the body are called _essential_ properties. The most important of these
essential properties are as follows:

1. Of osseous tissue, hardness, stiffness, and toughness. 2. Of muscular
tissue, contractility and irritability. 3. Of nervous tissue, irritability
and conductivity. 4. Of cartilaginous tissue, stiffness and elasticity. 5.
Of connective tissue, toughness and pliability. 6. Of epithelial tissue,
ability to resist the action of external forces and power to secrete.

[Fig. 1]


Fig. 1—Hand and forearm, showing the grouping of muscular and connective
tissues in the organ for grasping.


*Tissue Groups.*—In the construction of the body the tissues are grouped
together to form its various divisions or parts. A group of tissues which
serves some special purpose is known as an _organ_. The hand, for example,
is an organ for grasping (Fig. 1). While the different organs of the body
do not always contain the same tissues, and never contain them in the same
proportions, they do contain such tissues as their work requires and these
have a special arrangement—one adapted to the work which the organs
perform.

In addition to forming the organs, the tissues are also grouped in such a
manner as to provide supports for organs and to form cavities in which
organs are placed. The various cavities of the body are of particular
interest and importance. The three largest ones are the _cranial_ cavity,
containing the brain; the _thoracic_ cavity, containing the heart and the