2 CHEMICAL CONSTITUENTS OF BODY AND FOOD, 



The most useful classification of the more complex organic com- 

 pounds is the time-honoured one, into proteids, carbohydrates, and 

 fats. Taking this as our starting-point, we shall find that the other 

 substances present may be described either in subsidiary classes to 

 these, or as decomposition products of the more complex substances. 



The elements found in these compounds are carbon, hydrogen, 

 nitrogen, oxygen, sulphur, phosphorus, chlorine, iodine, fluorine, silicon, 

 sodium, potassium, calcium, magnesium, lithium, iron, and occasionally 

 manganese, copper, and lead. 



It will be on the whole most convenient to study the organic 

 compounds first, in the following order : 



1. Carbohydrates ; 



2. Fats, with which we shall stucfy the lecithins and cholesterins ; 



3. Proteids and albuminoids. 



In following out this plan we shall discuss some of the chemical 

 constituents of the food as well as those of the body. 



THE CARBOHYDRATES. 



The carbohydrates are found chiefly in vegetable tissues, and many 

 of them form important foods. Some, however, are found in or formed 

 by the animal organism, such as glycogen or animal starch, dextrose, and 

 lactose or milk-sugar. The carbohydrates may be conveniently but 

 loosely defined as compounds of carbon, hydrogen, and oxygen, the 

 two last-named elements being in the proportion in which they 

 occur in water. But this definition, if pushed, would include several 

 substances like inosite, acetic acid, and lactic acid, which are not 

 carbohydrates. 



The work of Fischer, 1 Tollens, 2 and many other chemists has, 

 moreover, shown that carbohydrates are not, as their name would 

 imply, simply compounds of carbon with water, but their constitutional 

 formula has been in many cases thoroughly worked out, and their 

 composition shown to be much more complex. This work has culminated 

 in the synthetical production of many of the sugars. 



From the chemical standpoint, the sugars (which are the simplest 

 of the carbohydrates) may be divided into two classes 



1. Those which, when digested with dilute acids, do not yield any 

 other sugar or sugars ; this class includes the glucoses ; and 



2. Those which, when so treated, do yield some other sugar or sugars ; 

 this class includes the members of the cane-sugar group. 



Further, the sugars are designated according to the number of 

 carbon atoms they contain ; thus we have trioses (e.g. glycerose), tetroses 

 (e.g. erythrose), pentoses (e.g. arabinose, xylose, rhamnose), hexoses (e.g. 

 glucose, mannose), heptoses, octoses, and nonoses, according as they 

 contain, three, four, five, six, seven, eight, and nine atoms of carbon 

 respectively in their molecules. 



The great majority of these sugars possess, however, but little 



1 See especially E. Fischer, Ber. d. deutsch. diem. Gesellsch., Berlin, Bd. xxiii. S. 2114. 



2 Tollens. " Kurzes Hftudbuch dor Kohlenhydrate," BresTau. 



