Toddler Liam Fee suffered heart injuries similar to those found on road crash victims after a "blunt force" trauma , a court has heard.
Dr Paul French carried out a post-mortem examination on the two-year-old, whose mother and her partner are accused of murdering him.
He told the jury that "similar heart injuries" have been found in road traffic accident victims and he detailed more than 30 external injuries he found on the toddler's body.
The specialist paediatric pathologist recorded the cause of death as blunt force injury to his torso, jurors were told.
He was giving evidence on day three of the trial of Liam's mother Rachel Trelfa or Fee, 31, and her civil partner Nyomi Fee, 28, who deny murdering the toddler at a house in Fife in March 2014.
Dr French said he found fractures to the boy's upper arm and thigh which were then analysed by a bone expert who said the child had sustained "two fracturing events".
These were three to five days previously and less than six hours before his death, he told the High Court in Livingston.
Going through the external injuries using a computer-generated model of a child, Dr French told jurors most of them were "in keeping with blunt force trauma".
These included an abrasion and laceration to the back of the head, bruises on his shin and thigh and external injuries to the genital area, but he did not record any injuries to the boy's neck.
He added that not all the bruises could be described as "the rough and tumble" of a toddler.
Dr French, 38, told the jury that when he examined Liam's body, he found 70ml of blood in the pericardial sac around the heart, which he described as a "significant volume".
Asked what impact that would have, he replied: "It would stop it working properly and cause death."
Advocate depute Alex Prentice QC, prosecuting, asked: "Does that cause rapid death?"
"Yes," the witness replied.
Asked to explain further, Dr French said: "You have a hole in the heart as a consequence of trauma."
"That led to rapid death?" asked Mr Prentice.
"Yes," he replied.
Dr French later added that the possibility the heart injury could be caused by chest compressions could not be "wholly excluded" but was "unlikely".
Mr Prentice asked: "In your opinion it (the injury) occurred in life?"
"Yes," he replied.
Dr French also said that during the post-mortem at Glasgow's Southern General hospital, he noted the toddler as having a low body weight for his age.
He said his weight had been lower at death than it had been recorded in notes eight months previously, which he said could indicate a "failure to thrive".
The pathologist added that the causes for this could be "natural chronic disease, poor diet, behaviour problems or potentially neglect".
Reading from his report, Dr French told the court: "No natural chronic disease to account for the failure to thrive has been identified at post-mortem."
The murder charge alleges that the couple assaulted Liam at a house in Fife on various occasions between March 15 and March 22, 2014.
They also face a catalogue of allegations of wilfully ill-treating and neglecting two other young boys, who cannot be named for legal reasons, over a period of more than two years.
The pair are accused of falsely telling others, including police, that one of the other young boys was responsible for Liam's death.
They deny all the charges.
The trial, before judge Lord Burns, continues on Thursday when Dr French will resume giving evidence.
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