ORIGir F THE FAT OF ANIMALS. 595 



which so large a proportion of vegetable food consists. This opinion, 

 however, has given way before the advance of analytical research. 

 Animals fatten quickest upon Indian corn, or oil cake, or oil mixed with 

 chopped straw, or upon oily seeds and nuts — or, as in the case of poultry, 

 on a mixture of meal or suet — because these kinds of food contain a large 

 proportion of fatty matter ready formed which the animal can easily ex- 

 tract, and after a slight chemical change can convert into a portion of its 

 own substance. 



The conversion of starch or sugar into fat in the animal body implies 

 a chemical change of a less simple nature — one which seems to impose 

 upon the vital principle a greater amount of labour than is implied in the 

 simple appropriation of the fat which exists ready fornjcd in the food. If, 

 then, there be in the food as mucli fat as is necessary to supply all that 

 the animal appropriates to itself, and if it is observed to lay on or appro- 

 priate more when the food is richer in fatty oils, we are led to believe 

 that the natural purpose served by the oil in the vegetable food is to supply 

 the fat of the animal body. In other words, the vegetable ministers to the 

 animal and lessens its labour by preparing beforehand the materials out 

 of which the animal is to build up the fatty parts of its body. 



But thf)Ugh this is the general source of the fat of animals, circum- 

 stances may occur in which the only vegetable food which the animal can 

 procure does not contain a sufficient proportion of fat to supply all the 

 wants of its body — or to enable it to perform the several natural functions 

 it is destined to fulfil. Thus wax is a kind of fat, and it has been shown 

 (Milne Edwards) that, when fed upon pure sugar, the bee is capable of 

 forming wax from its food. When fed upon such sugar, it not only lays 

 up a store of honey, but it continues to build its cells of wax. Now the 

 starch of the food is readily changed into sugar. It may be so changed 

 in the stomach of man and of otlier animals. That power which the bee 

 possesses they also may in cases of emergency be able to exercise. 

 Where a sufficient supply of oil for the necessary uses of the animal is 

 not contained in the food it eats, it may form an additional portion from 

 the starch or sugar in which its food abounds. 



According to the present state of our knowledge, therefore, the most 

 probable opinion in regard to the origin of the fat of animals seems to be 

 expressed in these two proposition. 



a. That the fat of animals is contained ready formed, and is usually 

 derived from the vegetable or other food on which they live — and that 

 when the food abounds largely in fat, the animal lays it more quickly 

 and abundantly upon its own body. 



h. That when the food does not contain a sufficient proportion of fat to 

 enable the animal comfortably to perform the various functions of its 

 body, it has the power to form an additional quantity from the starch or 

 sugar it eats — but that it will not readily fatten or lay on large additions 

 of fat upon its body when fed upon farinaceous, saccharine, or other foo^ 

 in which oil is not naturally contained.* 



* For tlie sake of the chemical reader I may be permitled here to show by what kind of 

 chemical changes— 1°, the fat of animals in general may be derived from the starch or sugar 

 of their food ; and 2°, how the peculiar kinds of fat contained in the body of any given ani- 

 mal maybe formed from the peculiar kinds of fat contained in its food. 



1°. How fat may be formed from starch or tugar .—'These two substances, as we have 

 already seen, may be represented by carbon ard water only— 



