THE VIDEO begins with the Route 66 bus rocking ever so slightly as its engine hums to life, heralding the start of a depressing two-person ballet.

A young mother is hunched forward in her seat, facing the aisle. Her expression never changes: Her eyes are shut, seemingly trapped inside of an invisible fog, while her mouth hangs wide open.

Her body droops forward, like a dying sunflower.

A little girl's voice cuts through the din. "Mama," she says. "Mama!"

She reaches across the aisle in her pink jacket and tries to use her tiny hand to prop up her mother's head.

"Let go!" the woman cries. The torturous, slow-motion ballet begins again.

The four-and-a-half-minute video, which was posted on the "People of Septa" Facebook page on Thursday, prompted the Philadelphia Police Department's Special Victims Unit and the Department of Human Services to each launch investigations yesterday, officials said.

As of last night, the video had been shared more than 38,000 times on Facebook alone.

Criminal charges have not been filed against the woman, who was apparently identified with the help of people who responded to the video on Facebook.

(The Daily News is not printing her name because she has not been accused of committing a crime. A person who answered the phone at the woman's house yesterday afternoon said she wasn't available to comment.)

Those tasked with getting to the bottom of the mother's condition - and whether the child was ever put in harm's way - were frustrated by the fact that no one on the bus bothered to try and help the little girl or contact authorities.

"When you have a person doing the 'heroin nod' and a little girl is trying to hold her head up, that's horrifying," said SEPTA Police Chief Tom Nestel.

"There's very little reason why 15 calls to 9-1-1 weren't received," he said. "I don't think there's anyone who can watch that and say, 'Ah, that's a shame,' and just walk away."

DHS officials echoed Nestel's sentiments in a news release that noted the mother appeared to have been "severely impaired by drugs or alcohol."

DHS Commissioner Anne Marie Ambrose said that while it was helpful that numerous people called and emailed the agency after the video went viral, others who watched the situation unfold in person did nothing.

"Child abuse is a community problem," Ambrose said. "If you see or know that a child is being abused or neglected, you should report it immediately."

Callers can anonymously report child abuse on the agency's 24-hour hotline: 215-683-6100.

John Warren, the man who filmed the video and later posted it on Facebook, could not be reached for comment.

He told Philadelphia Magazine that he did eventually get up to tell the bus driver about the situation, "but the daughter started pulling on the mom, yelling 'Our stop, our stop,' and she got off the bus."

In the video, a robotic voice calls out bus stops along Frankford Avenue - Tyson, Princeton, Cottman - while the woman repeatedly nods off.

The little girl playfully puts her face next to her mother's. "Come on," she says, "wake up."

One or two passers-by try waking the woman as well.

"Mom, no one can get through with the bags," the child says, while trying to lift the plastic bags that the woman has left dangling in the aisle.

Philadelphia Police spokesman Lt. John Stanford said he recognizes that some folks shy away from getting involved in other people's business.

"But you can still give us a call, or notify the bus driver, or the conductor if you're on a train," he said.

"It's disturbing to see that little girl," Stanford added. "She can't be more than 8 or 9 years old."