Confronted with potentially contradictory statements from his own textbook, a defense expert in author Michael Peterson's murder trial stuck to his conclusion Tuesday that Peterson's wife sustained fatal head wounds by falling down the stairs, not from being bludgeoned to death.

"We don't have a 'coup' or a 'contre-coup' injury here," prosecutor Jim Hardin Jr. asked, referring to a passage in Dr. Jan Leestma's 1987 textbook on the pattern of bruising in the front of the brain. The book notes that a contre-coup is one of the telltale signs of a fall onto the back of the head.

Peterson's head wounds are at the center of the case

"No we do not," admitted Leestma.

"Even though in your literature that's what we're supposed to have with a fall," pressed Hardin.

"Not necessarily," said Leestma, adding that the speed of the fall, the angle of the head, and a variety of other factors could cause death without leaving behind the injuries described in his out-of-print textbook, "Forensic Neuropathology."

Leestma spent almost four hours being cross-examined by the prosecution Tuesday, defending his interpretation of the lacerations on Kathleen Peterson's head, the neurological damage she sustained in her death, and his comparison of her case with 257 beating deaths in North Carolina between 1991 and 2002.

Prosecutors claim that Michael Peterson, 59, murdered his wife of five years with a blunt object at the bottom of the stairs to make her death appear like an accidental fall.  Peterson, who could spend life in prison if convicted of first-degree murder, says he found his wife dead after spending 45 minutes smoking outside by the pool.

On Monday, Leestma said Kathleen Peterson had likely sustained four blows to the head, not seven as the medical examiner testified, in two falls in the stairway of the couple's Durham, N.C., home.

The neuropathologist, who testified in a 1998 trial that au pair Louise Woodward had not shaken her employer's child to death (Woodward was found guilty), also faced questioning over his fees. Leestma explained that he charges $500 per hour of courtroom testimony, $350 for out-of-court examinations, including visiting the scene of the crime and reviewing records, and $75 for travel time during business hours.

The expert refused to estimate how much his tab was for the Peterson trial, but said he had, in the past, billed $25,000 for a single trial.

On Thursday, the jury is slated to visit the couple's North Carolina home, where they'll view the stairwell where Kathleen Peterson was found in a pool of her own blood.

Peterson's trial is being broadcast live by Court TV. 

It resumes Wednesday at 9:30 a.m., ET.