The third, or
albuminous group, includes all substances closely allied to albumen,
and hence containing a large proportion of nitrogen in addition to the
other three elements. The last group consists also of nitrogenized
substances, which resemble gelatine in many of their characteristics.
The first two groups are called non-azotized, as they contain no
nitrogen; the last two, azotized, containing nitrogen. "All articles of
food that are to be employed in the production of heat must contain a
larger proportion of hydrogen than is sufficient to form water with the
oxygen that they contain, and none are appropriate for the maintenance
of any tissues (except the adipose) unless they contain nitrogen."
Hence the obvious restriction of the first two classes to the
heat-producing function, and of the last two (or azotized) to the
reparation of the tissues.
We have, then, the two natural divisions of calorifacient and plastic
foods: the one adapted to sustain the heat of the body, and enable us
to maintain a temperature independent of that of the medium we may be
in; the other to build up, repair, and preserve in their natural
proportions the various tissues, as the muscular, fibrous, osseous, or
nervous, which compose our frames.
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