The trajectory of the bullet and gun powder on Trayvon Martin's body support George Zimmerman's account that the teen was on top of him when the defendant shot and killed Martin, an expert on gunshot wounds testified Tuesday as the defence approached the end of its case.

Forensic pathologist Dr. Vincent DiMaio also used photographs of Zimmerman to point out where he appeared to have been struck during testimony that took up a significant portion of the day's hearing. Defence attorneys, who said they may wrap up their case Wednesday, were hoping DiMaio's testimony would help convince jurors of Zimmerman's claims that he shot Martin in self-defence.

DiMaio, who was hired by the defence, said the muzzle of Zimmerman's gun was against Martin's clothing and it was anywhere from two to four inches from Martin's skin.

"This is consistent with Mr. Zimmerman's account that Mr. Martin was over him, leaning forward at the time he was shot," said DiMaio, the former chief medical examiner in San Antonio.

DiMaio testified that lacerations to the back of Zimmerman's head were consistent with it striking a concrete sidewalk. Later, when looking at photos of Zimmerman's injuries taken the night of the shooting, DiMaio identified six separate impacts to Zimmerman's face and head. He said he believed Zimmerman's nose had been broken.

"It's obvious he's been punched in the nose and hit in the head," he said.

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The death of 17-year-old Trayvon Martin in Sanford, Fla., in February 2012 sparked protests over concerns neighbourhood watch volunteer George Zimmerman wasn't arrested until weeks after the fatal shooting. (Associated Press)

Under cross-examination, DiMaio conceded that the gunshot could also be consistent with Martin pulling away from Zimmerman, and that he reached his conclusion without factoring in statements from some neighbors who say Zimmerman was on top of Martin. DiMaio, who has testified at high-profile trials including that of record producer Phil Spector, said witness accounts are often unreliable. The pathologist said he had been paid $2,400 by the defence.

DiMaio's testimony also addressed the difference between Zimmerman's account that he had placed Martin's arms out to his sides and a photo taken after the shooting that shows Martin's arms under his body. The pathologist said Martin would have been conscious for 10 to 15 seconds after the shooting as a reserve supply of oxygen ran out of his body, and during that time he could have moved his arms.

After DiMaio testified, the 911 calls that captured sounds of the fatal encounter were discussed again. Defence attorneys called Sanford City Manager Norton Bonaparte to the witness stand to describe the circumstances of how Martin's family came to hear the 911 tapes. Bonaparte said he played the 911 tapes while members of Martin's family sat together at City Hall. He played them as a courtesy before they were released publicly.

Defence attorneys are trying to show that Martin's family members may have influenced each other in concluding the screams are those of the Miami teen. Police officers testified for the defence that it's better for someone who is trying to identify a voice to listen to it alone.

Convincing the jury of who was screaming for help on the tape has become the primary goal of prosecutors and defence attorneys because it would help jurors evaluate Zimmerman's self-defence claim. Relatives of Martin's and Zimmerman's have offered conflicting opinions about who is heard screaming.

Zimmerman has pleaded not guilty to second-degree murder and says he shot Martin in self-defence during a scuffle in the townhome complex where he lived. Martin was there visiting his father and his father's fiancée.

Defence attorney Mark O'Mara also told Judge Debra Nelson that the defence would likely finish putting on its case on Wednesday. Zimmerman, so far, hasn't testified. But jurors saw repeated video recordings of Zimmerman telling his side of the story to police investigators. The defence started its case last Friday, and if it keeps to the schedule anticipated by O'Mara, its presentation will take about half of the time of the prosecution.

Nelson considered prosecutors' request to bar the defence from showing animation depicting the fight between Martin and Zimmerman. Nelson held an evidence hearing with jurors out of the courtroom, but ultimately postponed her decision and more arguments on the matter until later in the afternoon.

Prosecutors object to the animation, saying it isn't an accurate depiction.

Defence attorneys called the man who created the animation to testify. To recreate the fight, Daniel Schumaker went to the crime scene. He had employees in motion-capture suits re-enact what happened based on coroner photographs, police reports, the coroner's report, witness depositions and photos taken by responding police officers, he said.