h the portal vein to the liver, where it enters a
_second set of capillaries_ and is brought very near the liver cells. From
the liver it is passed through the hepatic veins into the inferior vena
cava, and by these it is emptied into the right auricle. This route then
includes the capillaries in the mucous membrane of the stomach and
intestines, the branches of the portal vein, the portal vein proper, the
liver, and the hepatic veins (Fig. 77). In passing through the liver, a
large portion of the food material is temporarily retained for a purpose
and in a manner to be described later (page 177).

*Absorption Changes.*--During digestion the insoluble foods are converted
into certain soluble materials, such as peptones, maltose, and
glycerine,--the conversion being necessary to their solution. A natural
supposition is that these materials enter and become a part of the blood,
but examination shows them to be absent from this liquid. (See Composition
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 combin