This is how hockey players start kicking the dog in their mind. You are a penalty killer, and you are reckoned to be one of the best of them in the whole NHL. Your team is up by a goal, having come back from two goals down in the third period. And then, with about four minutes left in the game, you get tangled up with a San Jose Shark and sent off for four minutes, and you, the penalty killer, have to sit there and watch your teammates try to kill off a penalty you committed. Luckily, they do it, and the Bruins beat the Sharks, 5-3, after David Krejci flips one into the open San Jose net, and you, Patrice Bergeron, the penalty killer, can smile about it after the game.

“That’s strange, that’s for sure” was Bergeron’s reaction to the events of last night’s game.

There are players I enjoy watching because their skills are obvious and transcendent, and I know that, on any of the proverbial (and largely mythical) given nights, I might see someone do something I’ve never seen before, something I thought beyond the capabilities of the human machine. There are players I enjoy watching simply because they are so physically enormous that the world seems briefly to waver in its normal perspective. There are players I enjoy watching because their personalities are so winning that even their failures are charming. And then there is Patrice Bergeron, whom I enjoy watching play hockey simply because of the craft with which he imbues his work.

He is the best two-way player in the league right now and, among other things, that is a triumph over everyday aerobics. He is a genius at winning faceoffs, and he has a wonderful sense of space, never losing touch of where he is on the ice on offense, or where the man he is supposed to be dogging is on defense. This won him the Frank J. Selke Trophy in 2012 and 2014, one of hockey’s more ungainly goblets, this one celebrating the league’s best defensive forwards. But what Bergeron does is not mere “grinding,” which the old hockey heads love almost beyond words. Bergeron works so hard and his two-way game is so complete that it has developed its own kind of elegance. If it is possible to make the backcheck a kind of art, Bergeron has done it.

“I’ve been fortunate enough to be on great teams,” Bergeron said. “I’m certainly not going to take the credit for all of that. I’ve been lucky enough, not to be along for the ride, but to be able to do my job and get the feeling of winning, and that’s what you want to keep doing.”

Part of the craft is playing hurt, and two seasons ago, he pushed even that envelope to its utmost. He played through the 2012-13 Stanley Cup final with a punctured lung, a separated shoulder, and a broken nose. And it is that which makes Bergeron special, because he almost had none of this. No Selke Trophies. No “triple gold” — the world championship, the Stanley Cup, and the Olympic championship. He became one of the first people to come out openly about what happens to the human brain when it is jostled around within the human skull, what in Bergeron’s native French is called a “commotion cerebral.” In English, a concussion.

Now, awareness of the dangers of repeated concussions has been sharpened to a very fine point. There is no more stupidity about “walking off” a concussion, no more testosterone-fueled joking about getting “your bell rung.” People are getting dementia at an early age. People are committing suicide because of the fog that has choked the life out of them. Researchers study with horror the brains of the dead. Do not ask for whom the bell tolls, sport.

Bergeron was one of the first of them. He was one of the first to talk about what happens when you get a concussion. His first one was very nearly his last. His first one nearly ended his playing career before it really got started. And Bergeron learned a new kind of toughness — the courage to evaluate honestly, and without the false and dangerous bravado common to athletes at this level, the chances you are willing to take.

“I realize how much hockey means to me,” he said. “And I definitely look at it from a different perspective than when I was 18 or 19 years old.”

♦♦♦

It was seven years ago. The Bruins were playing the Philadelphia Flyers at home. Bergeron was a 22-year-old playing the fourth season of what already was a very promising career. He had 31 goals in his first full season in the NHL, and the Bruins front office made a decision that it would build the team around him, and not around high-priced draft pick Joe Thornton, who was shuffled off to San Jose. The next season, Bergeron scored 70 points again — and, as his fourth year began, he had already put up three goals and four assists when the puck came loose near the boards.

Bergeron was belted — “freight-trained” is the way the hockey scouts refer to it — from behind by Flyers defenseman Randy Jones, slamming headfirst into the endboards. He lay on the ice, motionless, for several very long minutes before being taken away. At Massachusetts General Hospital, he was diagnosed with a broken nose, and with a grade-III concussion.

(According to the system developed in 2001 by Dr. Robert Cantu, a researcher at Boston University’s School of Medicine, a grade-III concussion is one that causes the victim to lose consciousness for longer than a minute or suffer post-traumatic amnesia for longer than a day, or that leaves the victim still manifesting the symptoms of a concussion after a week. Cantu’s were the latest in what were regular revisions to the standards for judging concussions. The Colorado Medical Society had come up with its own standards in 1991, and the American Academy of Neurology had created its own six years later. Those two systems consider any concussion involving loss of consciousness, no matter for how brief a period, a grade-III event.)

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Bergeron found himself plunged into a nightmare. He was hypersensitive to light. He became dizzy while walking more than about 200 feet. Then, 12 days after the game against the Flyers, he did something very unusual: He shared his story with the world. At a press conference at which Boston Globe writer Fluto Shinzawa said Bergeron walked with “the pace of an older man” and needed both hands to steady himself to mount the podium, Bergeron talked about sitting in a dark room, trying not to fall down. He told the assembled reporters that, even as he was sitting there, the symptoms were threatening to overwhelm him. The press conference was held to a strict time limit. The TV lights were dimmed. Bergeron kept talking. He said he was now dedicating himself to “changing things so that no one else has to go through this.” Then he left and went back into the dark, a cold compress on his head, and no real idea if he’d ever play hockey again.

It was a revelation to many people, inside the game and out. Very few in any sport had been as open and honest as Bergeron was that day. Concussions are internal. They strike at the places where the person is formed, where are stored the essential parts of what makes an individual unique to himself. With a concussion, you live inside the injury.

“I think I was trying to do what I thought was right,” Bergeron said Tuesday night. “I thought a lot of those hits could have been avoided. I was trying to send a message that guys had to be accountable and responsible on the ice. I know it’s a physical game, and I wouldn’t want to change that. Yeah, I think, when you live through it, you don’t want anyone to go through it again.”

♦♦♦

Bergeron didn’t play again until the beginning of the 2008 season. That December, he suffered another concussion, this time in a collision with Dennis Seidenberg of the Carolina Hurricanes. (Seidenberg is now Bergeron’s teammate in Boston.) That cost him a month. There were other episodes, too, the most recent one only two weeks ago, in Detroit, when Niklas Kronwall of the Red Wings drove him into the boards. Bergeron finished his shift — of course he did — and then went right to the Bruins locker room, where he underwent the NHL’s new protocols for diagnosing concussions, which include an exile to what has become known as the “quiet room.”

“It’s great to see the evolution of it,” Bergeron said. “And the research, too, that’s great to see. I think doctors are finding more and more about it, and that’s great. The crazy thing about it is that every concussion is unique; its symptoms are unique. You have to treat every one differently.

“You get a feeling of what you can do and what you can’t do, and you get a sense now of what it takes to heal it.”

He has a career now that is edging him very close to his sport’s Hall of Fame. (Bergeron’s game has a visceral appeal to hockey’s old guard, and it is stylish enough that he can fit in with whatever it is into which hockey seems to be evolving.) It is a career he might not have had. Patrice Bergeron knows its boundaries, how to play inside them, how to push them, if he has to do so. He has counted the cost more closely than most. He gives his game his informed consent.

This article has been updated to correct the following: Patrice Bergeron’s penalty in the San Jose Sharks game was for four minutes, not two, and he played injured during the 2012-13 Stanley Cup final, not the 2013-14 Stanley Cup final.