means of his sustenance.

*The Main Physiological Problems.*—The study of the body is thus seen to
resolve itself naturally into the consideration of two main problems:

1. _That of maintaining in the body a nutrient fluid for the cells._

2. _That of bringing the body into such relations with its surroundings as
will enable it to secure materials for the nutrient fluid and satisfy its
other needs._

The first problem is _internal_ and includes the so-called vital
processes, known as digestion, circulation, respiration, and excretion.
The second problem is _external_, as it were, and includes the work of the
external organs—the organs of motion and of locomotion and the organs of
special sense. These problems are closely related, since they are the two
divisions of the one problem of maintaining life. Neither can be
considered independently of the other. In the chapter following is taken
up the first of these problems.

*Summary.*—The individual parts, or units, that form the body organization
are known as cells. These consist of minute but definitely arranged
portions of protoplasm and are held together by the intercellular
material. They build up the body and carry on its different activities.
The tissues are groups of like cells. By certain general activities the
cells maintain their existence in the tissues and by the exercise of
certain special activities they adapt the tissues to their purposes in the
body. The body, as a cell organization, has its activities directed under
normal conditions toward a single purpose—that of maintaining life. In the
accomplishment of this purpose a nutrient fluid is provided for the cells