6. =Transversalis costarum= (M. ilio-costalis).—This long, segmental
muscle extends, as its name indicates, across the series of ribs, in
contact with the outer edge of the longissimus dorsi.
_Origin._—(1) The transverse processes of the second and third lumbar
vertebræ and the deep layer of the lumbo-dorsal fascia. (2) The anterior
borders and external surface of the last fifteen ribs.[45]
_Insertion._—The posterior borders of the ribs and the transverse
processes of the last two or three cervical vertebræ.
_Action._—Chiefly to depress and retract the ribs and so help in
expiration. Acting together, they may assist in extending this spine,
acting singly in inclining it laterally.
_Structure._—This muscle presents a distinct segmental arrangement. It
is composed of a series of bundles, the fibers of which are directed
forward and a little downward and outward. From these are detached two
sets of tendons. The superficial tendons spring from the outer edge of
the muscle. They are flat and are about half an inch in width. Each
crosses two or three intercostal spaces, to be inserted on the posterior
border of a rib. The deep tendons are detached from the dorsal part of
the deep face of the muscle. Each passes backward across one or two
intercostal spaces to its origin on the anterior border or outer surface
of a rib. Small bursæ may be found between the ribs and tendons.
_Relations._—Superficially, the dorsal serrati and the complexus;
deeply, the external intercostals and the ribs. The lumbar origin is
covered by the longissimus. The superior (deep) cervical and dorsal
vessels cross the surface of the muscle at the first and second
intercostal spaces respectively, and branches of the intercostal vessels
and nerves emerge between the transversalis and longissimus; here a
fascial layer dips in between the two.
_Blood-supply._—Intercostal arteries.
_Nerve-supply._—Dorsal branches of the thoracic nerves.
7. =Longissimus.=[46]—This is the largest and longest muscle in the
body. It extends from the sacrum and ilium to the neck, filling up the
space between the spinous processes medially and the lumbar transverse
processes and the upper ends of the ribs below; consequently it has the
form of a three-sided prism.
_Origin._—(1) The internal angle, crest, and adjacent part of the
ventral surface of the ilium; (2) the first three sacral spines; (3) the
lumbar and thoracic spines and the supraspinous ligament.
_Insertion._—(1) The lumbar transverse and articular processes; (2) the
thoracic transverse processes; (3) the spinous and transverse processes
of the last four cervical vertebræ; (4) the outer surfaces of the ribs,
except the first.
_Action._—Acting with its fellow, it is the most powerful extensor of
the back and loins; by its cervical attachment it assists in extending
the neck. By its costal attachment it may also assist in expiration.
Acting singly, it flexes the spine laterally.
[Illustration:
FIG. 181.—RIGHT PORTION OF CROSS-SECTION OF BACK OF HORSE. SECTION IS
CUT THROUGH SEVENTH THORACIC VERTEBRA.
_a_, Ligamentum nuchæ; _b_, trapezius muscle; _c_, cartilage of
scapula; _d_, latissimus dorsi; _e_, panniculus carnosus; _f_,
rhomboideus dorsi; _g_, serratus magnus; _h_, serratus anticus;
_h′_, lumbo-dorsal fascia, which divides below into three layers;
_i_, transversalis costarum; _k_, levator costæ; _k′_, internal
intercostal muscle; _l_, longissimus dorsi; _m_, _m′_, seventh
thoracic vertebra; _n_, head of eighth thoracic vertebra; _o_, head
of eighth rib; _p_, seventh rib; _r_, intercostal artery and nerve;
_s_, skin. The fasciæ are indicated by dotted lines. (After
Ellenberger, in Leisering’s Atlas.)
]
_Structure._—This is quite complex. The posterior part of the muscle is
greatly developed and constitutes the common mass of the loins. This is
covered by a strong aponeurosis which blends with the supraspinous and
sacro-iliac ligaments, and is attached to the crest and inner (sacral)
angle of the ilium and the first and second sacral spines; it furnishes
origin to the lumbar portion of the middle gluteus. In its course
further forward the muscle receives fasciculi from the lumbar and
thoracic spines, but diminishes somewhat in volume. At the withers it
divides into two parts. The dorsal division (spinalis et semispinalis),
reinforced by bundles from the first four thoracic spines, passes
forward under the complexus to be inserted into the spines of the last
four cervical vertebræ. The ventral division passes forward and downward
underneath the serratus magnus to be inserted into the ribs and the
transverse processes of the last four cervical vertebræ. Three sets of
fasciculi may be distinguished, viz.: (1) spinal, which are superficial
and internal; (2) transverse, attached to the transverse and articular
processes, which are internal and deep; (3) costal, which are external.
_Relations._—Superficially, the middle gluteus, the lumbo-dorsal fascia,
the latissimus dorsi, dorsal serrati, serratus magnus, and complexus;
deeply, the multifidus, intertransversales, external intercostals,
levatores costarum, the ligamentum nuchæ, and its fellow of the opposite
side (in the neck).
_Blood-supply._—Dorsal, superior cervical, intercostal, and lumbar
arteries.
_Nerve-supply._—Dorsal branches of the thoracic and lumbar nerves.
8. =Multifidus dorsi= (semispinalis of the back and loins).—This is a
long segmental muscle which covers the sides of the spinous processes of
the vertebræ from the sacrum to the neck.
_Origin._—(1) The lateral portion of the sacrum; (2) the articular
processes of the lumbar vertebræ; (3) the transverse processes of the
thoracic vertebræ.
_Insertion._—The spinous processes of the first two sacral, the lumbar,
thoracic, and last cervical vertebræ.
_Action._—Acting with its fellow, it is an extensor of the spine; acting
singly, it flexes it laterally.
_Structure._—It is composed of a series of bundles which are directed
obliquely forward and upward. Each fasciculus passes over several
vertebræ to its insertion. In the posterior part of the series the
bundles cross two or three vertebræ and are inserted into the summits of
the spines. Anteriorly, the bundles have a more horizontal direction,
cross three to five vertebræ, and are inserted into the sides of the
spines considerably below their summits. A further complication consists
in the fusion of several bundles into a common insertion.
_Relations._—Superficially, the longissimus; deeply, the vertebral
spines.
_Blood-supply._—Intercostal and lumbar arteries.
_Nerve-supply._—Dorsal branches of the thoracic and lumbar nerves.
9. =Intertransversales lumborum.=—These are very thin muscular and
tendinous strata, which occupy the spaces between the transverse
processes of the lumbar vertebræ except the fifth and sixth.
_Action._—To assist in flexing the loins laterally.
_Relations._—Superficially, the longissimus; deeply, the quadratus
lumborum.
_Blood-supply._—Lumbar arteries.
_Nerve-supply._—Lumbar nerves.