The three stomachs of the cetaceans are similar to those of the ruminants (Fig. 85). The first is a crop-like reservoir for the reception of the food when swallowed. The mucous membrane is entirely devoid of digestive glands. In the dolphins the mucous membrane is provided with a hard horny covering, which serves to break up the food mechanically by trituration. The second stomach and the gut-like pyloric prolongation constituting the third stomach contain gastric glands and are hence digestive in function.
(b) Stomach forms, in which a portion of the organ is converted into an apparatus for mastication, are seen especially in birds, in which animals, on account of the absence of teeth, mastication cannot be performed in the mouth.
The stomach of the bird is usually composed of two segments, one placed vertically above the other.
The first appears like an elongated dilatation of the œsophagus, forming the Proventriculus or glandular stomach.
The second is larger, round in shape, with very strong and thick muscular walls (Figs. 86 and 87).
The proventriculus furnishes the gastric juice exclusively.
The second or muscular stomach, devoid of gastric glands, functions merely as a masticating apparatus for the mechanical division of the food. The thick muscular walls of this compartment may measure several inches in diameter and carry on the opposed mucous surfaces lining the cavity a hard horny plate with corrugated and roughened surface (Fig. 88). These hard plates are designed to crush the food between them, as between two mill stones. The muscle stomach is best developed in herbivorous birds, while both the muscular wall and the horny plate are much weaker and thinner in carnivore wading and swimming birds (Fig. 89).
| Fig. 88.—Œsophagus and stomach of Gallus bankiva, hen. (Columbia University Museum, No. 1809.) | Fig. 89.—Stomach of Botaurus lentiginosus, bittern. (Columbia University Museum, No. 23/1810.) |
| Fig. 90.—Stomach of owl sp. (Nuhn.) | Fig. 91.—Stomach of Ardea cinerea, heron. (Nuhn.) |
