The foregoing are described with the other muscles which attach the
thoracic limb to the trunk (p. 251).
4. =Serratus anticus= (M. serratus dorsalis inspiratorius).—This is a
thin quadrilateral muscle, named from its toothed ventral border. It
lies beneath the rhomboideus, serratus magnus, and latissimus dorsi.
_Origin._—The lumbo-dorsal fascia and dorso-scapular ligament.
_Insertion._—The external surfaces of the fifth to the eleventh or
twelfth ribs inclusive.
_Action._—To draw the ribs on which it is inserted forward and outward,
thus assisting in inspiration.
_Structure._—The muscle arises by means of a thin aponeurosis which
blends with the lumbo-dorsal fascia and the aponeurosis of the
latissimus dorsi. The muscle-fibers pass downward and backward to be
attached to the ribs by seven or eight digitations below the outer edge
of the transversalis costarum.
_Relations._—Superficially, the rhomboideus, serratus magnus, latissimus
dorsi, and serratus posticus; deeply, the longissimus dorsi,
transversalis costarum, external intercostal muscles, and the ribs.
_Blood-supply._—Intercostal arteries.
_Nerve-supply._—Thoracic nerves.
5. =Serratus posticus= (M. serratus dorsalis exspiratorius).—This muscle
resembles the preceding one, which it partly covers.
_Origin._—The lumbo-dorsal fascia.
_Insertion._—The outer surfaces of the last seven or eight ribs.
_Action._—To draw the ribs backward, thus assisting in expiration.
_Structure._—Similar to the preceding muscle. The fibers are directed
downward and forward and terminate in seven or eight digitations, one or
two of which cover the posterior teeth of the anticus. The aponeurosis
blends with that of the latissimus dorsi.
_Relations._—Superficially, the latissimus dorsi and external oblique;
deeply, the longissimus dorsi, transversalis costarum, external
intercostals, serratus anticus, and the ribs.
_Blood-supply._—Intercostal and lumbar arteries.
_Nerve-supply._—Thoracic nerves.