical investigating on the street. While we would not be palpating anyone's thyroid or percussing their spleens, those simply walking toward us would be presenting themselves for brief inspection. Moving through s.p.a.ce, we reveal the ways we are functioning poorly: stiffness in a gait; an asymmetry to the swinging of our arms; a tendency to look overly closely when listening; a lugubriousness in carriage and expression.

Indeed, we were not yet ten yards out of the building before we saw a couple of men approaching. The investigation had begun. I took a quick survey of our surroundings. December already, it was late in the season to be as warm as it was. It had been raining. Philadelphia is already the color of rain, and she wore the damp comfortably. Yellow gingko leaves decorated stone-slab stairs and sidewalk squares. While talking, Lorber and I stole looks at the men approaching us. I began my discovery: What could be wrong? Coats? Check. Raingear? Nope-but it was not raining this moment. Um. Hmm. Have all their body parts? Seems so.

I had come to the end of my diagnostic tether: I had nothing. Lorber, though: "That gentleman needs his hip replaced."

That's all Lorber said, but as soon as he said it, I saw the limp on the fellow on our right. As my vision awakened, the limp seemed to get more p.r.o.nounced by the step. A huge limp! All I had noticed was a puffy jacket.

Gait is like the poker player's tell: revealing of all faults. We can think of walking as a kind of controlled falling, accelerating toward the center of a circle with our legs as radii and the journey of our hipbones drawing an arc on the circle that is ours alone.1 Despite the large range of body types among members of the species h.o.m.o sapiens, it is as easy as identifying features on a face to identify a normal gait. Researchers have quantified the order, duration, and phases of what are called "interactions between two multi-segmented lower limbs and the total body ma.s.s"-i.e., between your legs and you.

This is how it goes. You are standing. (Congratulations on that, by the way. Bipedalism is fairly rare among animals, and causes all sorts of organizational and balance issues for our bodies, which we spend our lives fine-tuning. Infants might take a full year from their birth to be upright without support, recapitulating in that year what it took our species millions of years to do. By fourteen months old, toddlers are taking approximately two thousand steps an hour. They are also falling-ka-boom!-about fifteen times an hour.) To begin walking, you lurch forward, nose first. You are aloft! One foot has begun to lift and swing, your weight shifts to the other leg, and you are already off-balance, both from front to back and from side to side. Your lightening foot rises from the knee, which itself requires the hip to lift. Your pelvis pivots back. If your abdominal muscles do not kick in right there, you begin to feel it in the muscles that are stronger: your lower back, your rear end. The toes of your raised foot are pointed down, but must lift, too, climbing above the plane of the foot and raising high, so as to send their blunt friend the heel toward the ground. Already the toes of the other foot are feeling the pressure of the motion, of holding your body's weight, and begin to clench to encourage that foot's readiness. Your heel strikes the ground, the rest of the foot slapping down after it, your knee flexed to absorb the shock. You rock from the outer edge of your foot forward and toward the inward edge, knee wobbling over the center line of the foot. And you are in what orthopedists will call a stance, with a foot on the ground. Actually, two feet. During walking, of course, we are no horses: we never have both feet off the ground. That is "running." During walking, the stride time with a foot in the air is shorter than the stride time with both feet on the ground. No wonder we don't get anywhere fast while walking: we're half standing still.2 One foot swung, the other races to keep up. Ideally, the second does the exact same thing as the first.

But that exact same thing almost never happens, and this is why gait a.n.a.lysis is such a ripe place to see an internal disorder manifest externally. Gait can go wrong in many characteristic ways. An asymmetric gait can reveal a spiraling host of one-sided troubles. The whole gait pattern-one "step"-is diagnostic, too. Someone who seems to waddle might do so because of a muscular disease: unable to stabilize his pelvic muscles, a person will tilt his entire pelvis with each step. Some hyperkinetic gaits, legs twitchy and restless, can indicate a problem in the basal ganglia of the brain. A hurried gait, along with shaking or tremors, might be a symptom of Parkinson's disease. If, on taking a step, the toes drag or the knee is lifted overly high to avoid dragging the toes, damage to the peroneal nerve could be indicated.

This last one I knew myself. Much to my chagrin, midway through taking these walks I incurred a back accident. Hoisting my young son into the backpack carrier we used when we needed to get someplace farther than a toddler's walking endurance, I felt a tweak. A few days later, I learned that "tweak" wasn't a bad description of what had happened: the disk between two vertebrae (L5/S1, for those of you with back injuries who collect the alphanumeric jargon) had herniated. My sciatic nerve was pinched, and with that, pain shot spastically and electrically through my left leg. A week, various steroids, and even more various narcotics mitigated the pain. But I was left with a compressed nerve, which takes weeks, months, maybe years to recover. In the meantime, various muscles, including those of my left foot and my left glute were not being innervated: they were numb and nearly useless. I spent a lot of time gazing at my foot, willing the toes to flex. They looked like perfectly good toes. They would bend if I pushed them manually, but they were cold to the touch: the muscles were deeply asleep, covered in pillows, blinds drawn and earplugs in.

Walking was an awkward, slow affair. The muscles a.s.signed to lift the toes and push the foot-and thus the body they carry-over and forward were not working. Nor was the muscle responsible for lifting the leg. So, using muscles in my back, I essentially threw my leg forward each step, then pivoted over it.

My gait had become, in the parlance of physical therapists, "disorderly." I learned about the other disorderly gaits after I had my own. Eventually, I had surgery on my back to relieve the compression on the nerve; six weeks later, I found myself barefoot and in shorts, walking down a long hallway in a physical therapist's office. At the end of the hallway was Evan Johnson. He sat with perfect posture on a low stool and studied me walking. The "gait test," a cla.s.sic, simple test of function, is widely used among physical therapists, and is surprisingly good at revealing disorder. It is also refreshingly low-key: you walk off, then you turn around and walk back.

Having looked at the innards of my spinal cord, excising a slice of my wayward disk, and sewing me up, my neurosurgeon (to whom I am forever thankful for doing those three things so well) had given me this diagnosis: "You could get full recovery" of the use of my leg, he said. "Or not. I can't tell." Really? My neurosurgeon can't tell? This was incredible and depressing news.

I was coming to accept that the prognosis for recovery from a nerve root injury is decidedly uncertain. But after seeing me walk off-and-back, Johnson popped up from his stool. "You'll be running again," he told me. "With time," he added.

He was right. I went through intensive rehabilitative therapy. It was so successful that, one day six months later as Johnson and I went out to take a walk from his office, neither of us even commented on the fact that I was walking with no apparent limp. Walking again represented that most desirable of conditions: ordinariness.

Johnson is tall and his smile is wide, neither feature revealing that he used to be a professional dancer. It was an injury while lifting another dancer that prompted him to seek his own care, and ultimately to earn a doctorate in physical therapy. Now he is the director of physical therapy at the Spine Center, a neurologic division of Columbia University Medical Center. He met me for a walk on a Friday before a long weekend, and the city was filled with people trying to get out of it. A rain had just scoured the air and pedestrians reveled in their shirtsleeves and coatlessness.

We looked left: people walking; we looked right: people walking. All seeming to be unwittingly submitting themselves for the gait test. I confessed to Johnson straightaway that I already felt there were two kinds of gaits: unremarkable and lame. People either seemed to get along fine or they had a limp, a weakness, or were afflicted with youth or age. It was not clear to me that there would be more that we could see.

Johnson spent the next ninety minutes disabusing me of that notion. Three steps into our walk, he had found his first subject.

". . . If you look at this woman while she's walking, she's carrying a very heavy bag"-on her right shoulder-"and her body is listed all the way over to the left as she negotiates her heels, her arm is swinging on the left to give her momentum. Notice her shoulder height on the right? Rather than have it relax and pull on her neck, she actually activates her upper trapezius and hikes the whole side of her body up . . . there's a good chance the scalene muscles on that side are going to be tight."

The woman was wearing high-heeled boots, a short dress, and looked like a very ordinary urban walker. But on closer inspection, it did appear as though her right side, holding the bag, was frozen in an uncomfortable position. And her left arm swung excessively to and fro, conducting an orchestra at her toes.

"She's holding on for dear life. A potential patient," he added.

I wondered if everyone begins to appear to be a potential patient when you are a physical therapist. As she swung away from us, I asked Johnson what the most common disorder he saw on the street was.

"One thing you see a lot is the habitual stooped posture, a forward head with a big kyphosis, especially in older individuals. Many wind up with stenotic spines, and when they lean forward, they actually create room for the tissues and the nerves in the back and it brings relief." Two seconds later, his exhibit materialized. "There, that individual . . ."

A tall, large man in a suit, his hair gray and thinning, was stepping off our sidewalk into the street. His neck brought his head forward rather than up, and his back was hunched, just as Johnson had said. The front of his suit hung lower than the back. I thought aloud how hard he must be to tailor.

"Indeed! That follows from his body: can you see how his hands are facing backwards, how the palms are facing you? That's a product of internal rotation of the shoulders. As he stoops, the back becomes rounded, the head goes forward, and the shoulders, too. And so does the front of his suit.

"It's basically our succ.u.mbing to gravity. It takes less musculature work to hang on the ligaments. Over time the ligaments yield; even the bones deform. If you have osteoporosis and the bones are soft, the vertebrae of the spine start to wedge to support that"-actually reshaping themselves so they are smaller in front and larger in back.

We were not just seeing a man crossing a street; we were seeing vertebrae in the process of wedging.

One minute later, clothing again served to accentuate a disorder in the making: "Look at the pants," he said, of a heavyset man. Okay, I did: the pants were blue. Also, they were so long that they gathered in rolls above the man's shoes.

Johnson patiently unpacked what he saw: "So look at how the cuffs are, and the shoes. The cuffs are bunched up more on the outside of the leg than the inside, and on the shoes you can see uneven wear where it looks like it's worn more on the outside than the inside. If you look at the way he walks, his knee is valgus-it's basically a moment where your knees come together and your feet go out. It effectively shortens your leg, particularly on the outside, while your inside becomes effectively longer. So your pelvis moves somewhat differently, and you end up wearing your shoes unevenly."

From disorderly cuffs and worn shoes, the astute gait observer can infer structural problems. Maybe his arch tends to drop, or maybe the ball of his hip is turned slightly forward in the socket of the pelvis. Over time, and after millions of steps, a subtle anatomical variation turns into an acquired deformity.

What had seemed like "unremarkable" gaits were looking more remarkable. Johnson and I moved onto the edge of the sidewalk and paused. A nearby subway unloaded a phalanx of walkers onto the sidewalk, and walkers hurried to make a short light across the street. This particular intersection frequently came up in discussions in his clinic, he said, because the street was extra-wide and the walk signal was extrashort. Many patients cannot make it across the street without hurrying or breaking into a run. Every gait disorder was accentuated under the stress.

Looking out onto the corner and the sidewalk, Johnson's a.s.sessments were rapid fire: "She hyperextends her knees, using the inert tissues-the ligament and the calcus-to absorb the shock. Her knees rotate in, see? She's a likely knee-injury candidate, hip injury candidate." He added, "She would not be a good candidate to be a runner."

Another: "If you look at that individual"-an older woman with thinning hair, an extralong jacket, and a defeated air-"she's waddling. Every time she steps on her right leg she leans way to the right and her left hip drops: that's called a Trendelenburg sign. It's weakness of the glute medius muscle and muscles of the side of the hip."

Followed by: "He's very thin"-an older man with a black hat, looking fragile-"and he's bowlegged on the right leg, which means it's taking a lot of weight on the inside of the right knee, and he's not bending the knee, he's landing on it, keeping it stiff. So he has a painful knee joint. He lacks muscle definition: that leads to a lack of control on that limb. His foot slaps down, and comes way inside the knee, where the heel lands. This contributes to his weaving back and forth, too."

Johnson found lots of so-called gait "faults," but he was also admiring of the people we saw: more than anything, one becomes aware of how many different but successful ways there are to propel oneself around one's day. Nor is every odd gait a pathology. A Hasidic man in too-large shoes flopped by us, prompting Johnson to remember a recent patient: ". . . an Orthodox gentleman who had a gait that was contributing to pain in his back: a tear of his annulus or his disk in his back, which is worse when you're leaning forward. So we worked a lot on posture, to get him into a more upright posture. But he refused to do it. He explained to me that it wasn't the posture of a humble man."

It was a revelation that gait might reveal religion. Or profession: a middle-aged man pa.s.sed by balancing a ladder by one rung over his left shoulder. "His gait speaks to the fact that he's walked with ladders like that quite a bit."

It is no surprise that "balancing a ladder" indicates that one might be "a person whose work employs ladders." But we could also see roughly how long he had been so employed. Because despite balancing an unwieldy object on one shoulder, it looked as though if we were able to surrept.i.tiously slide that ladder off of him, his gait would change not a whit. Were he inefficient in his gait when hoisting ladders, an injury would have had him retired long ago. In the same way, a furniture mover who can strap five boxes of books onto his back and head off down the street like a normal-if slow-moving-walker is one who knows what he is doing. Hire that man. He has found a walk that is efficient and low in stress, and he is unlikely to be injured moving your dictionaries.

Efficient was how Johnson defined the perfect gait. This is a word that comes up in dog-show judging, too, in which each entrant's gait is examined, usually at a trot. Many of the breed standards for gait are a version of Johnson's definition: "tireless and totally efficient" (malamute); "balanced, harmonious, sure, powerful, and unhindered" (rottweiler). Sometimes the descriptions range into the more lyrical: the "steady motion of a well-lubricated machine" (German shepherd); "true, precise, and not slurring" (Irish water spaniel); even "a perfect balance between power and elegance" (Rhodesian ridgeback). Despite the preponderance of potential patients among the pedestrians pa.s.sing us, Johnson pointed out plenty of balanced, precise-perfect-walking. On a hilly street, two men, diametrically opposed in style, approached us going downhill. One man was heavyset, wearing a loose cotton jumpsuit and cradling a sports drink in one hand. His whitening dreadlocks were pulled into a cap. The other was slim and shorn, wearing a shiny gray suit and a bright pink shirt. The first walked loosely and evenly, his knees bending to comfortably absorb each step, his pelvis rotating and his arms swinging smoothly. The gray suit was perfectly aligned in his steps: his ears over his shoulders, his shoulders over his hips.

Each, Johnson said, was a version of the ideal walker: their gaits had few asymmetries, were smooth and loose, and wasted no energy doing anything but going forward. From an evolutionary perspective, efficiency is the key. Our ancestors may have been easily outrun by any potential predator-we are not a particularly fast species-but we have endurance: those proto-humans who could keep running won their lives. And they could do that if their gait was efficient.

The gray suit lapel flapped in the breeze as its wearer jaywalked a diagonal across the middle of the street. The man in the jumpsuit ducked under a scaffolding. Neither was felled by predators on this day.

In Philadelphia, Lorber and I turned left onto Chestnut Street. Occasional raindrops were becoming less occasional. I had been back in this city exactly one day and was struck by how it was at once familiar and unfamiliar. Against a backdrop of urban design, shops, and citizens on the whole quite similar to those of my current city, the differences stood out as bas-relief. The sidewalks were narrower, befitting a place slightly older than my neighborhood. Buildings were on the whole shorter, allowing me to feel towering at five feet nine. The urban horizon was farther away: from some streets I could see to the next street, or to the next neighborhood, quite unlike the blindered, cavernous view one gets on a New York City street. Alleys interrupted long blocks, providing peeks onto the backsides of businesses. Peering down one alley, I wondered what kind of superhighway this was for one of John Hadidian's urban species.

I also recognized what I thought of as a "Philadelphia look": people with features reminiscent of my now-deceased grandmother, Johanna, who lived here for all of her eighty-six years. I can remember meeting her in a darkened restaurant on Chestnut Street for clam chowder. We sat in a hushed booth with velvet pillows, and she crumbled soda crackers into her soup bowl. And now I seemed to see people who resembled her, in the softness of her skin, the shape of her eyes, the pride in her walk. I could almost hear the jangles of the bracelets she wore on her arm. I asked Lorber, also a native of this city, if he knew this "look." He responded with a blank expression. Apparently the look was simply a nostalgic concoction of my own head.

We picked up our pace as the rain did, and began discussing the kind of shelter we might soon seek.

"That woman," Lorber interjected, his voice not changing tone from the previous sentence, "may have a genetic disorder."

"What?" My mind was still attending to the rain and my eyes were still looking at awnings.

"On the XY chromosome. The way her ears were set low, her short stature, and what was called 'webbing' under the face, that's indication of this disorder."

I looked behind us. There had been a woman; on reflection, I had indeed noticed that someone had walked by. Now she was retreating and soon disappeared around the corner from which we had come. She was broad and brunette. That was all I had noticed. Lorber, meantime, had seen a genetic deformity on her twenty-third chromosome.

I was amazed. In a vague, theoretical sense, I of course knew that we all wear our genes on our faces, bodies, and sleeves. My blue-eyedness is not a function of anything that I have done over my life: it was preordained once the sperm hit the egg. Still, eye colors seem categorically different from the kind of global diagnosis that Lorber was willing and able to make. This deformity would not only have physical ramifications, but behavioral ones. Seeing that woman's face, he was also seeing her probable behavior.

Lorber was confident but appropriately circ.u.mspect about his on-the-fly diagnosis. After all, he was not able to use some of the most useful and overlooked elements of an introduction to a new patient: simply hearing the patient tell her own history, revealing details cla.s.sified as "non-contributory," such as her profession, family life, and daily habits. Symptoms need a backstory.

Lorber himself likes to shake a patient's hand. He was hesitant to say just what it was that a handshake told him, but I got the impression that it was simply a way to open a conversation with touch, something both professional and personal. I made a note to shake his hand with conviction at our walk's end.

Nor were we close enough to the people walking by to really experience their bodies. Now, this may seem like a fine thing indeed, as to be close to a person is to smell that person, and often the smell of strangers' bodies can be repugnant. The smell of (quite a lot of) perfume used to cover the smell of bodies can be even more execrable. But by not getting close, we miss a fair amount of information. Lorber talked with a nostalgia usually reserved for childhood pancake Sundays and Aunt Leonie's madeleine cookies about how doctors used to smell a sample of suspect skin cells with their eyes closed, something few do anymore. When Lorber sees a patient who has had diseased tissue aspirated by another doctor, he always phones that doctor and asks them how the tissue smelled. What we might call bad breath is itself revealing of systemic or specific illness. There might be fishy, ammonia-like, musty, or b.l.o.o.d.y notes in bad breath-each indicating a different diagnosis. Lorber has written about a trio of his patients for whom their "putrid" breath-before pain in breathing, a cough, or fever appeared-was the only or first sign that they were harboring an anaerobic lung infection.

I clamped my mouth shut involuntarily.

Bodies do not only smell; they hum and whirr when everything is running smoothly, and especially when it is not. The nineteenth-century French physician Rene Laennec made a catalog of the sounds one might hear in a person's body, were one permitted to lean in close enough. Laennec curled a piece of paper upon itself to listen to his female patients' bodies, as to bend over a woman could be impracticable, if she was buxom, and was in any event indecorous. His simple paper cylinder later evolved into the stethoscope. Simply the m.u.f.fled thuds of the heart's valves snapping shut and the rhythmic wash of blood rushing away from the heart speak volumes about a person's health. There is plenty of literature about how the lub (closing of the mitrial/tricuspid valves from atria to ventricles) or the dub (closing of the aortic and pulmonary valves as the heart pushes blood out) can vary with health. When you slip on a blood pressure cuff in addition, someone listening through a stethoscope can hear the difference between the pressure of the flow of blood through your arteries when the heart is contracting and when it is relaxing. One imagines that with another simple tool, doctors may be able to hear the hoofsteps of our mortality approaching.

Laennec was especially interested in the sounds of disease. The list of sounds he heard through his paper tube reads like poetry. Of the various rattles from the bronchi, he heard pneumonia coming on as the sound of raw salt being gently heated in a pan. A pulmonary catarrh was so exactly like a pigeon's cooing that he might check under the bed for uninvited avian guests. With an obstructed bronchi, he heard "the chirp of a small bird and the slick squeal made when layers of oiled marble slabs were pulled brusquely apart." A cough like "a fly buzzing in a porcelain vase" might indicate lung disease. Other unfortunate illnesses made their presence known sonorously, through "sighing of the wind through a keyhole," "the murmur" from a toneless bellows, a pin "striking a porcelain cup," the sound of a spun top. Lorber and I stood under an awning, listening to the rain. Thinking about such purring, cooing, whistling, sawing, hissing, and crackling happening within me made me feel oddly musical as I sneezed.

As Lorber and I surveyed the street scene, each person who approached us became a demonstration of something new. The game of finding out what, exactly, they were presenting was a great one. My own version of the game was less medically informed, of course, and was instead imbued with the bravado of someone too new to a field to realize how little she knows. I was less able to identify specific problems than to locate potential subjects of interest. Bus stops were minefields for this kind of medical lens on the pa.s.sersby. We saw an older woman standing just apart from the other people waiting and wearing layers of ragged clothes with brand-new shoes. I am not sure what her health was like, but I could take a pretty good stab at the rest of her circ.u.mstance.

A young man waiting near her caught my eye. He was pacing, his head bent under a sweatshirt hood; his left hand held a phone to the side of his head. His gait was odd. It was as if his torso and hip were rigid as he moved, not fluidly rotating as they do in a comfortable stride. His toes were pointed outward. I pointed out my find to Lorber, who gently informed me that I had diagnosed a fashion statement. The fellow had his pants pulled down excessively, in the manner of lots of young men about his age-leading to a stiff swagger necessary to keep the pants from dropping to his ankles.

I looked for someone else whose gait might be based on their actual biology. There was a middle-aged man delicately crossing the street. Lorber was on it.

"I would guess that he has a disorder in his back, a spinal stenosis, which restricts the movement of his legs, and there is some atrophying of the muscles."

Sure enough, looking more closely as the man crept across the street, that seemed to be precisely the case: the man's trunk was solid, but his legs looked like they were dangling. He was not striding, but shuffling.

"It looks like he's not actually using his legs," I offered.