CHAPTER III. 



THE CAKBOHYDKATES. 



IT has been pointed out in the preceding chapter that while 

 plants are capable of effecting from relatively simple compounds the 

 synthesis of those complex albumins which are found in their 

 various tissues and organs, animals do not possess this power, 

 and are therefore dependent for their supply of nitrogen upon the 

 albuminous food-stuffs that have been elaborated by plants. The 

 carbohydrate supply of animals is also derived from plants, but for 

 the maintenance of life it is not necessary that this should be fur- 

 nished as such, as animals are not only capable of forming carbohy- 

 drates from the albumins as occasion demands, but, as we shall see 

 later, they can also form carbohydrates directly from the fats which are 

 stored in their tissues. The carbohydrates cannot therefore be re- 

 garded as essential food-stuffs, and we see, as a matter of fact, that 

 carnivorous animals, at least, are capable of existing on albuminous 

 food exclusively. They are important, however, as the stored 

 energy which is thus supplied to animals represents a considerable 

 caloric value, and they can hence protect the albumins from undue 

 destruction. The importance of the carbohydrates as food-stuffs is 

 thus secondary, and they are totally unable to take the place of the 

 albumins. All living matter requires a definite amount of nitrogen 

 so that life may be maintained, and if this is withdrawn death in- 

 evitably results. It is to be noted, however, that whereas animals 

 can exist without carbohydrate food, and whereas the albumins 

 largely predominate in their tissues, the reverse holds good for plants. 

 Here the carbohydrates prevail, while the albumins are much less 

 abundant. Consequently we may expect to find a far greater diver- 

 sity of carbohydrates in the vegetable than in the animal world. 

 This is actually the case. As it would lead too far, in a work of this 

 scope, to consider all those carbohydrates which occur in the vege- 

 table world, we shall confine our attention in the subsequent pages 

 to those forms which may be regarded as common food-stuffs, or 

 those which are more or less peculiar to the animal body. 



All carbohydrates consist of carbon, hydrogen, and oxygen, and 

 in most members of the group the elements hydrogen and oxygen 

 are present in such proportion as to form water. In others this 

 is not the case ; and there are substances, such as lactic acid 

 and acetic acid, which likewise contain hydrogen and oxygen in 

 this proportion, but which are manifestly not carbohydrates. As 



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