150 THE STUDY OF ANIMAL LIFE CHAP. 



is one of the largest organs in the body and one of the 

 most important. It has many functions and may be 

 described as a hard-worked organ, especially in man 

 and domesticated animals where much energy is expended 

 in dealing with the internal results of a life of ease. 

 There is practically no disease of the liver or of any other 

 organ in wild animals, except when some change of 

 circumstances (oftenest due to man) exposes them to 

 novel microbic or other parasites, to which they are 

 unprepared to offer resistance. 



All the products of digestion, except the fatty acids and 

 glycerine which result from the action of pancreatic 

 juice on fats, pass by the hepatic-portal system into the 

 liver, which has to bear the brunt of everything before 

 it is allowed to enter the general circulation. Thus 

 many poisonous substances are arrested in the liver and 

 prepared for elimination, and harmful substances are 

 changed into harmless ones, and excesses of useful sub- 

 stances are kept back, and so on. The liver functions 

 as a great selective sponge which keeps the composition 

 of the blood approximately constant. 



Starch in the food is digested into sugar, and this, 

 along with the sugar in the food, is carried by the hepatic- 

 portal system to the liver. During digestion there is 

 an increase in the proportion of sugar in the portal 

 vein going to the liver, yet there is no increase in the 

 hepatic veins which leave the liver. This is an instance 

 of the way in which the liver regulates the composition 

 of the blood. 



What becomes of the excess of sugar ? It is stored 

 in the liver, being synthetised into animal-starch or 

 glycogen, which can be readily re-transformed into 

 sugar, and passed on to the muscles to form explosives. 

 Or it may be that the glycogen is stored also in the 

 muscles. One of the great advances made by the French 

 physiologist Claude Bernard was his discovery of the 

 glycogenic function of the liver. 



The liver has several other functions, such as the 

 beginning of the preparation of nitrogenous waste-pro- 

 ducts for their final elimination by the kidneys. 



