DIGESTIVE ORGANS. 225 



The shape of the stomach is to some extent dependent upon the 

 shape of the body. In the elongate species it lies in the axis of the 

 trunk, especially in the lower vertebrates (fig. 227, a), but with increase 

 in the body width it becomes more transverse. This involves a bending 

 and a torsion of the tube, always to the right, and results in two faces 

 or * curvatures,' a lesser or anterior, and a greater or posterior, the 

 greater curvature often expanding into a so-called fundus region. 

 The end of the stomach which connects with the cesophagus is nearest 

 the heart and hence is called the cardiac end. 



In the fishes the stomach may be either straight or saccular, often assuming the 

 form of a blind sac (fig. 227, g). The line between cesophagus and stomach is not 

 well marked, as the cesophageal folds may continue into the stomach. The teleosts 

 exhibit the greatest variety in shape, in correlation to the differences in food. All 

 gastric glands are lacking in the cyprinoids, while Amia has both cardiac and pyloric 

 glands, and, like many teleosts, the stomach is ciliated. In the amphibians and 

 reptiles the distinctions between cesophagus and stomach are more marked, most 

 in the crocodiles. In the amphibians the ciliation of the mouth is continued into 

 the stomach. 



In the birds there is a differentiation of the gastric region into two 

 regions, an anterior glandular stomach or proventriculus, and a pos- 

 terior muscular gizzard. The proventricular glands secrete a diges- 

 tive fluid, and the food, mixed with this, is passed on to the gizzard. 

 The walls of the latter have their muscles developed into a pair of discs 

 with tendinous centres, while the glands of the gizzard form a secretion 

 which hardens into a horny (keratoid) lining, sometimes developing 

 into tubercular structures, of great use in grinding the food, thus in part 

 making good the absence of teeth. In the grain-eating birds small 

 pebbles are taken into the gizzard and are used in triturating the food. 

 (In the fossil pterodactyls small clusters of stones are sometimes found 

 in such a position as to lead to' the supposition that these reptiles also 

 had a gizzard.) The gizzard is best developed in the grain-eating 

 birds and is weakest in the birds of prey. In one species of pigeon 

 part of the wall of the gizzard is ossified. 



The mammalian stomach shows the greatest range of form (figs. 

 227, 228) and the greatest development of different kind of glands. 

 It may be a simple sac or it may be subdivided into a series of chambers. 

 It may be almost wholly cesophageal in character (Ornithorhynchus ', 

 fig. 228, A). Occasionally the cardiac glands may be absent. It 

 may be a simple sac, longitudinal or transverse in position, or it may be 

 15 



