s for cell life are provided in the body.

*The General Work of Cells.*--In handling the materials derived from the
lymph, the cells carry on three well-defined processes, known as
absorption, assimilation, and excretion.

_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
they have deposited.

*Production of the Intercellular Material.*--Though most of the cells of
the body deposit to a slight extent this material, the greater part of it
is produced by a single class of cells found in bone, cartilage, and
connective tissue. Cartilage, bone, and connective tissue differ greatly
from the other tissues in the amount of intercellular material which they
contain, the difference being due to these cells. In the connective tissue
they deposit the fibrous material so important in holding the different
parts of the body together. In the cartilage they produce the gristly
substance which forms by far its larger portion (Fig. 7). In the bones
they deposit a material similar to that in the cartilage, except that with
it is mixed a mineral substance which gives the bones their hardness and
stiffness.(4) The intercellular material, in ad