It turns green when it has been acted upon by acids, or exposed to the air.

So that the bile which you throw up when you are very sick at your stomach, is green because it has been acted upon by your gastric juice.
As you will remember, the blood which comes from the stomach and bowels is carried by the portal vein to the liver first and, through that, to the heart, instead of going directly to the heart, as all the other impure blood in the body does.

This is owing, in part, to the fact that this blood, being full of substances freshly taken or made from the food, is very likely to contain poisons; indeed, as a matter of fact, blood taken from these veins on its way to the liver, and injected directly into the blood vessels of an animal, acts like a mild poison.
In part, however, this blood goes first to the liver, because the liver, besides being a great blood purifier, is a "blood-maker" in the sense that it changes raw food-stuffs in the blood from the intestines into forms which are more suitable for use by the brain, the muscles, and the other tissues of the body.

Some of the sugars, for instance, the liver turns into a kind of animal starch (_glycogen_), which it stores away in its own cells.

It also turns both sugars and proteins in the portal blood into fat, part of which it pours into the blood, and part of which it stores away also in its own cells.