of the Blood, page 30.) There are present in the blood, however,
substances closely related to the peptones, maltose, glycerine, etc.;
substances which have in fact been formed from them. During their transfer
from the food canal, the dissolved nutrients undergo changes, giving rise
to the materials in the blood. Thus are the serum albumin and serum
globulin of the blood derived from the peptones and proteoses; the
dextrose, from the maltose and other forms of sugar; and the fat droplets,
from the glycerine, fatty acid, and soluble soap.

While considerable doubt exists as to the cause of these changes and as to
the places also where some of them occur, their purpose is quite apparent.
The materials forming the dissolved foods, although adapted to absorption,
are not suited to the needs of the body, and if introduced in this form
are likely to interfere with its work.(67) They are changed, therefore,
into the forms which the body can use.

*A Second Purpose of Digestion.*—Comparing the digestive changes with
those of absorption, it is found that they are of a directly opposite
nature; that while digestion is a process of tearing down, or
separating,—one which reduces the food to a more finely divided
condition—there is in absorption a process of building up. From the
comparatively simple compounds formed by digestion, there are formed
during absorption the more complex compounds of the blood. The one
exception is dextrose, which is a simple sugar; but even this is combined
in the liver and the muscles to form the more complex compound known as
glycogen. (See Methods of Storage, below.) These facts have suggested a
second purpose of digestion—that of reducing foods to forms sufficiently
simple to enable the body to construct out of them the more complex
materials that it needs. Evidence that digestion serves such a purpose is
found in the fact that both proteids and carbohydrates are reduced to a