332 



COLLEGE ZOOLOGY 



posterior end lying free. When a lymph 

 space beneath the tongue is filled, the 

 tongue is thrown forward for capturing in- 

 sects. The teeth are conical in shape; they 

 are borne by the upper jaw and two bones 

 in the roof of the mouth called vomers. 

 Teeth are used only for holding food and 

 not for chewing it. New teeth replace those 

 that may be lost. 



The esophagus opens into the mouth 

 cavity by a horizontal slit. Its inner surface 

 bears longitudinal folds, which give it the 

 remarkable powers of distension necessary 

 for swallowing large animals for food. His- 

 tologically it resembles the stomach. The 

 stomach is crescent-shaped and lies mostly 

 on the left side of the body. The anterior 

 or cardiac end is larger than the esophagus. 

 It decreases in size toward the posterior or 

 pyloric end, where it joins the small intes- 

 tine. The stomach is held in place by a dor- 

 sal fold of the peritoneum, and a ventral 

 fold of the peritoneum. 



The walls of the stomach are thick, con- 

 sisting of 4 layers: (1) the outer thin peri- 

 toneum, the serosa or serous membrane; (2) 



a tough muscular layer; (3) a spongy layer, 

 the submucosa; and (4) an inner folded 

 mucous layer, the mucosa. The mucosa or 

 mucous membrane is an inner lining, with 

 many glands, both one-celled and many- 

 celled. These glands are tubular in shape and 

 sometimes branched. They are formed by 

 the invagination of the epithelium of the 

 stomach lining. Those near the cardiac end 

 of the stomach are longer and differ histo- 

 logically from those near the pyloric end. 



The two largest digestive glands are the 

 pancreas and liver (Fig. 214). The pancreas 

 lies between the duodenum and the stom- 

 ach. It is a much branched tubular gland, 

 which secretes an alkaline digestive fluid and 

 empties it into the common bile duct. The 

 liver is a large, three-lobed, reddish gland, 

 which secretes an alkaline digestive fluid 

 called bile. This fluid is stored in the gall 

 bladder until food enters the intestine, then 

 it passes into the duodenum through the 

 common bile duct. 



The anterior portion of the small intestine 

 is known as the duodenum; this leads to the 

 much coiled ileum, which widens abruptly 



Peritoneum (serosa) 

 Lamina propria 



Mucosa 



Blood vessel 

 Submucosa - 



Circular muscle 



Longitudinal muscle- 

 OJI 



li 



/ 



;.va:»::||:-.:.- • :. fl 





r-.V?-.-: •..•.•.•::• ■•1:1 







■■ . .-ii-- I: ■■ -.: ■•: • ■ --^ ■•/. 



iJu-..:V :-. ■•■■ ■•■■■■\Sl 



jLiiv^ 



ithelial cell 

 Goblet cell 



Blood cells 



Lumen of intestine 



Muscle layer (muscularis) 



Figure 215. Diagram of a small portion of a cross section of the frog intestine showing its 

 histology. (After Laboratory Explorations in General Zoology, by Karl A. Stiles. Third edition. 

 Copyright 1955 by The Macmillan Company.) 



