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 study 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 bod} - . 
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 lias 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. dcutsch. clicm. Gesellsch., Berlin, Bd. xxiii. S. 2114. 
2 Tollens, " Kurzes Handbuch der Kolilenhydrate," Breslau. 
