[Footnote 2: Alcohol extract.]

These values illustrate both the method and its value in comparing
sources. Unfortunately experience has shown that polyneuritis is amenable
to other curative agents to a greater or less extent and it is difficult
to be sure whether the curative or preventive dose represents merely the
vitamine content of the unknown or is the sum of all the factors present
in the curative or preventive material. In comparing the value of
different chemical fractions it probably gives a fair enough basis for
evaluating their relative power but it is not entirely satisfactory as a
quantitive measure of vitamine content.

In America the comparison of vitamine content has been largely based on
feeding experiments with the white rat. No other animal has been so well
standardized as this one. Dr. Henry Donaldson of the Wistar Institute of
Philadelphia has brought together into a book entitled _The Rat_ the
accumulated record of that Institution bearing on this animal. This book
provides standards for animal comparisons from every view point; weight
relation to age, size and age, weight of organs and age, sex and age and
weight, etc. This book together with the experience of many workers as
they appear in the literature and especially the observations of Osborne
and Mendel have made the rat an extremely reliable animal upon which to
base comparative data. The omnivorous appetite of the animal, his ready
adjustment to confinement, his relatively short life span, all contribute
to his selection for experimental feeding tests. Another important reason
for his selection is that being a mammal we may reasonably consider that
his reactions to foods will be more typical of the human response than
would another type, the bird for example. It is perhaps necessary to sound
a warning here, however, and point out the danger of too great faith in
this comparability of rat and man or in fact of any animal with man. In