PLANTS FURNISH FAT. 249 



mences in the intestinal tube, the material from which it originates being 

 both nitrogenized and non-nitrogenized. Thus, when ducks have been 

 fed on albumen containing but little fat, the digested material in the in- 

 testine yields a larger proportion of fat than when they have been fed on 

 clay, or even on starch. If the glands of the intestine secreted fat from 

 the blood, it would be detected after feeding the birds with clay, and 

 hence we may conclude that the source of the increase observed is from 

 the albumen. 



But, in addition to the part they thus make, a large portion of the fat 

 of animals is undoubtedly obtained from the food. This is obviously the 

 case with carnivora, and the same may, indeed, be said of the herbivora. 

 Very many of the oleaginous bodies have a close chemical relationship 

 to one another, so that they may be regarded as affording a series, the 

 terms or members of "which arise from successive partial oxidations ; 

 and since the fats are soluble in one another, they freely mix together, 

 and therefore many of them may be found co-existing in the adipose tis- 

 sues, some of them less and some of them more advanced in their prog- 

 ress of oxidation. Whether they have been derived from pi an ts furnish 

 the food or by indirect processes made in the system, it is fatorthemate- 



. , , . , -, . " . rials from 



equally true in both instances that their primary source was w hich it is 

 in the vegetable kingdom. In the former case they occur- made ' 

 red in the plant-structure as hydrocarbons, in the latter as amylaceous 

 or nitrogenized bodies. Under the influence of the sunlight the vegeta- 

 ble tissues obtain them by decomposing carbonic acid and water, and to 

 those two substances they return after they have undergone destruction 

 in the animal organs, thus presenting a significant instance of the alter- 

 nate passage of atoms from the inorganic to the organic state, and back 

 again. 



The primary source of all fat substances is therefore in plants, which 

 obtain them from the decomposition of the inorganic constituents of the 

 air. The excess of hydrogen which characterizes this group of bodies in 

 most instances is undoubtedly derived from the decomposition of water, 

 and this explains the fact, frequently noticed, that the development of 

 such hydrocarbons in plants is often accompanied by the simultaneous 

 appearance of acids, for the hydrogen being appropriated by the former 

 class, the residual oxygen gives origin to acids or is set free. 



The quantity of fatty matter formed in the ordinary articles of food 

 used by domestic animals seems to be amply sufficient to Quantity of fat 

 meet all their wants. If a calculation be made of the amount 

 of such materials consumed by cattle during the process of mais. 

 fattening, it will be ascertained that the quantity used not only contains 

 sufficient to account for the increase of weight, but also furnishes an am- 

 ple supply for the portion which is destroyed by respiration. The fats 



