NUTRITION 95 



The fats which occur in the animal body are all derived 

 from fatty acids with an even number of carbon atoms. The 

 diabetic animal can form glucose from glycerol ; and a stage 

 in the intermediary metabolism of the fatty acids is the forma- 

 tion of keto-acids which further link up the metabolism of 

 fats with that of carbohydrates. Animals fed on carbohydrate 

 or protein diet deposit fat in their tissues, and the study of 

 hibernation indicates that the transformation of fat into 

 carbohydrates also occurs. In hibernating mammals (Pem- 

 brey) the respiratory quotient may be as low as 0*3, showing 

 that there is a conversion of substances with a small quantity 

 of oxygen (fat) into others with a larger amount (carbohydrate). 



Of carbohydrates, the polysaccharides starch and cellulose 

 are the principal representatives in animal diet. The role 

 of the former alone is understood in relation to the metabolism 

 of the mammal, where the breakdown of starch (and glycogen) 

 occurs in three stages during digestion. An amylase is present 

 in the saliva which is capable of carr}ang the hydrolysis of 

 starch in vitro through dextrins to the disaccharide malt 

 sugar. Actually the acidity of the gastric juice limits con- 

 siderably the extent of starch- digestion in its initial phase. 

 The same process is continued in the duodenum by the action 

 of an enzyme present in the pancreatic juice. There are in 

 addition present in the secretion of the duodenal glands 

 enzymes which complete the hydrolysis of maltose and the 

 other disaccharides (lactose and sucrose) into monosaccharide, 

 in which form they diffuse into the body. In the vertebrate 

 the blood, enriched with sugar, after digestion, has to flow 

 through the capillaries of the liver, where conditions occur 

 that permit synthesis of the storage carbohydrate glycogen 

 under the influence of an enzyme (glycogenase). During 

 starvation the reverse reaction predominates — glycogen is 

 transformed into sugar, which can be transported by the blood 

 to the muscle where it is also stored in the form of glycogen. 

 Glycogen is the universal storage form of carbohydrates in 

 animals. And the storage of glycogen is also a function 

 of the so-called liver of Crustacea. The amount of glycogen 

 in the crab's liver increases before each moult, and is used 



