As the day went on, more than 30 living bodies were turned into meat. I walked among the hanging slabs and saw quarter-cadavers, recognizing the same orthopedics of muscle and bone that I had seen in anatomy lab. Underneath our skins, we and the cattle are both glistening red outlined in white, strung like puppets by the names of a dead language.

Just as anatomical dissection has its proper procedure, which medical students can find neatly laid out in textbooks, Jewish traditional dietary law, or kashrut, provides a guide to the proper kosher dissection of meat and diagnosis of its cleanliness. The basic rules of kashrut are simple and widely familiar: Keep milk and meat separate and avoid shellfish and pork. But there is another criterion that is less well known: severe pneumonia can make a steer no longer kosher. Therefore, an examination of an animal’s lungs helps determine its cleanliness.

Pneumonia is an infection of the lungs and, in bad bouts, the infection can extend all the way to the lining of the lungs, which is called the “pleura.” The lungs and pleura normally slide freely past each other as the lungs expand and contract with each breath. When the two surfaces are inflamed by pneumonia, however, they stick together like an unlubricated piston in its shaft. As the pneumonia heals, a scar forms at the spot where the lung got stuck — a band of white fibrous tissue attaching the two surfaces. The rabbis look for these “adhesions,” each the footprint of past disease, and each a potential degradation of kashrut. According to Ashkenazi Jewish tradition, the number and size of these adhesions determine the grade of kosher. But most important, the rabbis must determine whether or not there is a hole hidden within the scar that reaches straight through the lung.

As a carcass hung freshly killed and cut open, a rabbi slid the lungs out of the chest cavity. His hand grasped the trachea as two fleshy lungs dangled below, and he walked over to the examining table. He placed an air hose into the animal’s trachea and inflated the lungs with a rush of air. They doubled in size like two large loaves of bread rising abruptly. The rabbi then cupped his hands around a scar tuft on the lung and filled his hands with water, being careful not to let any drain out. If there was a hole within the scar, air from inside the lungs would bubble up through the water, as when a mechanic investigates a flat tire for the puncture site. Such a hole from the outside to the inside shows the animal is not intact and therefore not kosher. These bubbles, then, are the diagnostic criteria for the rabbis.

And this is the heart of the situation: Kashrut’s concept of cleanliness and health relies on the sanctity of the barrier between the inside of the body and the outside world. Maintaining cleanliness means keeping the outside out, much as people in many cultures remove their shoes before entering a house or a place of worship. When we breathe, air enters our lungs and whooshes all the way down to the alveoli — but this is not truly inside the body. The air in the lungs is still continuous with the atmosphere and all of its dust, spores and smoke. The real threshold of the physical self is the lining of those deep alveoli where the body meets the atmosphere. The lungs are like the skin — a boundary with the rest of the world — but outside-in. A hole connecting the inside of the lungs to the pleura is a way for the dirt of the outside world to get in, truly inside, the body, and once that sacred barrier has been breached, innocence and purity are soiled.