AND PRACTICALLY.

When one takes into account the large number of muscles employed in
respiration, and remembers that these muscles must act in perfect
harmony with each other if the great end is to be attained, he
naturally inquires how this complex series of muscular contractions
has been brought into concerted action so as to result in that
physiological unity known as breathing.

It is impossible to conceive of such results being effected except
through the influence of the nervous system, which acts as a sort of
regulator throughout the whole economy. All the parts of the
respiratory tract are supplied with nerves, which are of both
kinds--those which carry nervous impulses or messages from and those
which convey them to the nervous centres concerned; in other words, to
and from the bodies of the nerve-cells whose extensions are termed
nerves. These centres are the central offices where the information is
received and from which orders are issued, so to speak.

The chief respiratory centre--_the_ centre--is situated in that
portion of the brain just above the spinal cord, in its continuation,
in fact, and is known as the _medulla oblongata_, or _bulb_. But
while this is the head centre, at which the ingoing (_afferent_)
impulses are received and from which the outgoing (_efferent_) ones
proceed, it makes use of many other collections of nerve-cells, or
subordinate centres--_e.g._, those whose nerve-extensions or
nerve-fibres proceed from the spinal cord to the muscles of
respiration.

[Illustration: FIG. 15. The purpose of this diagram is to indicate the
relation between ingoing (afferent) and outgoing (efferent) nervous
influences (impulses)--in other words, to illustrate _reflex action_.
The paths of the ingoing impulses are indicated by black lines, and
those of the outgoing ones by red lines, the point of termination
being shown by an arrow-tip. The result of an ingoing message may be
either favorable or unfavorable. The nervous impulse that reaches the
brain through the eye may be either exhilarating or depressing. The
experienced singer is usually stimulated by the sight of an audience,
while the beginner may be rendered nervous, and this may express
itself in many and widely distant parts of the body. An unfavorable
message may reach the diaphragm or intercostal muscles, and render
breathing shallow, irregular, or, in the worst cases, almost gasping.
The heart or stomach, even the muscles of the lary