0 combined to create this elaborate animation. initially the state objected. they wanted it edited even after the edited after it was roofed down to still photographs but the state decided we want to throw it out. the gentleman on the stand right now, dr. vincent di marx -- maio. he is the defense's first witness of the day. >> craig melvin reporting outside the courthouse in sanford and we will take you inside the courthouse now. this is the forensic pathologist, the gunshot expert, dr. di maio. >> 1981 when you began your work at the bexar county medical examiner's office, were you able to work as a consultant privately in other cases outside bexar county? >> yes, sir. >> what kind of work would you do in that regard? >> mostly civil cases and then a few criminal cases. outside bexar county, i could testify for the defense for prosecution. i did -- you don't do many private criminal cases, such as like what i'm doing now. i'd run about, you know, two to four a year at the most. >> you have been working as a consultant since the beginning of 2007 then, if i've done the math correctly. i'm sorry. exclusively as a -- >> right, since january of 2007. >> in other words, you've been in private practice as a consultant since january of 2007? >> yes. >> and the majority of your work is in connection with civil cases? >> yes, sir. >> does that sometimes still include gunshot injuries, though? >> yes. usually, it's cases, insurance cases involving alleged suicides, whether it's a suicide or not. on occasion, you'll get a case of accidental discharge to maybe a defect in the firearm. >> you mentioned that you were in the military. did you do any research when you were in the military? >> yes, sir. that is when i started my firearms research. we studied the mechanisms of firearms and review gunshot wound cases. we had with micro flash and high speed photography. you see pictures occasionally of a gun going off with a bullet in the air or something. that is the type of photography we did, but we we shoot to evaluate different types of very, very quick. you can take photographs of things like moving through the air, bullets going through the air, 2,000, 3,000 feet per second and you can just kind of freeze them when the flash goes off. >> would you take a moment, please, and explain to the jury the mechanics of what happens when a bullet is fired from a gun in terms of what comes out of the gun and when? >> okay. it debt nature and send of jets and flame through either one or two holes in the primer into the main body of the cartridge case innighting the gun powder. the gun powder then begins to burn. it doesn't explode, it burns. and it's converted to gas and then the pressure will be gradual to build up. if the front of cartridge case, you have the bullet and that is kind of fixed in. so the pressure then begins to push against the bullet, to push it out of the cartridge case into the barrel, and then to be gripped by the rifling of the barrel and it takes a while. it has to build up. now during this time, some gas will get around the bullet and will actually get in front of it. so now what you have is you have some of this gas in front of the bullet. then you have the bullet. then you have the bulk of the gas. now, the barrel of the gun, of course, is filled with air. as the bullet goes down the barrel, it pushes the gas ahead of it and it pushes the air. it's kind of like, you know, a hydraulic system. the first thing you get out of the gun, if you do micro flash, you'll see it, you'll get a little powder of gas, not very big, and then pretty soon after that, you get the bulk of the gas. it looks like a mushroom lying on its side. okay? and in that big gas cloud, there will be a bullet and there will be unburnt or burning grains of powder, so it's gas, bullet, big gas cloud with -- big gas cloud with powder grains. all coming out in 245 order. >> can the gas that comes out first, that is part of the firing process, combined -- the burning material, combined with the air that's in the barrel? >> well, it will to a certain degree but that is mostly the larger cloud of gas. what happens is is it's kind of like you have a column of gas and air being pushed straight ahead. so if you hold the gun against, say, like clothing, what happens is this column of gas and air tears a hole through the clothing and then the bullet gets to the clothing and then the gas with the soot gets to the clothing as well as the powder. and that material, if it's in contact, all of the thing after the hole is made goes through the hole and comes you out the other side. >> if i heard you correctly then, fountain muzzle of the gun is against clothing, there will be a hole made in the clothing by the gas first? >> and the air, right. >> and the air. followed by the bullet, and then followed by the soot or the burning powder behind it? >> the powder, right. and that is why blanks, people think blank cartridges are dangerous -- are not dangerous. you could kill yourself with them. if talk a gun with a blank cartridge and put it against your chest, the gas formed, will just tear right through your skin and muscle into your lung. >> we continue to watch the first witness of the defense today called to the stand is dr. vincent di maio a gunshot expert and forensic pathologist. lisa bloom has been watching the proceedings this morning. a lot of people are independence in the testimony this doctor will give and crucial warchs may recognize he has been in drew peterson's murder trial and phil expe spector's murder trial. what is the weight he is bringing to the defense? >> he is absolutely the gunshot expert. he has written the book. he was a medical examiner who performed in many thousands of aups and has taken some heat for colorful statements he made in the high profile statements you referenced. both of those defendants were convicted of murder, notwithstanding vincent di maio testimony for the defense. most defendants are convicted in american criminal courts but he is testifying about a very important point for george zimmerman in this case and that is that the gunshot residue and the pattern of the bullet going through two shirts that trayvon martin was wearing and into his body, the defense says proves their theory that trayvon martin was on top angled over george zimmerman. that is the import of the doctor's testimony. >> lisa bloom, stand by. we will take a break and back with much more on msnbc. for their family. that's why i created the honest company. i was just a concerned mom, with a crazy dream. a wish that there was a company that i could rely on, that did all of the hard work for me. i'm jessica alba, and the honest company was my dream. 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