The unbroken curved line above the dotted ones indicates the position
of the diaphragm (only approximately, of course) after expiration.]

Attention will now be directed to some facts that it is of the utmost
importance to clearly understand, if one is to know how to breathe and
the reasons for the method employed. The lungs are contained in a
cavity the walls of which are made up of a domed muscular (and
tendinous) structure below, and elsewhere of bony and cartilaginous
tissues filled in with soft structures, chiefly muscles. This cage is
lined within by a smooth membrane which is kept constantly moist by
its own secretion. The lungs are covered by a similar membrane, both
of these fitting closely like the hand to a glove, so that there are
two smooth membranes in opposition. It cannot be too well remembered
that these two, the inner surface of the chest walls and the outer
surface of the lungs, are in the closest contact. This is so whatever
the changes that take place in the size and shape of the chest. The
lungs are concave below, and so fit accurately to the fleshy partition
between the chest and the abdomen which constitutes the lower boundary
of the chest, if we may use the term "chest" somewhat loosely. Above,
suiting the shape of the chest, the lungs are somewhat conical.

The pressure of the air tends of itself to expand the lungs, which are
highly elastic, even when one does not breathe at all. But if more
air is to enter there must be additional space provided; hence greater
expansion of the lungs can only follow an enlargement of the chest
cavity in one or in all directions. These are spoken of as
_diameters_. It follows that it is possible to conceive of the chest
being enlarged in three, and only three, directions; so that it may be
increased in size in its vertical, its transverse, and its
antero-posterior diameter, or diameter from before backwards.

This expansion, as in the case of all other movements, can be effected
only by muscles, or, to speak more accurately, by neuro-muscular
mechanisms. Exactly what muscles are employed may be learned from the
accompanying illustrations and by observation. While it is highly
important to know in a general way which muscles are chiefly
concerned, or, rather, where they are situated, it cannot be deemed
essential for every reader to learn their names, attachments, etc.,
down to the minutest details, as in the case of a student of anatomy
proper. The author d