240 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



by its greater diameter, but in some of the fishes (fig. 253, a) there 

 is no distinction in size. The stomach lies in the coelom and hence is 

 covered externally by the serous membrane (peritoneum), but the 

 oesophagus usually extends a short distance into the body cavity and 

 then its lower end has the same coat. 



The true stomach is characterized by the presence of glands, de- 

 veloped from the mucous layer and emptying into the lumen. Of 

 these glands there are at most (mammals) three kinds: cardiac, near 

 the entrance of the oesophagus, which secrete an albuminoid fluid; 

 pyloric, near the pylorus, which form mucus; and the most character- 

 istic, the fundus glands, which secrete a digestive ferment, pepsin. 

 (For the structure of these glands reference should be made to histo- 

 logical text-books.) Tested by glands, many vertebrates (dipnoi, 

 cyprinoids) lack a true stomach, while the sturgeons have the gastric 

 glands extending into the oesophagus. On the other hand, a part of 

 the enlargement called the stomach in mammals often includes a part 

 of the oesophagus (fig. 254, A, E). 



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. 253, a), but with in- 

 crease 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 

 oesophagus is nearest the heart and hence is called the cardiac end. 



In the fishes the stomach may be either straight (fig. 256), or sacctJar, often 

 assuming the form of blind sac (fig. 253, g). The line between oesophagus and 

 stomach is not well marked, as the oesophageal 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, has the 

 stomach ciliated. In the amphibians and reptiles the distinctions between 

 oesophagus 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 digest- 

 ive 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 



