Indeed, he is so general a favourite, Mr.Cleek, that there isn't an attendant connected with the show who would not, and, indeed, has not at some time, put his head in the beast's mouth, just as the chevalier does in public, certain that no harm could possibly come of the act.
"You may judge, then, sir, what a shock, what a horrible surprise it was when the tragedy of two weeks ago occurred.

Often, to add zest to the performance, the chevalier varies it by allowing his children to put their heads into Nero's mouth instead of doing so himself, merely making a fake of it that he has the lion under such control that he will respect any command given by him.

That is what happened on that night.
Young Henri was chosen to put his head into Nero's mouth, and did so without fear or hesitation.

He took the beast's jaws and pulled them apart, and laid his head within them, as he had done a hundred times before; but of a sudden an appalling, an uncanny, thing happened.

It was as though some supernatural power laid hold of the beast and made a thing of horror of what a moment before had been a noble-looking animal; for suddenly a strange hissing noise issued from its jaws, its lips curled upward until it smiled--smiled, Mr.Cleek!--oh, the ghastliest, most awful, most blood-curdling smile imaginable--and then, with a sort of mingled snarl and bark, it clamped its jaws together and crushed the boy's head as though it were an egg-shell!" He put up his hands and covered his eyes as if to shut out some appalling vision, and for a moment or two nothing was heard but the low sobbing of the victim's sister.
"As suddenly as that change had come over the beast, Mr.Cleek," Scarmelli went on presently, "just so suddenly it passed, and it was the docile, affectionate animal it had been for years.