This is a corrosion cast of the blood vascular system of the human brain. Basically, they fill the blood vessels with a kind of plastic material and then remove the organic tissue, so you can see all the blood vessels without anything else to get in the way. We know that the brain is a huge drain on oxygen and so needs a lot of blood, but I think this is really cool that you can see how well vascularized it really is and how very many blood vessels and capillaries there are in the brain.

[Image Source]