These consist of seven muscles or sets of muscles, which are attached to
the thoracic vertebræ, to the ribs and their cartilages, and to the
sternum. Functionally, they are muscles of respiration.
1. =Levatores costarum.=—These constitute a series of small muscles
which occupy and overlie the upper ends of the intercostal spaces.
_Origin._—The transverse processes of the thoracic vertebræ.
_Insertion._—The external surfaces of the upper ends of the ribs
posterior to the vertebral origin.
_Action._—To draw the ribs forward in inspiration.
_Structure._—Arising by tendinous fibers, each muscle passes backward
and outward and expands at its insertion. Some fibers pass over one rib
and are inserted on a succeeding one. At the first and last spaces the
muscle cannot be distinguished from the external intercostal, of which
it is in reality only a specially developed part.
_Relations._—Superficially, the longissimus dorsi; deeply, the ribs,
internal intercostal muscles, and the intercostal vessels and nerves.
_Blood-supply._—Intercostal arteries.
_Nerve-supply._—Intercostal nerves.
2. =External intercostals= (Mm. intercostales externi).—Each of these
occupies an intercostal space, from the levatores to the sternal
extremity of the rib. They do not occupy the intercartilaginous spaces.
_Origin._—The posterior borders of the ribs.
_Insertion._—The anterior borders and external surfaces of the
succeeding ribs.
_Action._—To draw the ribs forward in inspiration.
_Structure._—The fibers are directed downward and backward. There is a
considerable admixture of tendinous tissue. The thickness of the muscles
gradually diminishes toward the lower ends of the spaces.
[Illustration:
FIG. 184.—DORSAL AND LUMBAR REGIONS OF HORSE, DORSAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
_Relations._—Superficially, the serratus magnus, latissimus dorsi,
serratus anticus and posticus, longissimus dorsi, transversalis
costarum, rectus thoracis, deep pectorals, obliquus abdominis externus,
and panniculus; deeply, the internal intercostals and (in the upper part
of the spaces) the intercostal vessels and nerves.
_Blood-supply._—Intercostal and internal thoracic arteries.
_Nerve-supply._—Intercostal nerves.
3. =Internal intercostals= (Mm. intercostales interni).—These occupy the
entire length of the intercostal spaces, including their interchondral
portion.
_Origin._—The anterior borders of the ribs and their cartilages.
_Insertion._—The posterior borders of the preceding ribs and cartilages.
_Action._—To draw the ribs backward in expiration. It seems probable,
however, that the intercartilaginous portion is inspiratory.[47]
_Structure._—The direction of the fibers is oblique downward and
forward. There is a smaller amount of tendinous tissue than in the
external set, and the thickness diminishes from below upward. In the
upper part of the spaces fibers sometimes cross a rib in a fashion
similar to the subcostals of man. A thin aponeurosis separates the
internal from the external intercostal muscle in each space.
_Relations._—Superficially, the levatores costarum and the external
intercostals; deeply, the endothoracic fascia and pleura, the
transversus thoracis, diaphragm, transversus abdominis, and the internal
thoracic and asternal vessels. In the upper part of the intercostal
spaces the intercostal vessels and nerves lie between the internal and
external intercostal muscle, but below they lie chiefly on the deep face
of the inner muscle.
_Blood-supply._—Intercostal and internal thoracic arteries.
_Nerve-supply._—Intercostal nerves.
4. =Retractor costæ.=—This is a small triangular muscle which lies
behind the last rib, chiefly under cover of the serratus posticus.
_Origin._—The transverse processes of the first three or four lumbar
vertebræ by means of the lumbar fascia.
_Insertion._—The posterior border of the last rib.
_Action._—To retract the last rib.
_Structure._—The muscle arises by a thin aponeurosis. Its fibers are
parallel to those of the adjacent internal oblique.
_Relations._—Superficially, the serratus posticus and external oblique;
deeply, the transversus abdominis.
_Blood-supply._—Lumbar arteries.
_Nerve-supply._—Lumbar nerves.
5. =Rectus thoracis= (M. transversus costarum; lateralis sterni).—This
is a thin muscle which lies under cover of the deep pectoral muscles. It
is directed obliquely backward and downward, and crosses the lower part
of the first three intercostal spaces.
_Origin._—The outer surface of the first rib, below the scalenus.
_Insertion._—The cartilage of the third or fourth rib. The aponeurosis
usually joins the rectus abdominis. It may reach the sternum.
_Action._—It may assist in inspiration or concur with the rectus
abdominis.
_Relations._—Superficially, the deep pectoral muscles; deeply, the
intercostal muscles and the ribs.
6. =Transversus thoracis= (Triangularis sterni).—This is a flat muscle
situated on the thoracic surface of the sternum and the cartilages of
the sternal ribs.
_Origin._—The sternal ligament.
_Insertion._—The cartilages of the ribs, from the second to the eighth
inclusive.
_Action._—It draws the costal cartilages inward and backward, thus
assisting in expiration.
_Structure._—Each muscle has the form of a scalene triangle, of which
the base is the strongly serrated external border. The muscle contains a
good deal of tendinous tissue. The anterior bundles are directed forward
and outward; the posterior backward and outward.
_Relations._—Superiorly, the endothoracic fascia and pleura; inferiorly,
the costal cartilages, the internal intercostal muscles, and the
internal thoracic vessels.
_Blood-supply._—Internal thoracic artery.
_Nerve-supply._—The intercostal nerves.
7. =Diaphragm.=—This is a broad, unpaired muscle which forms a partition
between the thoracic and abdominal cavities.[48] In outline it has some
resemblance to a palm-leaf fan. In form it is dome-shaped, compressed
laterally. On a median section it is seen to have a general direction
downward and forward from the lumbar vertebræ to the xiphoid cartilage.
The thoracic surface is strongly convex, and is covered by the pleura.
The abdominal surface is deeply concave, and is covered for the most
part by the peritoneum. The muscle consists of a peripheral fleshy
portion, two muscular crura, and a tendinous center.
_Attachments._—(1) =Costal part=: The cartilages of the ninth to the
fifteenth ribs, and the last three ribs at an increasing distance from
their sternal ends.
(2) =Sternal part=: The upper surface of the xiphoid cartilage.
(3) =Lumbar part=: (_a_) The =right crus= is attached to the inferior
common ligament, and by this means to the first four or five lumbar
vertebræ. (_b_) The =left crus= is attached in a similar fashion to the
first and second lumbar vertebræ.
_Action._—It is the principal muscle of inspiration and increases the
longitudinal diameter of the chest. The contraction produces a general
lessening of the curvature of the diaphragm. In the expiratory phase the
costal part and crura lie directly on the body walls, so that the bases
of the lungs are in contact with the tendinous center and sternal
portion only. In ordinary inspiration the fleshy rim recedes from the
chest-wall, so that the bases of the lungs move backward to a line about
parallel with the curve formed by the cartilages of the asternal ribs,
and about four or five inches (ca. 10 to 12 cm.) therefrom. It is stated
that the inspiratory movement affects the tendinous center much less
than the fleshy part, since the posterior vena cava is firmly attached
to the former. It should be noted, however, that the direction of the
thoracic part of the vena cava in the expiratory phase is oblique upward
and backward. Thus it would seem that there is no anatomical reason why
the diaphragm should not move as a whole in ordinary inspiration at
least.
_Structure._—The =costal part= (Pars costalis) consists of a series of
digitations which meet, or are separated by a very narrow interval from,
the transversus abdominis; between the two are the asternal vessels.
From the tenth rib backward the attachments are to the ribs at an
increasing distance above the costo-chondral junctions. Thus at the last
rib the attachment is four to five inches (10 to 12 cm.) from the lower
end. Anteriorly, the origin extends along the ninth costal cartilage to
the xiphoid cartilage. From these points of origin the fibers curve
inward and forward to join the tendinous center. The =right crus= (Crus
dextrum) is about twice as thick as the left one and is also longer. It
arises by a strong tendon from the lumbar vertebræ (by means of the
inferior common ligament). The tendon is succeeded by a rounded belly
which leaves the vertebral column, at the last thoracic vertebra.
Passing downward and forward, its fibers spread out and join the
tendinous center. The =left crus= (Crus sinistrum) arises by a thin
tendon from the inferior common ligament at the first and second lumbar
vertebræ. This is succeeded by a triangular belly which joins the
central tendon. Between the crura and the attachment to the last rib the
edge of the muscle crosses the ventral surface of the psoas muscles
without attachment, forming the so-called lumbo-costal arch; here the
thoracic and abdominal cavities are separated only by the serous
membranes and some areolar tissue. The =tendinous center= (Centrum
tendineum) resembles the periphery in outline, but is more elongated. It
is partially divided into right and left halves by the descent of the
crura into it. It is composed largely of radiating fibers, but many
interlace in various directions; this is specially evident around the
foramen venæ cavæ, which is encircled by fibers. A strong tendinous
layer extends across below the hiatus œsophageus.
[Illustration: