=M. serratus posterior superior= (Fig. 73, _l_).--A thin sheet of muscle
and tendon beneath the serratus anterior (_i_) on the dorsal part of the
thorax and neck.
_Origin_ by fleshy slips from the outer surfaces of the first nine ribs
just ventrad of their angles. The origin may extend as far as the tenth
or eleventh ribs. The first slip is usually delicate. The fibres are
directed dorsad and craniad and unite to form a continuous sheet. This
ends along a longitudinal line opposite the vertebral transverse
processes in the aponeurosis which covers the longissimus dorsi and
other muscles in this region. Its insertion is into the median dorsal
raphe between the axial spinous process and the tenth thoracic spinous
process.
_Relations._--Outer surface with the serratus anterior (_i_), the
latissimus dorsi (Fig. 68, _m_), and the rhomboideus. Inner surface with
the external intercostals (Fig. 73, _m_), the iliocostal (Fig. 69, _h_),
the longissimus dorsi (Fig. 69, _f_), the spinalis dorsi (Fig. 69, _g_),
and the splenius (Fig. 73, _b_).
_Action._--Draws the ribs craniad.
=M. serratus posterior inferior= (Fig. 73, _n_).--The serratus posterior
inferior is a thin muscle lying caudad of the serratus posterior
superior (_l_) and sometimes overlying the caudal end of the latter.
_Origin_ by four or five heads from the last four or five ribs. In some
specimens only three heads may be present. The separate bundles pass
dorsocaudad and unite to form a continuous sheet which ends in a
continuation of the aponeurosis of the serratus posterior superior
(_l_).
_Insertion_ into the lumbar spinous processes and the intervening
interspinous ligaments.
_Relations._--Outer surface with the latissimus dorsi (Fig. 68, _p_),
and at the origins with the intercostales externi (Fig. 73, _m_). Inner
surface with the intercostales interni (Fig. 69, _k_) at the origin and
with the intercostales externi (Fig. 69, _i_), iliocostal (Fig. 69,
_h_), longissimus dorsi (Fig. 69, _f_), and the caudal end of the
spinalis dorsi (Fig. 69, _g_).
[Illustration: FIG. 73.--BODY MUSCLES EXPOSED AFTER REMOVAL OF THE
FORELIMB AND THE MUSCLES SHOWN IN FIG. 68.
_a_, M. occipitoscapularis (cut); _b_, M. splenius; _c_, M. levator
scapulæ ventralis (cut); _d_, M. cleidomastoideus (cut); _e_, M.
longus capitis; _f_, M. scalenus (_f′_, middle division; _f″_, dorsal
division; _f‴_, ventral division; _f_, cervical portion); _g_, M.
longissimus capitis; _h_, M. levator scapulæ (cut); _i_, M. serratus
anterior (cut); _j_, M. transversus costarum; _k_, M. rectus abdominis
(_k′_, its tendon); _l_, M. serratus posterior superior; _m_, M.
intercostales externi; _n_, M. serratus posterior inferior; _o_, M.
obliquus abdominis internus; _p_, lumbodorsal fascia.]
_Action._--Assists the serratus posterior superior.
=M. transversus costarum=, or =sternocostalis externus= (Fig. 73,
_j_).--This is a thin muscle applied to the cranial part of the side of
the thorax, covering the cranial end of the rectus abdominis (_k_).
_Origin_ by tendon from the side of the sternum between the attachments
of the third and sixth ribs. The muscle passes dorsocraniad to its
_Insertion_ on the first rib and the lateral portion of its costal
cartilage.
_Relations._--Outer surface with the pectoralis minor (Fig. 65, _o_).
Inner surface with the scalenus (Fig. 73, _f‴_), the rectus abdominis
(Fig. 73, _k_), and the intercostales externi (Fig. 73, _m_). At its
insertion it is united with a portion of the scalenus (Fig. 73, _f‴_).
_Action._--Draws the sternum forward.
=Mm. levatores costarum.=--Small muscles having _origin_ on the
transverse processes of the thoracic vertebræ, passing caudoventrad, and
becoming _inserted_ on the angle of the rib lying immediately caudad of
the origin. They are continuous with the external intercostals.
_Relations._--Outer surface with the longissimus dorsi (Fig. 69, _f_),
and craniad with the iliocostal (Fig. 69, _h_). Inner surface with the
internal intercostals (Fig. 69, _k_).
_Action._--Pull the ribs dorsocraniad.
=Mm. intercostales externi= (Fig. 73, _m_; Fig. 69, _i_).--The external
intercostals are placed in the outer portion of the intercostal spaces.
They are composed of bundles of fibres attached by their ends to the
adjacent borders of the ribs and having in general the direction of the
external oblique muscle, i.e., they pass from their cranial ends
caudoventrad. They occupy the intercostal spaces between the true ribs
and extend even caudad into the spaces between the false ribs. They are
lacking between the ventral ends of the costal cartilages of the first
six to eight ribs, so that the internal intercostals (Fig. 69, _k_) are
here exposed. The more caudal external intercostals are more nearly
craniocaudal in direction.
_Relations._--Outer surface with obliquus abdominis externus (Fig. 68,
_p_), latissimus dorsi (Fig. 68, _m_), serratus posterior inferior (Fig.
73, _n_) and superior (Fig. 73, _l_), serratus anterior (Fig. 73, _i_),
scalenus (Fig. 73, _f_), and iliocostal (Fig. 69, _h_). Inner surface
with the internal intercostals (Fig. 69, _k_).
_Action._--Protractors of the ribs.
=Mm. intercostales interni= (Fig. 69, _k_).--The internal intercostals
are similar to the external intercostals, beneath which they lie. Their
fibres pass between the ribs at nearly right angles to those of the
external intercostals and have nearly the direction of the fibres of the
internal oblique. They occupy all the intercostal spaces from the first
to the thirteenth ribs.
_Relations._--Outer surface with the external intercostals (Fig. 69,
_i_), and ventrad with the scalenus (Fig. 73, _f_), transversus costarum
(Fig. 73, _j_), and rectus abdominis (Fig. 73, _k_). Inner surface with
the pleura and the transversus thoracis.
_Action._--Retractors of the ribs.
=M. transversus thoracis= (triangularis sterni; sternocostalis
internus).--This represents a thoracic portion of the transversus
abdominis. It consists of five or six flat muscular bands lying on the
inner surface of the thoracic wall.
_Origin_ on the lateral borders of the dorsal face of the sternum,
opposite the attachments of the cartilages of the third to the eighth
ribs. The six bands thus formed, each about one centimeter wide, pass
laterad and are _inserted_ into the cartilages of the ribs near their
junction with the ribs, and into the fascia which covers the inner
surface of the internal intercostals in this region.
_Relations._--Outer surface with the internal intercostals and the
cartilages of the ribs. Inner surface with the pleura.
=Diaphragma= (Fig. 74).--The diaphragm consists of a central so-called
=semilunar= tendon (_e_) and of muscular fibres which pass radially from
the body wall to the tendon. It forms a complete oblique partition
between the abdominal and thoracic cavities. The dorsal end is farther
caudad than the ventral.
The central tendon (_e_) is thin and irregularly crescent-shaped, with
the convexity ventrad and the horns of the crescent prolonged as two
tendinous bands (_e′_) which end in two triangular =membranous= portions
(_d_) of the diaphragm, one on each side of the spinal column. It is
pierced by an opening for the vena cava (_f_).
[Illustration: FIG. 74.--DIAPHRAGM, CAUDAL SURFACE SEEN OBLIQUELY FROM
THE RIGHT.
_a_, right crus; _b_, left crus; _c_, _c′_, sternocostal part of
diaphragm; _d_, _d′_, membranous portions of the diaphragm; _e_,
central tendon; _e′_, prolongations of central tendon; _f_, opening
for posterior vena cava; _g_, œsophagus; _h_, aorta; _i_, M.
transversus abdominis.]
The muscular portion is in two parts: (_a_) The =vertebral= portion (_a,
b_) arises by a single tendon from the ventral surface of the second,
third, and fourth lumbar centra. The tendon diverges into two, the right
one (_a_) of which is much stronger, and from each of the two arise
muscle-fibres. Each mass of fibres is one of the two =crura= (sing.
=crus=) of the diaphragm. The aorta (_h_) enters the abdomen between the
crura dorsally. The fibres of each crus diverge to be inserted into the
central tendon and the dorsal continuation of its horn. The two sets of
fibres unite ventrad of the opening of the aorta. Between this opening
and the central tendon is another opening for the œsophagus (_g_); this
lies entirely in that part of the diaphragm which arises from the right
crus (_a_). The fibres are again united ventrad of the œsophageal
opening.
(_b_) The =sternocostal part= (_c_, _c′_) takes origin from the
xiphoid process and the last five ribs, by fleshy bundles which
interdigitate with those of the transversus abdominis (_i_). The fibres
converge to the central tendon (_e_). Between the crus of each side and
the most dorsal of the costal fibres is the membranous interval (_d_)
mentioned above.
4. =Abdominal Muscles.= =M. obliquus abdominis externus= (Fig. 68,
_p_).--A large, thin sheet of muscle covering the whole abdomen and part
of the thorax ventrally.
_Origin._--(_a_) From the last nine or ten ribs by means of as many
tendons, which are interconnected to form arches that span the slips of
the serratus anterior. The muscle-fibres arise from these tendons and
from their intervening arches. (_b_) From the lumbodorsal aponeurosis
common to it and the internal oblique. The cranial fibres pass nearly
ventrad, the caudal fibres caudad, and the intervening fibres take an
intermediate course. The fibres end in a thin aponeurosis of insertion
along a curved line which passes at first caudad and then laterodorsad.
The aponeurosis fibres continue in the direction of the muscle-fibres to
the
_Insertion_ into the median raphe ventrad of the sternum from the
insertion of the seventh costal cartilage to the xiphoid process, into
the linea alba from the sternum to the pubic tubercle, and into the
tubercle and the cranial border of the pubis. Caudad of the xiphoid
process the aponeurosis is closely united to the superficial layer of
the internal oblique, where it forms the outer layer of the sheath of
the rectus abdominis. Laterad of the pubic tubercle the tendon is
perforated by the inguinal canal. In the cat neither the caudal part of
the muscle nor its tendon is attached to the ilium, as it is in man and
the dog, so that no Poupart’s ligament, or inguinal ligament, is formed.
_Relations._--Outer surface with the cutaneus maximus, the integument,
and near the origin with the latissimus dorsi (Fig. 68, _m_). Inner
surface with the obliquus internus (Fig. 73, _o_), the rectus abdominis
(Fig. 73, _k_), the intercostales externi (Fig. 73, _m_), a small part
of the serratus posterior inferior (Fig. 73, _n_), and by its dorsal
tendon with the longissimus dorsi (Fig. 69, _f_).
_Action._--Constrictor of the abdomen.
=M. obliquus abdominis internus= (Fig. 73, _o_).--A thin sheet similar
to the preceding but of less extent. Its fibres cross those of the
external oblique nearly at right angles and lie beneath them.
_Origin._--(1) Between the fourth and seventh lumbar vertebræ from the
lumbar aponeurosis which is common to it and the external oblique. The
lumbar aponeurosis takes origin from the lumbar spinous processes and
the interspinous ligaments, and is continuous craniad with the
aponeurosis of the serratus posterior inferior. Laterad it splits into
three sheets, two for the above-named muscles and a third which passes
to the vertebral transverse processes and forms the fascia covering the
supraspinous muscles of the lumbar region. (2) By a similar aponeurosis
from the ventral half of the iliac crest. (3) By fleshy fibres from the
three =crural arches=. These are three delicate ligamentous arches which
stretch from the crest of the ilium to the pubic spine. The dorsal one
gives exit to the iliopsoas muscle, the middle to the femoral vessels,
and the ventral to the spermatic cord. In the female the middle and
ventral arches may fuse. The pillar between the dorsal and middle arches
is attached to the iliopectineal eminence.
_Insertion._--The fibres pass cranioventrad and end along a longitudinal
line in a thin aponeurosis of insertion, which is united in the linea
alba to those of the external oblique and transversus. At the caudal end
of the aponeurosis all its fibres pass outside of the rectus abdominis
(Fig. 73, _k_). At its cranial end the fibres divide into two sheets or
laminæ, one of which passes outside of the rectus and the other inside.
The outer lamina unites with the aponeurosis of the external oblique,
while the inner lamina unites with that of the transversus. There is
thus formed a sheath for the cranial part of the rectus muscle.
_Relations._--Outer surface with the obliquus externus (Fig. 68, _p_).
Inner surface with the transversus abdominis (Fig. 69, _l_), and by its
ventral tendon with the rectus.
_Action._--Compressor of the abdomen.
=M. transversus abdominis= (Fig. 69, _l_).--A thin sheet covering the
whole surface of the abdomen and lying beneath the internal oblique. Its
fibres are nearly transverse.
_Origin._--(1) By fleshy fibres or by a thin aponeurosis from the
cartilages of all the false and floating ribs, by interdigitation with
the fibres of the diaphragm. (2) From the tips of all the lumbar
transverse processes. (3) From the ventral border of the ilium. (4) From
the dorsal and middle of the three crural arches, where it may blend
partly with the internal oblique. The muscle is continuous craniad with
the transversus thoracis muscle. Near the lateral border of the rectus
abdominis the muscle ends in a thin aponeurosis of insertion which is
continued (its fibres having the direction of the muscle-fibres) to the
_Insertion_ in the linea alba.
_Relations._--Outer surface with the internal oblique (Fig. 73, _o_) and
the rectus abdominis (Fig. 73, _k_), dorsad also with the longissimus
dorsi (Fig. 69, _f_). Inner surface with a thin fascia covering the
peritoneum and ventrocaudad with the rectus (Fig. 73, _k_). The dorsal
edge of the muscle touches the iliopsoas and longissimus dorsi; the
ventral edge touches the muscle of the opposite side.
_Action._--Constrictor of the abdomen.
=M. rectus abdominis= (Fig. 73, _k_).--A rather thick, flat muscle which
lies near the median ventral line separated by the linea alba from its
fellow of the opposite side and stretching from the pubis to the first
costal cartilage. Opposite the first lumbar vertebra it is approximately
four centimeters wide. It narrows at both ends.
_Origin_ by a strong tendon from the tubercle of the pubis. The muscle
passes craniad at first between the peritoneal fascia and the
transversus aponeurosis, then in the sheath formed by the internal and
external oblique and the transversus aponeurosis. It emerges from the
sheath opposite the xiphoid process and passes ventrad of the costal
cartilages. Between the third and fourth costal cartilages it ends in a
thin tendon which passes beneath the transversus costarum and is
_Inserted_ into the first costal cartilage near its middle, into the
second costal cartilage near its sternal end, and into the sternum
between the first and fourth cartilages.
_Relations._--Outer surface of the cranial part of the muscle and its
tendon with the pectoralis minor (Fig. 68, _o_) and the transversus
costarum (Fig. 73, _j_); caudad the muscle is covered by the outer layer
of the rectus sheath. Lateral edge with the obliquus externus (Fig. 68,
_p_), obliquus internus (Fig. 73, _o_), and transversus abdominis (Fig.
69, _l_). Medial edge with the muscle of the opposite side. Inner
surface with the internal intercostals (Fig. 69, _k_) and the rib
cartilages; the inner layer of the rectus sheath and the peritoneum.
_Action._--Retracts the ribs and sternum and compresses the abdomen.