36 PHYSIOLOGY CHAP. 



Cholesteriu resembles the fats in certain of its characteristics, 

 though absolutely unlike them in its chemical constitution ; it is 

 regularly found in every animal and plant cell, particularly in the 

 brain and liver. Since it is a secretion from the skin of man and 

 other animals, it is found in the epidermal structures (hair, fur, 

 feathers, nails, etc.), for which it forms a kind of protective grease. 

 Cholesterin is a monatomic alcohol of unknown constitution, which 

 crystallises from alcoholic solution in laminae like mother-of-pearl. 

 Like glycerin, it forms with fatty acids compounds which corre- 

 spond to the fats. 



From a chemical point of view the carbohydrates are aldehydic 

 or ketonic derivatives of polyhydric alcohols. They may be divided 

 into three groups: (a) monosaccharides, (&) di-saccharides, (c) poly- 

 saccharides. 



(a) Among the monosaccharides are more particularly grape 

 sugar (glucose or dextrose) and fruit sugar (fructose or laevulose), 

 which are abundant in plant juices ; the first also occurs in animal 

 tissues. They turn the plane of polarised light to the right or 

 left. They are readily oxidised ; they are fermented by yeast, and 

 converted into alcohol and carbonic acid : 



C 6 H 18 6 = 2C S H 6 OH + SCO,,. 



They have the property of readily abstracting oxygen from the 

 surrounding medium, and behave as reducing agents to oxidised 

 compounds. This property is utilised in detecting the presence of 

 sugars, and also in estimating them. The tests most used are 

 Troinmer's and Bottger's. In the former the sugar solution, 

 rendered alkaline with caustic potash or soda, on adding a few drops 

 of dilute copper sulphate, and heating, reduces the copper oxide 

 to cuprous oxide, a suboxide which forms a reddish-yellow pre- 

 cipitate. In the second test a few drops of bismuth subnitrate 

 are added to the alkaline solution of sugar, which is turned black 

 by the reduction of the bismuth salt to the metallic state. 



Besides these two tests, which, since they are based on the 

 reducing property of glucose, are not, strictly speaking, specific to 

 this compound, but are common to all the reducing substances, 

 three other specific tests are known for glucose, namely Moore's 

 test, the phenyl-hydrazine test, and that of alcoholic fermentation 

 (biological test). 



In the first the solution of glucose is warmed, after diluting it 

 with about a quarter of its volume of caustic soda or potash. The 

 mixture first turns yellow, and then successively (according to the 

 content of sugar) orange, brown, dark brown, giving off the char- 

 acteristic odour of burnt sugar or caramel, which becomes more 

 intense on acidification. 



The second test consists in warming the glucose solution with 



