Dr Alan Williams counted 22 bruises, splits and grazes on James's face and head, and 20 more wounds on his body.
The prosecution at Preston Crown Court claims the accused boys - who deny all charges - attacked him on 12 February at a railway embankment in Walton, Liverpool. It claims they knew they were doing wrong and intended to cause James serious injury.
Dr Williams said that post- mortem examinations revealed that James had been struck by at least 30 blows, and he had endured 'a short period of survival' after the attack.
'It may only have been minutes,' Dr Williams said on the seventh day of the trial. James was dead before a train severed his body.
Dr Williams speculated on how James suffered so many injuries, so many that none could be isolated as the fatal blow.
The deep bruising, the cut on his forehead down to the skull and the damage to the back of the head together with many other injuries suggested bricks and an iron bar had been used, Dr Williams said.
Earlier, the court had admitted as exhibits a catalogue of debris found around the body. Exhibit 17, a box of 27 bricks; exhibit 19, a blood- stained stone found near a child's scarf; exhibit 28, his underpants; and exhibit 34, an iron bar.
A woman juror recoiled when offered the opportunity to lift the bar, a part-rusted railway fishplate, 20ins long and weighing 22lbs.
Dr Williams spent 33 minutes describing the external injuries suffered by James. Many to his legs had been inflicted when he was naked. The prosecution claims the killers stripped him from the waist. It is also claimed a grotesque indecency was inflicted on James: the foreskin of his penis was pulled back.
Many of the injuries covered an area of less than one square inch. But others were massive. Dr Williams traced the lines of skull fractures, his left hand mapping the attack from the jaw, over the forehead, across virtually all of the skull. Brain damage was extensive, including a haemorrhage at the centre.
One other implement was responsible for James's suffering. Injury number 13 was bruising to the area around the right cheek and ear. It was a severe blow which, Dr Williams said, left a pattern, a grooved mark. It was probably caused by stamping or kicking.
The court was also told that among the mourners laying flowers at the scene of James Bulger's murder was one of his alleged killers.
Boy A, who may not legally be identified, joined a neighbour who went to the railway embankment three days after the discovery of the child's body. 'I laid my flowers, and he walked away, looking at the other flowers,' Brenda Jeremy, 47, said in a statement read to the jury.
James, stolen from his mother's side at a Bootle shopping centre, was led 2 1/2 miles to the embankment, the court has been told.
Witness H, a 15-year-old girl, said that at 5.20pm on 12 February, she saw a toddler with two older boys. They were yards from Walton Lane police station, at the foot of the railway embankment. She watched them for only 10 seconds before she walked on.
The child was being pushed towards the road. He was laughing, H said. One of the older boys was climbing toward the embankment, the other apparently following, with the infant in his arms.
The prosecution claims witness H was the last to see James alive before he was stoned to death.
Dorothy Davies and Joanne Hinton, two video shop assistants who know A, told the court he entered their shop with another boy after 6pm on the day James disappeared. They were looking at videos when a woman entered the shop, grabbing hold of A and shouting at the other boy.
The jury was told the woman was B's mother, and she led both boys from the shop. Minutes earlier, she had reported her son missing to the police.
Mrs B told the police at 7.15pm she had found her son; at 7.48pm, Mrs A complained to police that her son had been assaulted by Mrs B.
Two days later, during the afternoon of 14 February, four youths discovered James's body.
The case continues today.