jointed behind to the vertebrae in such a way that muscles attached to them
can, by shortening, elevate them; the effect is that the longer ribs are
raised, and pushing forward the breastbone and cartilages, the thoracic
cage enlarges from before back; but being elastic, the hoops will give a
little and cause some expansion from side to side; moreover, when the ribs
are raised, each one is rotated on its axis in such a way that the lower
border tends towards eversion; the total effect of this rotation is a
lateral expansion of the whole thorax. Between the ribs and the cartilages
the space is filled by the intercostal muscles (_vide_ fig. 2), the action
of which, in conjunction with other muscles, is to elevate the ribs. It is,
however, unnecessary to enter into anatomical details, and describe all
those muscles which elevate and rotate the ribs, and thereby cause
enlargement of the thorax in its antero-posterior and lateral diameters.
There is, however, one muscle which forms the floor of the thoracic cage
called the diaphragm that requires more than a passing notice (_vide_ fig.
2), inasmuch as it is the most effective agent in the expansion of the
chest. It consists of a central tendinous portion, above which lies the
heart, contained in its bag or pericardium; on either side attached to the
central tendon on the one hand and to the spine behind, to the last rib
laterally, and to the cartilages of the lowest six ribs anteriorly, is a
sheet of muscle fibres which form on either side of the chest a dome-like
partition between the lungs and the abdominal cavity (_vide_ fig. 2). The
phrenic nerve arises from the spinal cord in the upper cervical region and
descends through the neck and chest to the diaphragm; it is therefore a
special nerve of respiration. There are two--one on each side supplying the
two sheets of muscle fibres. When innervation currents flow down these
nerves the two muscular halves of the diaphragm contract, and the floor of
the chest on either side descends; thus the vertical diameter increases.
Now the elastic lungs are covered with a smooth pleura which is reflected
from them on to the inner side of the wall of the thorax, leaving no space