Sense of course of a working life first attested 1803. v. 1590s, to charge at a tournament, from career (n.). The meaning move rapidly, run at full speed (1640s) is from the image of a horse passing a career on the jousting field, etc. Related: Careered; careering.
Contractions of the stomach muscle, called peristaltic waves, grind, crush and mix the food producing a liquefied substance called chyme.. The pyloric sphincter separates the stomach from the duodenum. This valve keeps food and gastric juices in the stomach until liquefied enough to proceed into the duodenum.
There are digestive juices in the stomach that help break down the food and the muscles there mix the food up. After the stomach has done its job, there's another valve, called the pyloric valve, that allows food to move from the stomach and into the first part of the small intestine, which is called the duodenum.