406 



VERTEBRATE LIFE AND ORGANIZATION 



Mtise-ntiiry 



Serosa 



LoTT^itudiiial 

 muscle 



Bloodvessel 



Mxiscu]a.ris 

 mucoSa.c 



Mucosa.- 

 iSubmitc osa." 



Gaslricoland- 



Muscularis 



'Circular 

 muscle. 



Sabmucosa. 



"Miicosa 



B 



Figure 21.10. Diagrams of cross sections through the frog's stomach. A, Low mag- 

 nification; B, an enlargement of tlie segment of the preceding lying between the dotted 

 lines. 



narrow, tubular gastric glands continue toward the muscularis mucosae. 

 These glands contain several large, clear, mucus-producing cells and 

 other secretory cells filled with granules. The protein-splitting enzyme 

 pepsin is secreted by these glands in the anterior part of the stomach and 

 the adjacent portion of the esophagus, but hydrochloric acid, needed to 

 activate pepsin, is secreted by glands in more posterior parts of the 

 stomach. 



Multicellular glands are absent from the mucosa of the frog's intes- 

 tine. The intestine receives digestive juices from the liver and pancreas, 

 and into a laryngotracFieal chamber (comparable to the larynx and 

 The intestinal mucosa is thrown into many longitudinal and transverse 

 folds which slow up the passage of the food and increase the digestive 

 and absorptive surface. 



The pancreas and liver are large glands that develop embryonically 

 as outgrowths from the intestine. The pancreas produces a variety of 

 enzymes that are discharged through a pancreatic duct into the common 

 bile duct. Certain of its cells also produce the hormones insulin and 

 glucagon (p. 615). The liver's secretion, known as bile, leaves the liver 

 through hepatic ducts, is stored temporarily in the gall bladder, then 

 is discharged into the intestine through the cystic and common bile 

 ducts. Bile contains no digestive enzymes, but its bile salts emulsify fats 

 and aid in their absorption. In addition, the liver has an important role 

 in determining the concentration of certain constituents of the blood. 



1 86. Respiratory System 



The respiratory system of the frog includes the skin and the mucous 

 membranes lining the mouth and pharynx as well as the lungs. AD of 

 these are moist, vascular, semipermeable membranes exposed to the en- 

 vironment, through which gases can diffuse in both directions between 

 the blood and the environment. Lungs are the characteristic organs of 



