

CHAPTER III. 

 HTDROCARBONACEOUS SUBSTANCES. 



members of this class are distinguished from the preceding by 

 - their organic origin. They appear as products of the nutritive actions 

 of organized beings, and are not introduced ready formed from the in- 

 organic world. They exist both in vegetables and in animals. In the 

 former they are produced as new combinations, under the influence of 

 the vegetative process ; and even in animals, which feed upon vegeta- 

 bles or upon other animals, they are so modified by digestion and 

 assimilation that they present themselves, as final constituents of the 

 body, under new and specific forms. They all consist of carbon, hydro- 

 gen, and oxygen, of which carbon is present by weight in especially 

 large proportion, forming from 44 to 84 per cent, of the entire sub- 

 stance. Owing to the absence of nitrogen, which is an important ele- 

 ment in organic matters of the following class, they are known as 

 " non-nitrogenous " substances. They are divided into two principal 

 groups, namely : the carbo-hydrates, or substances containing carbon, 

 with hydrogen and oxygen in the proportions to form water ; and the 

 fatty matters, in which the proportions of carbon and hydrogen are 

 increased, while that of oxygen is diminished. The group of the carbo- 

 hydrates includes starch, glycogen, and sugar. 



Starch, C 6 H 10 O 5 . 



A special physiological interest attaches to starch from the fact 

 that it is the first organic substance produced, in vegetation, from inor- 

 ganic materials. The animal body is incapable of forming organic 

 matter, and must be supplied with these substances in the food. But 

 vegetables have the power of combining inorganic elements in such a 

 way as to produce a new class of bodies, peculiar to the organic world, 

 and capable of serving for nutrition. This is shown by numerous ex- 

 periments, in which seeds or young plants, artificially cultivated in a 

 soil of clean sand, moistened only with solutions of mineral salts, have 

 germinated, grown, and fructified, increasing, many times over, the 

 quantity of organic material which they contained at the beginning. 



This production of organic matter takes place in the leaves and other 

 green tissues of growing plants, under the influence of the solar light ; 

 and the first substance which makes its appearance under these condi- 

 tions is nearly -always starch. It is produced from two inorganic mat- 

 ters absorbed from without, namely, carbonic acid and water, which are 

 deoxidized by the vegetable tissues, and their elements combined to 

 form a carbo-hydrate. This is proved by the fact that oxygen is ex- 



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