The reason is that the venom snakes use is a mixture of proteins. Proteins are made up of building blocks of what's called amino acids. They're the same stuff, effectively, as makes up the meat in your Sunday roast. That means that if you were to eat them - say I ate a snake and ate the poison sacks - it would go into my digestive system where my stomach acid and the enzymes in my stomach would just break down the protein, so it would fall apart and it would be harmless.

Venom is only actually toxic if it gets beyond the gut and it gets into the circulation of the body. That's why one snake could, for instance, eat another snake, and it wouldn't be poisoned by it.

In the same way, a person who has diabetes may need to inject the hormone insulin. Insulin is a protein, so if you were to take it by mouth the digestive tract would break it down. That's why people have to inject insulin to make it work.

So why don't snakes poison themselves, given they have the venom in their bodies? It's exactly the same reason that you remain healthy despite the fact that your pancreas makes a deadly cocktail of digestive juices which, if they got into your bloodstream, would kill you very rapidly.

People who get the condition acute pancreatitis have a very high mortality rate. It kills people because they literally eat themselves from the inside out. The reason that they don't do that normally when you're healthy is because the enzymes are made in cells in an inactive state. They're exported from the cell into a duct which is lined with special protecting cells that stop it going back in to the body's own tissues and doing any damage. The only place it can go is down the duct and then out into the digestive tract.

If you put that into the context of the snake, it's got a gland which knows how to make the proteins in the venom. They get exported into this duct which is a special holding bay, protected from the venom. It can't go back the wrong way or into the snake's bloodstream. When the snake bites you there are tiny muscle cells around those ducts and it squirts the venom down its fangs (there are ducts in there) and into the holes that the teeth have made in you. That's why the snake doesn't die from its own venom because it keeps it in a specially adapted part of the body so it can't get into the circulation.

I did ask a snake venom researcher this question once and he said you can also find antibodies in snakes to their own venom, to a certain extent. They kind of have their own antivenom built it. Possibly because they have exposed themselves at low level, or been exposed during altercations with other snakes. I don't know how protective that is so there's two mechanisms there why a snake doesn't poison itself.