Color-coded denim cloths cover the row upon row of black body bags atop cold metal tables. Blue means a body that eventually will go into a common grave. Tan, the family wants those remains back for burial, eventually.

These are bodies donated to science, awaiting one of the most sensitive rites in becoming a doctor. Before first-year medical students lay their hands on the living, they learn anatomy from the dead.

Week after week, for six months, teams of students will file into in a laboratory at Georgetown University to slowly take apart "their" body.

First goes the skin on the back, peeled away from the yellow globs of fat that made up what in life someone may have called love handles.

They lay bare the spinal cord and marvel at how its lower roots resemble the tail of a horse.

Carefully probing a lump inside one chest, a team unearths what at first looks like a metal button — a port through which this man once received chemotherapy.

The room quiets as students unwrap the protective covering over each hand. One torso, they quickly learn, looks pretty much like another. But a hand is unique, somehow more intimate, as they hold it with their own blue-gloved hands. Many of the women's nails still bear polish. One year, shockingly, students found a wedding ring.

"You will be working with somebody's grandmother, father or wife," Dr. Carlos Suarez-Quian tells his 200 students before they unzip those body bags for the first time.

They're beginning a balancing act: How to steel their emotions so they can help people, without losing their compassion.

Dissecting cadavers is an evolving tradition. No, sophisticated simulators and the plastic-infused organs of museum exhibits can't replace seeing and touching and lifting real bodies. In fact, demand is growing for whole-body donations.

What's changing is how they're used. Nearly one-third of medical schools have begun integrating nuts-and-bolts anatomy with clinical training spaced throughout their first year. That means Georgetown students dissect the heart, for example, the same week they begin learning how to tell the "lub-DUB" of a healthy heartbeat from the "lub-SHOOP" of a blocked valve.

"There's a very big difference between talking about chromosomes and having your knife in fat," says student Sarah Buchman of Bethesda, Md., as she eases through fat that, yes, looks like the squishy goo encountered on raw chicken.

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Day 1 opens with an interdenominational prayer: "Make us grateful for these gifts these donors have given us," says the Rev. Paul McCarren as students wait silently amid the bodies.

Then Suarez-Quian has some class rules. Wear goggles when sawing bone, he warns. All the medical devices people have implanted today can mean flying bits of metal. And absolutely no photos of the bodies on Facebook.

"We've been anticipating this moment a long time," says Christopher Chen of Irvine, Calif., anxious to get started. "It hits you — wow, you're a medical student."

But unzipping the lab's 44 body bags brings a surprise, and hesitation. The cadavers aren't lying on their stomachs, ready for students to make their initial cuts on the back. They have to be flipped over. The first lesson: How stiff and heavy a dead body is, especially one filled with at least eight gallons of embalming fluid.

"I was really worried the body would fall off the table," Chen admits.

Finally his team's cadaver is in position, and Chen asks to make the first cut, tracing the faintly visible spine with his scalpel.

"This skin's pretty thick. Can I have the forceps?" he asks, getting adjusted to how much force it can take to penetrate a body.

Suarez-Quian moves from table to table, showing students how to patiently peel back skin and then gently lift the triangular trapezius muscle without accidentally destroying the first nerve they'll encounter, the greater occipital nerve.

"This is not dissection by grenades. You have to go just deep enough," he calls to the room.

He stops to calm a nervous student. "If you make mistakes, that's fine. Your patient's not going to complain."

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"All consciousness is past. Use this body well to enhance your knowledge and lessen the pain of another," says a letter one donor wrote to the medical school.

Who donates their bodies? In some programs, women more than men. They come from every demographic. But there are no national statistics or even federal monitoring of whole-body donations, although one estimate suggests there may be 20,000 a year.

This year, Suarez-Quian got a startling phone call a few weeks before class began: The morgue was almost full. Georgetown usually receives about 60 whole-body donations annually yet this year received between 80 and 90.

Schools don't pay for a body donation, but Suarez-Quian still worries that the economy may have played a role — with some families perhaps deciding on donation to avoid funeral costs — instead of a straightforward desire to contribute to science.

There's no evidence of a nationwide donor increase, says Dr. Richard Drake of the Cleveland Clinic, who took an informal survey for the American Association of Anatomists. About half of anatomy programs require that donors themselves, not their surviving relatives, make the decision.

The demand is growing with an increase in medical school enrollment, plus additional programs that use cadavers to allow surgeons, paramedics and other health providers to learn and practice new procedures.

Whole-body donors usually are past retirement age, and because of disease or age can't offer their organs for transplant into someone else. One impediment is weight. Georgetown has quit accepting bodies that weigh more than about 200 pounds. Just last month, the university's embalmer injured his back preparing a heavy cadaver.

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Back in the lab, "My hands are getting numb already, " Nicholas Bonazza of Pittsburgh murmurs to his classmates, about 20 minutes into the day's dissection.

Embalming fluid is seeping through his surgical gloves. The skin sensation will wear off, reassures Suarez-Quian, but they must wash out their eyes quickly if any splashes.

Bonazza once worked at a funeral home, so "I've seen them," he says of bodies, "but cutting is a very different experience."

Then there's the pungent odor of phenol, a key chemical for long-term preservation. The smell can stimulate saliva, surprising students with feelings of hunger they find inappropriate.

"I couldn't eat meat for a good month or two" after a brief introductory anatomy course over the summer, Buchman says.

Forget the swooning stereotypes. In Suarez-Quian's 23 years teaching anatomy, no student ever fainted before this year. It happened the day the class used electric saws to cut the spine.

Starting on the anonymous back actually eases many students' nerves. A few days later, the bodies are flipped again to start working on the chest. Pretty soon, one team finds hard, irregular breast tissue in an elderly woman. Breast cancer, a student exclaims, although it will take a look under the microscope to be sure.

With bodies age 50 to 93, they'll stumble upon a variety of disease.

"If you don't believe me that smoking causes cancer, trust me, you'll see the evidence here," Suarez-Quian promises the class.

He reminds students to look beyond body structure and not forget "the humanity of anatomy."

Buchman gets the message. "You're going to be working with vulnerable people. There's nothing more vulnerable than a dead body."

Often, the students look over their shoulders to see McCarren, the lead chaplain at this Catholic university. He's there if they want to talk, but also because he's fascinated by this glimpse into the workings of the human body that most people never get.

"You don't lose sight of the mystery of it," McCarren says. "It's very moving — this is a person."

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It will be April before these students see their cadavers' faces. The faces are wrapped, in cloth and plastic, to keep the more delicate tissue of the head from drying out. Like with the rest of the body, the skin will be peeled back. They'll remove the skull cap to lift out the brain. Then they'll halve the face, the only way to see sinuses.

"Seeing someone's face is a very tough situation," says Vinny DiMaggio of Brooklyn, N.Y. "It's really a gift" all these donors have given.

He is in charge of organizing a memorial service the class will host for their donors' families in the spring, when dissection is done. That's when students thank families, one on one, for this big step in their education.

About half of the donors' families want the remains returned after dissection is done. Students working on those bodies keep every bit of tissue, for Georgetown to cremate.

The remaining bodies will be cremated, too, although Suarez-Quian retains some organs for additional classes. Those ashes are buried in a local cemetery plot, under a tombstone that reads: "In Memoriam Those who Gave of Themselves that Others Might Benefit."