WHY HAVE four boxers, including a 13-year-old boy, been fatally injured in American prize rings since October?

Because they were participants in a sport premised upon the ability of one man to knock another into unconsciousness.

Every year boxing, which is ranked as the 15th most injury-producing sport, accounts for nine to 10 deaths worldwide. It is simply inevitable that if enough powerful men hit other men on the head often enough, a few of those being hit will be killed.

For although the brain is a wondrously protected organ, encased as it is in fluid, membrane and bone, it can take only a certain amount of punishment.

While fight fans often get the impression one fighter is working over the head and face of another with repeated punches, studies by Dr. Harry Kaplan, a professor of neurology at the College of Medicine and Dentistry of New Jersey, have shown that on average a fighter is struck solidly on the head only twice during a professional fight. That's two good whacks out of the 1,000 traded blows.

But those two punches can do one of two things -- they can cause the brain to receive such a battering inside the skull that it is permanently damaged; or, they can cause the fighter to fall -- banging his head on the canvas or striking one of the ring posts.

A study published in 1969 of four fighters who had died as a result of injuries sustained in fights in New York City revealed that:

None of the fighters died instantly, but all lost consciousness in the ring and died 55 hours to nine days later without ever coming to.

All four men died despite surgical attempts to save them.

All had swelling within the brain, caused by internal bleeding.

According to Dr. MAX M. Novich, a ringside physician at the 1976, '72 and '68 Olympics and president of the Ringside Physicians' Association, "all the great killings in boxing in the last decade have been in the fight previous to the killing, leading to the saying, 'he was killed in the fight before.' The boxer never really recovers, they don't get enough rest or the diagnosis isn't made" and the next pummeling finishes the fighter off.

Many believe that's what happened to Willie Classen, who died Nov. 28 after taking a straight right during a fight five days earlier. Classen had reportedly suffered a terrible beating in London in late October, and many observers believe he hadn't recovered from that when he climbed into the ring for the last time.

Ironically, the skull, which affords the brain protection, is also the instrument of its destruction. The inside of the skull has numerous, sharp, bony protrusions, and when a person's head is hit hard enough, the brain, banging back and forth within its protective envelope, can become impaled on one of those sharp points.

Such an injury, which often occurs when a fighter's head bounces on the floor or ring post, causes bleeding within the brain, and pressure builds up.

"The blood builds up a tremendous amount of pressure," says Novich, and "the mid-brain is just suffocated by the buildup, so there's no blood supply feeding it. And that's where the control centers are located."

According to Novich, the most lethal punch is a hook, "because you can't see it coming and you can't get out of the way.

"When you're hit with a hook you swivel your head and neck, and that's when knockouts occur," he continued. "There's something about the head and neck that can't take that torsion."

The knockout mechanism is not totally understood, said Novich, but it is generally believed that the initial loss of consciousness occurs because of a kind of overload of signals to the nervous system.

One of the reasons the hook can be particularly lethal, he said, is that such blows often strike the temple, the thinnest part of the skull.

According to Novich, proposals to increase the weight of gloves used in pro boxing from eight to 10 ounces will do nothing to reduce the loss of life. In fact, he said, the increased glove weight could result in an increase in the power of the punches "because it would increase the force" of the blows.

Novich believes that deaths might be reduced, however, by limiting the number of rounds fought to 10, and cutting the length of each round from three to two minutes.

"But the most important thing," he said, "is to get these kids into shape . . . There's something about the conditioning that's missing. They don't spend the four or five years in the gym hardening the flesh, conditioning. Boxing's a hard sport. It's not easy. Conditioning has improved in other sports, but not boxing."

But boxers, said Novich, a defender of the sport, "are pantywaists compared to football players. Nobody gets hurt worse than football players."