be mounted in the usual way, but with
_hard_ balsam. Prepare and study both transverse and longitudinal
sections, making drawings. The sections should be prepared from bones that
are thoroughly dry but which have not begun to decay.

*To show the Structure of a Joint.*--Procure from a butcher the joint of
some small animal (hog or sheep). Cut it open and locate the cartilage,
synovial membrane, and ligaments. Observe the shape and surface of the
rubbing parts and the strength of the ligaments.

CHAPTER XV - THE MUSCULAR SYSTEM

As already stated, the skeleton, the nervous system, and the muscular
system are concerned in the production of motion. The skeleton and the
nervous system, however, serve other purposes in the body, while the
muscular system is devoted exclusively to the production of motion. For
this reason it is looked upon as the special _motor_ system. The muscular
tissue is the most abundant of all the tissues, forming about 41 per cent
of the weight of the body.

*Properties of Muscles.*--The ability of muscular tissue to produce motion
depends primarily upon two properties--the property of irritability and the
property of contractility. _Irritability_ is that property of a substance
which enables it to respond to a stimulus, or to act when acted upon.
_Contractility_ is the property which enables the muscle when stimulated
to draw up, thereby becoming shorter and thicker (a condition called
contraction), and when the stimulation ceases, to return to its former
condition (of relaxation). The property of contractility enables the
muscles to produce motion. Irritability is a condition necessary to their
control in the body.

*Kinds of Muscular Tissue.*--Three kinds of muscular tissue are found in
the body. These are known as the _striated_, or striped, muscular tissue;
the _non-striated_, or plain, muscular tissue; and the _muscular tissue of
the heart_. These are made up of different kinds of muscle cells and act
in different ways to cause motion. The striated muscular tissue far
exceeds the others in amount and forms all those muscles that can be felt
from the surface of the body. The non-striated muscle is found in the
walls of the food canal, blood vessels, air passages, and other tubes of
the body; while the muscular tissue of the heart is confined entirely to
that organ.

*Striated Muscle Cells.*--The cells of the striated muscles are slender,
thread-like structures, having an average len