CHAPTER X 

 THE PHYSIOLOGY OF AN ANIMAL 



HAVING now studied the structure of multicellular animals, 

 we will consider briefly the functions of the various organs, with 

 special reference to the frog. 



Alimentary System. The primary purpose of the alimentary 

 canal is the digestion and absorption of food. The food of 

 animals is always organic, since animals are unable to utilize 

 mineral substances upon which plants subsist. Animals feed 

 upon the substances manufactured by plants: starch, the first 

 product of photosynthesis, may serve animals for food, and the 

 same is true of sugar, fats, and proteids. These foods are usually 

 in a solid form when taken into the animal's mouth, and in order 

 to be of any use they must pass from the alimentary canal into 

 the blood vessels. Solid food is incapable of passing through the 

 intestinal walls, and must be changed so that it can be dissolved 

 in the liquids of the alimentary canal, a process called digestion. 

 Digestion is brought about by digestive fluids which are secreted 

 by digestive glands within the alimentary tract. The frog has 

 no salivary glands such as man possesses, and the first digestive 

 glands are in the walls of the stomach. These are microscopic 

 in size and are called gastric glands. They are present in large 

 numbers and secrete the gastric juice, which is poured directly 

 upon the food after it reaches the stomach. A second digestive 

 gland is the pancreas, lying just below the stomach and pouring 

 its secretion, the pancreatic juice, by a special duct into the 

 intestine, close to the stomach. This is mixed with the food 

 just as it leaves the stomach and after it has been acted upon by 

 the gastric juice. By the action of these two digestive fluids the 

 solid foods are changed in their nature and rendered partly 

 soluble. They are then dissolved in the intestinal liquids, becom- 

 ing a thick, rather slimy mass of dissolved material. The 

 different foods eaten by the animal are subject to different 



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