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
and proper relations between the body and its surroundings are
established.

*Exercises.*--1. If a tissue be compared to a brick wall, to what do the
separate bricks correspond? To what the mortar between the bricks?

2. Draw an outline of a typical cell, locating and naming the main
divisions.

3. How do the cells enable the body to grow? Describe the process of
cell-division.

4. How does the general work of cells differ from their special work?
Define absorption, excretion, and assimilation as applied to the cells.

5. Compare the conditions surrounding a one-celled animal, living in
water, to the conditions surrounding the cells in the body.

6. What is meant by the term "environment"? How does man's environment
differ from that of a fish?

7. What is the necessity for a nutrient fluid in the body?

8. Why is the maintenance of life necessarily the chief aim of all the
activities of the body?

9. State the two main problems in the study of the body.

PRACTICAL WORK

*Observations.*--1. Make some scrapings from the inside of the cheek with a
dull knife and mix these with a little water on a glass slide. Place a
cover-glass on the same and examine with a compound microscope. The large
pale cells that can be seen in this way are a variety of epithelial cells.

2. Mount in water on a glass slide some thin slices of cartilage and
examine fi