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 combined action of the mouth, stomach,
liver, intestines, and other organs. A number of organs working together
for the same purpose form a _system_. The chief systems of the body are
the digestive system, the circulatory system, the respiratory system, the
muscular system, and the nervous system.

*The Organ and its Work.*—A most interesting question relating to the work
of the organ is this: Does the organ work for its own benefit or for the