The stomach has several pockets. They are called gastric bags and there are 3. The stomach is the main digestive organ as the killer whale does not chew its prey and does not have salivary glands (which is the first chemical decomposition element of the food). Because the oesophagus is very large, what is swallowed arrives as is in the stomach, or to be more precise, in the first pocket.
The first pocket, or mechanical stomach is actually a kind of crusher, which will pound and chop the prey, making them more easily digestible. This pocket has no salivary glands though. It’s a kind of post oesophagus. However, the works from Flower & Lyddeker (1891) showed that, occasionally, a small quantity of digestive juices from the second pocket passes on to the first pocket, starting the digestion process. It’s worth noting that pebbles and shells are sometimes found in this mechanical stomach, aiding in the crushing of the food.
Food is then transferred to the second pocket, the central one which is called the chemical stomach, or main stomach. Hence the walls are thicker and of course with digestive glands. The hydrochloric acid together with the digestive juices will deteriorate the large molecules so they can be absorbed by the mucous membranes of the digestive tube.
Then comes the last pocket, or pyloric stomach (because it is situated in the pylorus zone, before the intestine). It is a kind of waiting room.