92

FOOD OF ANIMALS.

A very slight alteration in the relative proportion ofthe elements of which they consist causes very greatdifferences in their nature and properties ; and in conse-quence of the very peculiar nature of nitrogen (91), allsubstances which contain it are exceedingly liable tochange.

281. On comparing together the various substanceswhich constitute animal and vegetable matter, it isobserved that lignin, gum, sugar, and starch, are allperfectly distinct in nature and properties from any ofthe substances usually found in the bodies of animals :a remarkable similarity, however, exists between thegluten, albumen, and oily substances of plants, andcertain forms of animal matters.

282. The albumen and gluten of plants closely resem-ble the albumen and fibrin of animals; indeed, some ofthe varieties of these substances may be said to beabsolutely identical; they consist of the same elements,and possess the same properties: thus, for example,there is no chemical difference between that variety ofalbumen which exists in peas, beans, and other legumi-nous seeds, which is called legumine, and the animalalbumen which is found in milk, cheese, &c., and calledcasein.

283. The knowledge of this fact, that the gluten andalbumen of plants are identical in composition with someof the most common forms of animal matter, throwsgreat light on the nutrition of animals. It shows thatthe gluten and albumen of plants used as food may