Food first enters the oral cavity. This is the "mouth" of the penguin and it is the initial site where food begins its digestion. However, very little digestion occurs in the oral cavity -- probably none. There are, however, mucous glands that secrete mucus, aiding the passage of food. The oral cavity assumes a role of connecting the outside world to the esophagus, which is a conduit from the oral cavity to the crop. The esophagus is a tube-like structure that has a muscular wall. Through movements known as peristalsis, wave-like contractions propel food from the oral cavity into the crop where food sits if it is to be regurgitated or if the stomach is full. It is my understanding that the crop is merely a diverticulation (distended pocket) of the esophagus and may not have any specialized functions.
Food leaving the crop either reverses the esophagus and out the oral cavity or it passes through the proventriculus. it secretes digestive enzymes that allow digestion of fish and crustaceans to begin. The proventriculus is the first part of the penguin "stomach." The second part of the penguin "stomach" is the gizzard. it grinds and pulverizes fish and crustaceans into smaller components that are more easily digested by penguins. The gizzard has a thick muscular wall (this is necessary to produce the contractions which grind up food). The internal surface of the gizzard is similar to sandpaper since it contains a grainy keratinous substance known as koilin. Histologically, this is known as the cutica gastrica. Once partially digested food exits the gizzard it encounters secretions of the liver and the pancreas. The liver, located inferolateral to the heart, produces and secretes bile, a collection of acids, pigments and cholesterol that are vital to the digestion and absorption of fat. To curb the acidity, the pancreas will secrete sodium bicarbonate to neutralize acid. This all occurs once partially digested food enters the intestines. Food travels along this curvy organ while it is being slowly digested and absorbed. When material can no longer be absorbed, it is delivered to the cloaca
