ility 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
lungs; and the _abdominal_ cavity, containing the stomach, the liver, the
intestines, and other important organs (Fig. 2). Smaller cavities serving
different purposes are also found.

[Fig. 2]

Fig. 2--Diagram of a lengthwise section of the body to show its large
cavities and the organs which they contain.

*Organs and Systems.*--The work of the body is carried on by its various
organs. Many, in fact the majority, of these organs serve more than one
purpose. The tongue is used in talking, in masticating the food, and in
swallowing. The nose serves at least three distinct purposes. The mouth,
the arms, the hands, the feet, the legs, the liver, the lungs, and the
stomach are also organs that serve more than one purpose. This introduces
the principle of economy into the construction of the body and diminishes
the number of organs that would otherwise be required.

The various organs also _combine_ with one another in carrying on the work
of the body. An illustration of this is seen in the digestion of the
food--a process which requires the com