dition to be easily separated one from another. Thus a piece of
muscle contains all the various organic compounds just mentioned, but they
are combined, and in different cases the amount will vary. Again, fat may
exist in the muscles even though it is not visible to the naked eye, and a
microscope is required to show the minute fat cells.

10. Protoplasm. The ultimate elements of which the body is composed
consist of "masses of living matter," microscopic in size, of a material
commonly called protoplasm.[2] In its simplest form protoplasm
appears to be a homogeneous, structureless material, somewhat resembling
the raw white of an egg. It is a mixture of several chemical substances
and differs in appearance and composition in different parts of the body.

Protoplasm has the power of appropriating nutrient material, of dividing
and subdividing, so as to form new masses like itself. When not built into
a tissue, it has the power of changing its shape and of moving from place
to place, by means of the delicate processes which it puts forth. Now,
while there are found in the lowest realm of animal life, organisms like
the amoeba of stagnant pools, consisting of nothing more than minute
masses of protoplasm, there are others like them which possess a small
central body called a nucleus. This is known as nucleated protoplasm.

[Illustration: Fig. 1.--Diagram of a Cell.

A, nucleus;
B, nucleolus;
C, protoplasm. (Highly magnified)
]

11. Cells. When we carry back the analysis of an organized body as
far as we can, we find every part of it made up of masses of nucleated
protoplasm of various sizes and shapes. In all essential features these
masses conform to the type of protoplasmic matter just described. Such
bodies are called cells. In many cells the nucleus is finely granular or
reticulated in appearance, and on the threads of the meshwork may be one
or more enlargements, called nucleoli. In some cases the protoplasm at the
circumference is so modified as to give the appearance of a limiting
membrane called the cell wall. In brief, then, a cell is a mass of
nucleated protoplasm; the nucleus may have a nucleolus, and the cell
may be limited by a cell wall. Every tissue of the human body is formed
through the agency of protoplasmic cells, although in most cases the
changes they undergo are so great that little evidence remains of their
existence.

There are some organisms lower down in the scale, whose whole activi