AMPHIBIA 279 



It also has slight amylotic Q.n6. proteolytic ferments. Desaturation of 

 fats is one of the most important functions of the liver. 



The principal function of the liver is the formation of glycogen, 

 sometimes called " animal starch." The glycogen is given out into 

 the blood in the form of dextrose, into which it changed by an enzyme 

 in the hepatic cells. The liver thus acts as a reservoir for food, 

 storing it up when it is in excess, expending it gradually to tide over 

 periods of fasting, such as hibernation. Glycogen is also found in 

 muscles, ovaries, the central nervous system and the skeleton. 

 The pancreas regulates the liberation of sugar (page 455). 



As an organ of excretion the liver collects u7-ea from the muscles 

 and so changes it that the kidneys can extract the injurious sub- 

 stances more readily. 



When food passes from the stomach into the duodenum of the 

 intestine it has an acid reaction due to gastric juice. In the duo- 

 denum it is mixed with bile and pancreatic juices which are alkaline 

 and so is neutralized. Proteins not acted on by pepsin are acted 

 upon by the trypsin of the pancreatic juice and converted into 

 peptones. The pancreatic juice changes starch into sugar. The 

 soluble food is absorbed through the walls of the intestines into the 

 blood and lymph. 



Spleen. — Since the spleen is found attached to the digestive tube 

 by mesentery it is usually identified and drawn with the digestive 

 system. It is, however, ductless and important in connection with 

 the circulatory system, but without digestive function. During 

 embryonic life the spleen produces red corpuscles. Even in the 

 adult amphibian spleen leucocytes (white blood corpuscles) and 

 occasional spindle cells are formed. The destruction of old, worn- 

 out, red corpuscles and the ingestion of wastes with pigment 

 granules seem to be important splenic functions. 



Circulatory System. — The functions of the circulatory system 

 are to carry food material and oxygen to all parts of the body, and 

 to remove the CO2 and other waste products of tissue metabolism 

 to the organs where they are eliminated. Two fluids, blood and 

 lymph, perform these functions. 



The vascular system of the vertebrate is a closed system of tubes 

 of vessels filled with blood, and ramifying through all parts of the 

 body. Its main parts are: The heart, which contracts and con- 

 tinually drives the blood around the system of vessels, the arteries, 

 which take the blood from the heart to all parts of the body, the 



