This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The mucous membrane lining the stomach is seen, when its surface is examined with a common magnifying glass, to be covered with shallow pits. A more powerful microscope shows on the bottom of each one of these pits the openings of several minute tubes, the gastric glands, which lie imbedded in the mucous membrane, packed closely, side by side (Fig. 48). These glands secrete the gastric juice.
The Muscular Coat Of The Stomach lies outside the mucous membrane, and is made up (Fig. 34) of plain muscular tissue, whose fibres run in different directions. By its contractions it stirs up the food and mixes it with the gastric juice. Around the pyloric orifice of the stomach is a thick ring of muscle (the pyloric sphincter), which usually is contracted, closing the passage between the stomach and the commencement of the small intestine. During digestion in the stomach the pyloric sphincter relaxes from time to time, and allows food, more or less digested, to pass on into the intestine.