62 The Vertebrate Organ Systems 



Throughout the digestive tract, various enzymes reduce the food 

 to absorbable forms. The chief digestive glands are the liver and 

 pancreas. Their products enter the duodenum through the common 

 bile duct. 



VDMEMNE TEETH 



CtSTIC DUCT. 



GALL BLAOOER. 



SMALL INTESTINE 



COUMON BILC DUCT 

 MESENTERY 



LAWiC INTESTINE 



URINAAY BLAOOCR 



LIVER 



PANCREATIC DUCT 



SPLEEN 



Fig. 15. — The digestive system of the bullfrog. The lower jaw has been removed. 



THE RESPIRATORY SYSTEM 



For respiration, the adult frog relies on its lungs, mucous mem- 

 branes of the mouth, and moist skin. The lungs are small saclike 

 structures with but a few folds for increasing the surface area. Within 

 these folds are the small chambers or alveoli which are supplied with 

 capillaries. 



For pumping the air in and out of the lungs, the frog lacks the 

 specialized chest muscles of mammals. Instead it must literally swal- 

 low its air. When air is brought in through the external nares, the 

 glottis is closed and the mouth cavity is increased in size by depress- 

 ing the floor. To get air into the respiratory system, the external 

 nares close, the mouth cavity decreases in size, and the glottis opens. 



