28 CLASSIFICATION OP FOOD. 



So, too, as respects the albumenoid bodies, of which it would be incor- 

 rect to speak as though they were limited to nutrition. In their decay 

 or descending metamorphosis in the organism, they give rise to the evo- 

 lution of heat, and are at last dismissed under the aspect of products of 

 oxidation. They are, therefore, as far as this goes, as much respiratory 

 food as are the fats themselves. 



Other ciassifi- Perhaps the most convenient subdivision of food articles 

 cations of food. j s presented in the four following groups : 



1st. Carbohydrates, or compounds in which carbon is united with 

 hydrogen and oxygen, their proportion being that for forming water. 

 Starch, sugar, gum, cellulose, are examples. 



2d. Hydrocarbons. Compounds containing unoxidized hydrogen. 

 The oils, fats, and alcohol, are examples. 



3d. Albumenoid bodies. These contain nitrogen. Albumen, fibrin, 

 casein, are examples. 



4th. Salts. Any classification of food articles which does not con- 

 tain this group is imperfect ; for salts are not only absolutely essential 

 to organic processes, but also to the construction of many tissues. As 

 an example of the former case, the chloride of sodium may be mentioned ; 

 and of the latter, the phosphate of lime. 



It has been supposed that the tissue-making power of any kind of 

 Value of food food depends on the quantity of nitrogen it contains, and 

 does not de- that .jfs value may therefore be determined by chemical anal- 



pend wholly on . ^ . J 



its composi- ysis. Upon this principle tables have been constructed, 

 tlon - showing the agricultural worth of different articles of forage 



for domestic animals. But, as will be found hereafter, when we consider 

 the physiological effect of the allotropism of bodies, these tables are not of 

 the use supposed. Without entering into details at present, the case of 

 gelatin may be taken as an example ; this, though a substance abounding 

 in nitrogen, possesses no tissue-making value, but in reality belongs to the 

 calorifacient class, and therefore its administration in the sick-room, under 

 the various well-known forms of jellies, soups, etc., is altogether deceptive 

 as regards any nutritive power, since it undergoes speedy oxidation in 

 the system, and the products of its change escape by the kidneys and the 

 lungs. The value of food is not only dependent on the occurrence of 

 certain chemical elements ; they must also be present in certain allotropic 

 states. 



The same remark applies to the tables which have been constructed, 

 showing the amount of caloric furnished by different varieties of heat- 

 making food. The quantity of heat set free during the combustion of a 

 substance depends not only on the nature of the elements composing it, 

 but also on the particular states in which they occur. Combustibles may 

 have the same chemical composition, but very different heating power. 



