acid reaction takes place here, and they
acquire the usual faecal smell and color, which increases as they
approach the rectum. Some physiologists have supposed that a second
digestion takes place in the upper portion of the large intestine. The
lacteals, filled with chyle, pass into the mesenteric glands with which
they freely unite, and afterward enter the _receptaculum chyli_, which
is the commencement of the thoracic duct, a tube of the size of a
goose-quill, which lies in front of the backbone. The lymphatics, the
function of which is to secrete and elaborate lymph, also terminate in
the _receptaculum chyli_, or receptacle for the chyle. From this
reservoir the chyle and lymph flow into the thoracic duct, through which
they are conveyed to the left subclavian vein, there to be mingled with
venous blood. The blood, chyle, and lymph, are then transmitted directly
to the lungs.

The process of nutrition aids in the development and growth of the body;
hence it has been aptly designated a "perpetual reproduction." It is the
process by which every part of the body assimilates portions of the
blood distributed to it. In return, the tissues yield a portion of the
material which was once a component part of their organization. The body
is constantly undergoing waste as well as repair. One of the most
interesting facts in regard to the process of nutrition in animals and
plants is, that all tissues originate in cells. In the higher types of
animals, the blood is the source from which the cells derive their
constituents. Although the alimentary canal is more or less complicated
in different classes of animals, yet there is no species, however low in
the scale of organization, which does not possess it in some form.[2]
The little polyp has only one digestive cavity, which is a pouch in the
interior of the body. In some animals circulation is not distinct from
digestion, in others respiration and digestion are performed by the same
organs; but as we rise in the scale of animal life, digestion and
circulation are accomplished in separate cavities, and the functions of
nutrition become more complex and distinct.

* * * * *

CHAPTER V.

PHYSIOLOGICAL ANATOMY.

ABSORPTION.

[Illustration: Fig. 35.
Villi of the small intestine greatly magnified.]

_Absorption_ is the vital function by which nutritive materials are
selected and imbibed for the sustenance of the body. Absorption, like