Ravens defensive end Pernell McPhee simultaneously furrowed his brow, shook his head and frowned.

It was a telling reaction triggered by McPhee recounting instances of how the defense has consistently failed to stop the run this season, a bizarre departure from a traditional hallmark of the reigning AFC North champions.

"We're real disappointed," McPhee said. "It's about the name that the Baltimore Ravens holds, and we aren't playing that kind of defense, especially run defense. It's been crazy. It's frustrating. It's a lot of fundamental things going wrong, but it ain't effort. We're trying hard."

Prior to being sidelined for Sunday's game at M&T Bank Stadium against the Oakland Raiders and last week against the Cleveland Browns because of knee and thigh injuries, McPhee was among the culprits for the Ravens' defensive breakdowns.

He started at left defensive end against the Kansas City Chiefs, Dallas Cowboys and the Houston Texans, as the Ravens endured an unprecedented stretch with the beleaguered defense allowing a total of 622 rushing yards.

Historically stout against even the most dangerous running backs, the Ravens are plummeting to a new low mark in franchise history.

Allowing 139.5 rushing yards per contest and ranked 28th in run defense heading into Sunday's game against Oakland, the Ravens are on pace to allow a franchise-record 2,232 rushing yards and 18 touchdowns this season.

The previous worst performance was during their inaugural season in 1996 when injured middle linebacker Ray Lewis was a rookie. The Ravens finished 23rd in run defense that year, surrendering 120 rushing yards per game and a total of 1,920 rushing yards for the season.

"The Ravens' defense doesn't look anywhere near as aggressive as they've been," NFL Films analyst Greg Cosell said. "There's been an incremental change. It's strange to watch how they've been playing compared to what you're accustomed to seeing."

Whether it's Browns rookie running back Trent Richardson plowing ahead for 105 yards last week, giving up a franchise single-game record 227 rushing yards to the Cowboys, or being shredded by Chiefs running back Jamaal Charles and Texans star runner Arian Foster, it's been a series of embarrassments for a once-stingy outfit.

Just last season, though, the Ravens finished second in run defense overall. They had been in the top 10 in rushing defense for the past nine years, with six top-five rankings during that span. Barring a dramatic change in the second half of the season, that streak will be broken.

"Nobody has pressed the panic button, and nobody is going to push the panic button," strong safety Bernard Pollard said. "We are a prideful group. It's been times we've had to eat that humble pie. That's one of the worst things in front of the fans, on national television."

Here's a look at key elements causing the run-stopping regression:

Missed tackles

The most glaring example of the Ravens' struggles against the run was on display during a 31-29 victory over the Cowboys on Oct. 14 when they unraveled defensively.

It didn't matter who carried the football against Baltimore, as the Cowboys piled up yards against a defense struggling to bring runners down to the ground. Dallas averaged 5.4 yards per carry even though starting running back DeMarco Murray left the game with a foot injury.

Both Pollard and free safety Ed Reed were stiff-armed repeatedly as backs routinely gained yards after first contact.

"It's something we're trying to correct," outside linebacker Paul Kruger said. "It's not going to ever be completely fixed. There's always going to be a guy that breaks a tackle, but we're getting better. We're working hard."

The Ravens' lapses have included failing to wrap up runners, not driving through the ball carrier and trying unsuccessfully to deliver arm-tackles.