_Absorption_ is the process of taking water, food, and oxygen into the
cells.

_Assimilation_ is a complex process which results in the addition of the
absorbed materials to the protoplasm. Through assimilation the protoplasm
is built up or renewed.

_Excretion_ is the throwing off of such waste materials as have been
formed in the cells. These are passed into the lymph and thence to the
surface of the body.

Absorption, assimilation, excretion, and also reproduction are performed
by all classes of cells. They are, on this account, referred to as the
_general work of cells_.

*The Special Work of Cells.*—In addition to the general work which all
cells do in common, each class of cells in the body is able to do some
particular kind of work—a work which the others cannot do or which they
can do only to a limited extent. This is spoken of as the _special work of
cells_. Examples of the special work of cells are found in the production
of motion by muscle cells and in the secretion of liquids by gland cells.
It may be noted that while the general work of cells benefits them
individually, their special work benefits the body as a whole. Another
example of the special work of cells is found in the

[Fig. 7]


Fig. 7—Cartilage cells, surrounded by the intercellular material which