194 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



The ducts of the pancreas subdivide considerably, and finally 

 terminate in blind tubules lined by glandular epithelium. 



Nutrition. The food of the adult frog consists mainly of 

 insects. These are secured by the tongue, which can be rapidly 

 protruded and retracted. Its peculiar mode of attachment enables 

 it to be put out to some distance, and being covered with a viscid 

 secretion, insects are easily secured. The secretion of the inter- 

 maxillary gland is especially sticky, and the tip of the tongue 

 brushes past its openings when protruded. The teeth are not 

 used for chewing, but to prevent the escape of prey. The cilia 

 of the mouth-cavity work so as to carry the food to the gullet, 

 and from this point the contractions of the muscular wall of the 

 alimentary canal propel it backwards. The chemical agents acting 

 upon the foods are principally (1) Gastric juice; (2) Pancreatic 

 juice; and (3) Bile. 



The gastric juice is secreted by the peptic glands, and in 

 virtue of a ferment, pepsin, which it contains, converts some of 

 the proteids into peptones. The ferment can only act in an 

 acid solution, and the secretion contains a small amount of free 

 hydrochloric acid. The cuboidal cells of the peptic glands 

 secrete pepsin, while the acid is formed by the ovoidal cells. 

 The pancreatic juice contains ferments which (a) convert starch 

 into grape-sugar; (b) proteids into peptones; and (c) split up 

 fats. Alkalinity is necessary for this action, and the bile 

 neutralizes any acid passing over from the stomach. It also 

 emulsifies fats, and facilitates their absorption. 



The ridges and folds into which the lining of the alimentary 

 canal is raised largely increase its absorptive surface. Some of 

 the products of digestion pass at once into the blood-system, 

 others reach it indirectly by way of the lacteals, subvertebral 

 sinus, and lymph-hearts. The refuse is ejected at the cloacal 

 aperture. 



Most of the products of digestion, by way of the portal vein, 

 pass through the liver, and this organ absorbs the carbo-hydrates, 

 stores them up (as glycogen), and returns them to the system as 

 required. 



5. The circulatory organs consist of a closed Hood-system 

 with which a lymph-system communicates. 



(1) Blood System. The blood is a bright-red, coagulable fluid, 

 composed of a clear plasma in which float colourless corpuscles of 



