cal environment and serves various purposes.
The process by which gaseous materials are made to pass between the body
and the atmosphere is known as

*Respiration.*--As usually defined, respiration, or breathing, consists of
two simple processes--that of taking air into special contrivances in the
body, called the lungs, and that of expelling air from the lungs. The
first process is known as _inspiration_; the second as _expiration_. We
must, however, distinguish between respiration by the lungs, called
_external respiration_, and respiration by the cells, called _internal
respiration_.

_The purpose of respiration_ is indicated by the changes that take place
in the air while it is in the lungs. Air entering the lungs in ordinary
breathing parts with about five per cent of itself in the form of oxygen
and receives about four and one half per cent of carbon dioxide,
considerable water-vapor, and a small amount of other impurities. These
changes suggest a twofold purpose for respiration:

1. To obtain from the atmosphere the supply of oxygen needed by the body.

2. To transfer to the atmosphere certain materials (wastes) which must be
removed from the body.

The chief organs concerned in the work of respiration are

*The Lungs.*--The lungs consist of two sac-like bodies suspended in the
thoracic cavity, and occupying all the space not taken up by the heart.
They are not simple sacs, however, but are separated into numerous
divisions, as follows:

1. The lung on the right side of the thorax, called the right lung, is
made up of three divisions, or _lobes_, and the left lung is made up of
two lobes.

2. The lobes on either side are separated into smaller divisions, called
_lobules_ (Fig. 33). Each lobule receives a distinct division of an air
tube and has in itself the structure of a miniature lung.

[Fig. 33]

Fig. 33--*Lungs and air passages* seen from the front. The right lung shows
the lobes and their divisions, the lobules. The tissue of the left lung
has been dissected away to show the air tubes.

3. In the lobule the air tube divides into a number of smaller tubes, each
ending in a thin-walled sac, called an _infundibulum_. The interior of the
infundibulum is separated into many small spaces, known as the _alveoli_,
or air cells.

The lungs are remarkable for their lightness and delicacy of
structure.(30) They consist chiefly of the tissues that form th