_Structure._—The walls of the gizzard are very thick and are made up of three coats: mucous, muscular, and serous. The cavity of the gizzard is covered by a thick skin-like structure possessing a heavy stratified squamous epithelial layer which is thrown somewhat into ridges (Fig. 32, _B_). This membrane becomes thinnest near the edges. The mucous membrane, being cornified and readily detachable, is by some anatomists considered as a special membrane (Fig. 37, _B_). At the pyloric opening there is a valve formed by a fold of the mucous membrane, which prevents grit and large particles of food from passing out of the gizzard. [Illustration: FIG. 33.—View of viscera of the left side of a hen. 1, The base of the heart. 2, Proventriculus. 3, Left lobe of the liver. 4, Gizzard. 5, End of the duodenal loop. 6, Pancreas. 7, Free portion of the small intestine. 8, Lungs. 9, Diaphragm. ] [Illustration: FIG. 34.—View of the viscera of the right side of a hen. 1, Base of the heart. 2, Lungs. 3, Right lobe of the liver. 4, Duodenal loop. 5, Pancreas. 6, Cæcum. 7, Large intestine or rectum. 8, Free portion of the floating small intestine. 9, Gall-bladder. 10, Right kidney. 11, The ribs. Note the lungs pushing up between them. ] On each side it has a powerful fleshy muscle, the _muscularis lateralis_. These musculares laterales are hemispherical, consist of very closely packed fibers extending transversely, and are attached to strong anterior and posterior ligamentous tendons. They are joined at the edge of the organ by a strong aponeurosis. The muscular fibers are red but do not possess cross striations; they are of the smooth type. The _muscularis intermedii_, thinner and less developed than the musculares laterales, occurs on each of the anterior and the posterior parts of the gizzard. The serous coat covers over the greater part of the external surface of the gizzard and closely adheres to the muscular coat. _Function._—The hard callous pads of the gizzard, operated by the powerful muscles above described, together with grit, act like mill stones and make reduction to fineness very complete. At the posterior part there is a sacculated portion containing glands of the long tubular type which secrete a fluid ferment similar to that secreted by the glands of the pyloric portion of the stomach of mammals. These glands also exist in a small band near the entrance of the gizzard. Here the food is mixed with strongly acid secretion containing pepsin which makes gastric digestion perfect. [Illustration: