266 
An Essay on Fat and Muscle. 
gardens, as they find that swine fed on acorns only, seldom 
thrive. 
29. Our essay is now brought to a close ; and, from what has 
been stated, it must be evident that constant and progressive 
change appears to be one of the leading characteristics of life ; 
the whole seems like one vast laboratory, where mechanism is 
subservient to chemistry — where chemistry is the agent of the 
higher powers of vitality. In considering the digestive functions 
of animals, we have seen the manner in which vegetable food 
is assimilated into blood and tissues ; we see that the materials 
have to pass through a great number of intermediate stages before 
they can attain their final state. We can perceive all these, but 
still we have as yet a very imperfect knowledge of the nature of 
the vital agents concerned in producing those chemical changes 
which the food must necessarily undergo during its assimilation. 
The living principle, whether of a vegetable or animal, is so 
adapted that it can elaborate its body out of the materials which 
are around it ; but neither can create out of nothing that matter 
of which its organisation, during its appointed time, is composed. 
These materials, but few in number, are first elaborated from the 
air, the earth, and the waters, into the substance of plants, for the 
food of herbivorous and graminivorous animals, which, in their 
turn, are eaten by carnivorous animals ; and when, after a time, 
the spirit has left its tenement, the organised body is resolved 
into its original inorganic substances — carbonic acid, water, and 
ammonia — these elements being either returned to the atmos- 
phere, whence they were derived, or imbedded in the parent soil, 
again to constitute races of vegetables, and to contribute to the 
nourishment of organised beings. Even those portions of organic 
matter which, in the course of decomposition, escape in form of 
gases, and are widely diffused through the atmosphere, are not 
wholly lost to living creatures ; for, in the course of time, they 
also re-enter into the vegetable kingdom, resuming the solid form, 
and re-appearing in organic products, destined again to pass 
through the same never-ending cycle of vicissitudes and trans- 
mutations. This is the most important page in the whole book 
of material nature, for thus is grass changed into mutton and 
beef, which afterwards are changed into the flesh of man. 
