gans, and
nerve tissues), which acts as a protection against the destruction of
the red blood cells; (b) phosphorized fat (lecithin), the universal
distribution of which, according to Starling, seems to indicate that
it plays an important part in the metabolic process of the cells,
serving as a source of phosphorus which is required for the building
up of the complex nucleoproteins of the cell nuclei.

PROTEINS

Upon investigation it was found that neither the fats nor
carbohydrates were the chief constituents of the active tissues. It
was found, in fact, that the carbohydrates occurred in very small
quantities only in the muscles, and that frequently the quantity of
fat was likewise limited. Other substances, containing nitrogen and
sulphur in addition to carbon, oxygen, and hydrogen, which were
invariably present, and which are essential constituents of all
tissues and cells, both in animals and in plants, must be necessary to
all known life. To these substances, believed at the time to be the
fundamental constituents of all tissues, Mulder gave the name Protein,
from the Greek, meaning "to take first place." Later investigations
proved that, while the proteins were essential to the building and
repairing of the tissues and cells in general, they were not the only
factors concerned in the work; that certain mineral salts were
necessary constituents of all tissues, and must be present in order
for any normal growth and development to occur.[4]

~Composition of Proteins.~--The average nitrogen content of common
proteins is about 16%; that is, in 100 grams of protein there will be
approximately 16 grams of nitrogen, or in 6.25 grams of protein there
will be 1 gram of nitrogen. To estimate the protein content of a food
when the percentage of nitrogen is known, it is necessary simply to
multiply the percentage of nitrogen present, by the nitrogen factor,
6.25; or, if the amount of nitrogen is desired, when the percentage of
protein is given, to divide by same factor.

~Construction of Proteins.~--In plant structure the building up of the
proteins is accomplished by the plants from inorganic substances
existing in the soil and air; but in the animal body this is not
possible, because the construction of the tissues requires the use of
other proteins--the most available ones being found in food. Each
animal (or species) forms the proteins characteristic of its own
tissues,--while the proteins of food are similar to th