The =phrenic nerve= (N. phrenicus) (Figs. 428, 429, 433), the motor
nerve to the diaphragm, is formed by the union of two or three roots
which cross the superficial face of the scalenus muscle obliquely
downward and backward. The chief roots come from the ventral branches of
the sixth and seventh cervical nerves. The root derived from the fifth
nerve is small and inconstant. The root from the seventh cervical comes
by way of the brachial plexus. The course of the nerve is not the same
on both sides. On the =right side= the nerve enters the thorax by
passing between the brachial artery and the anterior vena cava. It then
courses backward and somewhat downward over the right face of the
anterior vena cava, crosses the pericardium, and continues along the
posterior vena cava to the diaphragm. In the latter part of its course
it is inclosed in a special fold of the right pleura and inclines
gradually to the ventral face of the vein. On the =left side= the nerve
enters the thorax between the left brachial and inferior cervical
arteries, and then runs its entire course in the mediastinum. In the
anterior mediastinum it lies along the brachiocephalic artery ventral to
the left vagus and cardiac nerves, and crosses over the dorso-cervical
vein. It then passes over the upper part of the pericardium and runs
backward in the posterior mediastinum to reach the tendinous center of
the diaphragm considerably to the left of the median plane. Each nerve
is usually connected near its origin with the first thoracic ganglion of
the sympathetic by a ramus communicans, and each terminates by dividing
into several branches which are distributed to the corresponding part of
the diaphragm.