THE VERTEBRATE ANIMAL — SUBPHYLUM VERTEBRATA 383 



which is attached to one of its lobes. The liver also serves to convert 

 carbohydrates to glycogen (animal starch) and store it for future 

 ener^ production. It is also in the liver that protein wastes are 

 converted into urea and uric acid in order that they may be excreted 

 from the blood in the kidneys. The large intestine which is shorter 

 than the small intestine possesses no villi or digestive glands. It 

 receives the fecal matter and delivers it to the anus. In many 

 forms of vertebrates the posterior portion of the large intestine is 

 modified to become a cloaca, which receives also the products from 

 tlie urinary and reproductive organs. 



The chief function of this entire system is that of dissolving and 

 converting complex food materials into a form which may be absorbed 

 and assimilated by the protoplasm of cells throughout the body. 

 The materials commonly used for foods have large molecules, usu- 

 ally called colloidal in nature. Digestion then must serve to break 

 up these large molecules into smaller ones, thus forming solutions of 

 substances in order that they will readily diffuse through mem- 

 branes. Digestive enzymes are responsible for placing the food 

 materials in solution. So, proteins are converted to soluble amino 

 acids, starches and sugars to maltose and finally glucose, and fats 

 to fatty acids and glycerin. 



In general, an enzyme is an organic substance which by its pres- 

 ence under certain conditions \vill cause or hasten chemical reaction 

 between other substances without itself being consumed. The en- 

 zymes are formed in the protoplasm of cells and their action is 

 similar to that of a catalyst, since they accelerate chemical action. 

 There are different types of enzymes each capable of producing spe- 

 cific kinds of reactions. There are oxidizing enzymes (oxidases) 

 capable of bringing about oxidation ; reducing enzymes (reductases) 

 which produce reduction in tissues; coagulating enzymes (coagu- 

 lases) which cause clotting or coagulation; and hydrolysing enzymes 

 (hydrolases) act by causing a reaction between a substance and water. 

 Most of the digestive enzymes fall in this latter class. Most enzymes 

 consist of a parent substance or precursor (zymogen) which becomes 

 active only in the presence of a certain other substance, termed acti- 

 vating agent or coenzyme. As an example, the precursor of pep- 

 sin is pepsinogen which becomes activated in the presence of dilute 

 acid. 



