ok - so all filament based 3D printing will cause some opacity - for large solids you will not get glass!
I think the best option, if you want a glassy object for your anatomy, is to use the printer to print a multi-piece mould - and then pour clear resin into the mould. Your release agent (and the mould itself) will leave a surface pattern - so it will still have a surface texture - but it will be far more satisfactory, even though it is a multiple step process (print the mould then pour the resin). If you print the mould with PLA then make sure it is quite hollow as you need a little flex to release the model. Do not try to use a two-part mould - and model them ensuring that they are proper booleans of each other. (add your pegs as a part of the model!) Use a fine layer of beeswax (or soft wax) to get flush fit, and use (a small amount!) of release agent! make sure the final mould stands upright (and clamps easy) with no air gaps that do not have air sprues - then you can pour from the top. I would go for some easier test models first and get used to the process!
moulds allow for all sorts of materials - such as soft silicon, plaster, or even very low-temp. metal.