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Self-defense claim is on shaky grounds. so this evidence is extremely important. >> are you saying you can't tell based on those photos, the medical examiner examining from the photos, whether or not those were enough lacerations to explain being slammed multiple times? >> i think you can actually take a fall, bang your head against the sidewalk, and come down with this kind of hematoma situation. i think it -- certainly, the wounds appear to be super official but that doesn't mean there isn't some underlying trama more serious -- >> but if your head is slammed repeatedly on a sidewalk, wouldn...

Head that you say hit cement? >> impact. >> impact. so it's your position that there are at least three impacts between that head and cement? >> yes. >> okay. >> concrete. >> reporter: earlier, zimmerman's best friend took the stand. mark ostraman convinced zimmerman to buy a gun and housed zimmerman and shelley after the shooting and wrote a book about the case. >> you wrote a book and were you quoted what the defendant george zimmerman told you, correct? >> correct. >> and you wrote do you have a problem, that's what you said trayvon martin said? >> right, correct. >> reporter: he said marti...