A STUDY OF THE FROG 7 



of the liver and opens into the duodenum, the first part of the 

 small intestine, through the bile duct. The pinkish, many- 

 lobed pancreas lies between the duodenum and the stomach. 

 It also secretes a digestive fluid which is poured into the duo- 

 denum through the common bile duct. The food, which con- 

 sists chiefly of insects and worms, is first acted on by the fluid 

 secreted by the mucous layer of tissue lining the esophagus. 

 As it passes into the stomach the acid gastric juice that is 

 secreted by the walls of the stomach also acts upon it and 

 digests out some of the proteid matter. In the duodenum the 

 bile and the pancreatic juice act upon the fats and starches. 

 The food, thus digested and made available, is taken up by the 

 walls of the intestine and carried by the blood and lymph to 

 all parts of the body to build up new tissue and increase 1 he size 

 of the body, or to renew tissue which has been worn out by the 

 various activities of the animal. Some reserve food is stored 

 in the liver in a form that is available for use when necessary, 

 as during the winter while the frog is hibernating. Nutri- 

 ment is also stored in the large many-branched yellowish fat 

 bodies which are closely connected with the reproductive organs. 



The lungs are thin-walled, sac-like bodies. The area of the 

 inner surface is increased by many folds which form minute 

 spaces, the alveoli, the walls of which are abundantly supplied 

 with blood capillaries. The waste carbon dioxide in the blood 

 is given off and the oxygen taken up through the thin walls of 

 these capillaries. The air passes through the nostrils or 

 external nares into a slightly enlarged chamber, the olfactory 

 chamber, thence through the posteror nares into the mouth. 

 The nostrils are then closed, the floor of the mouth is raised, and 

 the air is forced through the glottis into the short larynx and 

 into the lungs. The air is forced out from the lungs by the 

 contraction of the muscles of the body- wall. While respiration 

 is carried on chiefly by the lungs, the skin, particularly in the 

 frog, acts in the same capacity, the transfer of gases taking 

 place through the capillaries there as in the lungs. 



The Circulatory System. The blood of the frog is a liquid 

 plasma which contains three kinds of corpuscles. The com- 

 paratively large, flattened, elliptical, red corpuscles are most 



