The =superficial fascia= covering the lateral and ventral walls of the
abdomen is continuous dorsally with the lumbo-dorsal fascia, in front
with the thoracic fascia, and behind with the gluteal fascia. In the
inguinal region it forms part of the fascia of the penis or of the
mammary glands. At the lower part of the flank it forms a fold which is
continuous with the fascia of the thigh near the stifle joint. In this
fold are the precrural lymph glands. Medially it blends with the linea
alba. It contains the abdominal portion of the panniculus carnosus.
The =abdominal panniculus= (M. cutaneus maximus) covers a large part of
the lateral surface of the abdomen and thorax. The general direction of
its fibers is longitudinal. Its posterior extremity forms the basis of
the fold of the flank. Its anterior extremity is inserted by a thin
tendon into the internal tuberosity of the humerus, with the posterior
deep pectoral muscle. Its dorsal edge may be indicated by a line drawn
from the upper end of the thirteenth rib to the fold of the flank. Its
ventral limit corresponds to a line drawn from the fold to a point about
a handbreadth external to the umbilicus, and from here to a point a
little above the level of the elbow. Behind the shoulder the fibers
become oblique and blend with the scapular portion. The aponeurosis
extends ventrally to the linea alba, dorsally to the supraspinous
ligament. The muscle is intimately adherent to the skin, so that special
care is necessary in removing the latter. Its deep face, on the other
hand, is loosely attached to the underlying structures by a quantity of
areolar tissue which is more or less loaded with fat (panniculus
adiposus) in animals in good condition. The large external thoracic
(“spur”) vein is partially embedded in the lower part of the muscle. Its
action is to twitch the skin.
The =deep fascia= is represented chiefly by the =abdominal= tunic
(Tunica flava abdominis). This is a sheet of elastic tissue which
assists the muscles in supporting the great weight of the abdominal
viscera. It is practically coextensive with the obliquus externus, which
it covers. Ventrally it is thick, and is intimately adherent to the
aponeurosis of the muscle. Laterally it becomes thinner and is more
easily separated, although fibers from it dip in between the
muscle-bundles. It is continued for some distance upon the intercostals
and serratus magnus. Traced forward, it passes as a thin layer beneath
the posterior deep pectoral muscle. Posteriorly it is attached to the
external angle of the ilium. In the inguinal region it forms the deep
fascia of the prepuce or of the mammary glands.
The =linea alba= is a median fibrous raphé which extends from the
xiphoid cartilage to the symphysis pubis. It is formed chiefly by the
junction of the aponeuroses of the oblique and transverse muscles, but
partly by longitudinal fibers. A little behind its middle is a cicatrix
which indicates the position of the umbilical opening of the fœtus.
1. =Obliquus abdominis externus= (great oblique; external oblique of the
abdomen).—This is the most extensive of the abdominal muscles. It is a
broad sheet, irregularly triangular in shape, widest behind. Its fibers
are directed chiefly downward and backward.
_Origin._—(1) The outer surfaces of the last fourteen ribs, and the
fascia over the external intercostal muscles; (2) the lumbo-dorsal
fascia.
_Insertion._—(1) The linea alba and the prepubic tendon; (2) the
external angle and shaft of the ilium; (3) the internal femoral fascia.
_Action._—(1) To compress the abdominal viscera, as in defecation,
micturition, parturition, and expiration; (2) to flex the trunk (arch
the back); (3) acting singly, to flex the trunk laterally.
_Structure._—The muscle is composed of a fleshy portion and an
aponeurosis. The =muscular portion= lies on the lateral wall of the
thorax and abdomen. It arises by a series of digitations, the anterior
four of which alternate with those of the serratus magnus. The origin
may be indicated by a slightly curved line (concave above) drawn from
the lower part of the fifth rib to the external angle of the ilium. The
fibers are directed downward and backward and terminate on the
aponeurosis, except in the flank, where they are almost horizontal in
direction. The line of junction is a curve (concave above) extending
from the upper edge of the posterior deep pectoral muscle toward the
external angle of the ilium. The =aponeurosis= is intimately attached to
the abdominal tunic, and its fibers are largely interwoven ventrally
with those of the aponeurosis of the internal oblique. By this fusion is
formed the outer sheath of the rectus abdominis, which blends at the
linea alba with that of the opposite side. In the inguinal region the
aponeurosis divides into two chief layers; one of these curves upward
and backward and is inserted into the external angle of the ilium and
the prepubic tendon. Between these points the aponeurosis is much
strengthened and is called the =inguinal= (Poupart’s) =ligament=
(Ligamentum inguinale). This curves upward and somewhat forward, becomes
thin, and blends with the iliac fascia. It forms the posterior wall of
the inguinal canal. About an inch (ca. 2 to 3 cm.) in front of the pubis
and about two inches (ca. 4 to 5 cm.) from the median plane the
aponeurosis is pierced by a slit-like opening,[50] the =external
inguinal ring= (Annulus inguinalis subcutaneus). This is the external
orifice of the inguinal canal. Its long axis is directed outward and
forward, and is about four inches (ca. 10 cm.) in length. The inner
angle is rounded and is well defined by the junction of the inguinal
ligament with the prepubic tendon, but the outer angle is not so sharply
defined. The borders or pillars are constituted by arciform fibers of
the aponeurosis of the external oblique (Crus mediale, laterale). The
=femoral layer= of the aponeurosis (Lamina femoralis) passes on to the
inner surface of the thigh, where it blends with the femoral fascia. A
thin =iliac layer= (Lamina iliaca) passes over the outer margin of the
iliacus to the external border of the ilium.
_Relations._—Superficially, the skin, the panniculus carnosus, the
abdominal tunic, and the posterior deep pectoral muscle; deeply, the
ribs and their cartilages, the intercostal muscles, the internal
oblique, the contents of the inguinal canal, and the sartorius and
gracilis.
_Blood-supply._—Intercostal and lumbar arteries.
_Nerve-supply._—Intercostal and lumbar nerves.
2. =Obliquus abdominis internus= (small oblique; internal oblique of the
abdomen).—This muscle is situated under the preceding one. Its fibers
are directed downward, forward, and inward. It forms a triangular curved
sheet with the base behind.
[Illustration:
FIG. 186.—VENTRAL MUSCLES OF HORSE, AFTER REMOVAL OF GREATER PART OF
PANNICULUS CARNOSUS AND ABDOMINAL TUNIC. SARTORIUS AND GRACILIS
REMOVED FROM RIGHT THIGH.
_c_, Mastoido-humeralis; _g_, anterior superficial pectoral; _g′_,
posterior superficial pectoral; _h_, posterior deep pectoral; _l′_,
aponeurosis of obliquus abdominis externus; _t_, pectineus; _u_,
ilio-psoas; _v_, semimembranosus; _w_, gracilis; _x_, sartorius;
_y_, vastus internus; _z_, _z′_, adductor; _14_, sternum; _14′_,
cariniform cartilage. (After Ellenberger-Baum, Anat. f. Künstler.)
]
_Origin._—The external angle of the ilium and the adjacent part of the
inguinal (Poupart’s) ligament.
_Insertion._—(1) The cartilages of the last four or five ribs; (2) the
linea alba and the prepubic tendon.
_Action._—Similar to that of the preceding muscle.
_Structure._—Like the external oblique, it is composed of a fleshy
portion and an aponeurosis. The fleshy portion is fan-shaped, and is
situated chiefly in the flank. At its iliac origin it is covered by a
glistening aponeurosis. Traced inward and downward along the abdominal
surface of the inguinal ligament, the muscular origin is found to become
much thinner, and also becomes loosely attached to the ligament. About
four or five inches (ca. 10 to 12 cm.) from the linea alba the muscle
separates from the ligament and forms the anterior wall of the inguinal
canal. The abdominal orifice of the canal, the =internal inguinal
ring=[51] (Annulus inguinalis abdominalis), is found here. It is
normally a narrow slit, bounded in front by the edge of the internal
oblique, and behind by the inguinal ligament. The =aponeurosis= is to a
great extent blended with that of the external oblique, being, indeed,
considerably interwoven with it ventrally. Where it covers the rectus
abdominis it is attached to the tendinous inscriptions of that muscle.
_Relations._—Superficially, the external oblique; deeply, the rectus
abdominis, transversus abdominis, and the peritoneum.
_Blood-supply._—Circumflex iliac, lumbar, and intercostal arteries.
_Nerve-supply._—Ventral branches of the lumbar nerves.
3. =Rectus Abdominis.=—This muscle is confined to the ventral part of
the abdominal wall; it extends from the lower part of the chest-wall to
the pubis.
_Origin._—The cartilages of the fifth to the ninth ribs inclusive, and
the adjacent surface of the sternum.
_Insertion._—The pubis, by means of the prepubic tendon.
_Action._—Similar to that of the oblique muscles. It is specially
adapted to flex the lumbo-sacral joints and the lumbar and thoracic
parts of the spine.
_Structure._—The fibers of the muscle are directed longitudinally. Nine
to eleven transverse bands of fibrous tissue extend in an irregular
manner across the muscle. These are termed =inscriptiones tendineæ=.
They strengthen the muscle and serve to prevent separation of its
fibers. The width of the muscle is greatest about its middle.
_Relations._—Superficially, the aponeuroses of the oblique muscles
(which constitute the external rectus sheath), and the posterior deep
pectoral; deeply, the transversus, intercostals, the cartilages of the
ribs, and the sternum. The posterior abdominal artery runs along the
outer edge of the muscle posteriorly, and the anterior abdominal artery
on or in its anterior part.
_Blood-supply._—Anterior and posterior abdominal arteries.
_Nerve-supply._—Intercostal and lumbar nerves.
4. =Transversus abdominis.=—This muscle, named from the general
direction of its fibers, is a triangular curved sheet. Its lateral part
is muscular, its ventral aponeurotic.
_Origin._—(1) The inner surfaces of the distal ends or the cartilages of
the asternal ribs, meeting the costal attachment of the diaphragm; (2)
the transverse processes of the lumbar vertebræ, by means of the deep
layer of the lumbo-dorsal fascia.
_Insertion._—The xiphoid cartilage and the linea alba.
_Action._—Similar to that of the oblique muscles.
_Structure._—The =muscular part= is a sheet of parallel bundles of
fibers, directed downward and inward. It is thickest over the cartilages
of the ribs, and from here it thins out greatly toward the aponeurosis
and the lumbar region. The fibers of the =aponeurosis= directly continue
those of the fleshy part. Posteriorly it becomes extremely thin and
blends with the aponeuroses of the oblique muscles. It covers the deep
face of the rectus, so forming the internal rectus sheath.
_Relations._—Superficially, the oblique and straight muscles, the
retractor costæ, the cartilages of the asternal ribs, and the internal
intercostal muscles; deeply, the transversalis fascia and the
peritoneum. The =transversalis fascia= is little developed in the horse,
and is very thin in emaciated subjects, but in animals in good condition
it contains a good deal of fat. It blends with the iliac fascia and
descends into the inguinal canal. The asternal artery runs along the
interval between the origin of the transversus and the costal part of
the diaphragm. The intercostal nerves pass down over the external
surface of the muscle, to which they give branches. Branches of the
first three lumbar nerves are similarly disposed further back.
_Blood-supply._—Intercostal, lumbar, and asternal arteries.
_Nerve-supply._—Intercostal and lumbar nerves.
5. =Cremaster externus.=—This small muscle may be regarded as a detached
portion of the internal oblique, with which it blends at its origin
(Figs. 272, 450).
_Origin._—The iliac fascia, near the origin of the sartorius.
_Insertion._—The tunica vaginalis communis.
_Action._—To raise the tunica vaginalis, and with it the testicle.
_Structure._—The muscle arises by a thin aponeurosis which is succeeded
by a flat muscular belly about one and a half to two inches (ca. 4 to 5
cm.) in width.[52] It passes down the inguinal canal on the
postero-external surface of the tunica vaginalis, to which it is rather
loosely attached. On reaching the point where the tunic is reflected on
to the tail of the epididymis, the muscle is firmly attached to the
outer surface of the tunic by short tendinous fibers.
_Relations._—The muscle lies between the peritoneum and the fascia
transversalis in front and the iliac fascia and inguinal ligament
behind. On reaching the internal ring it descends the inguinal canal on
the postero-external surface of the tunica vaginalis communis.
_Blood-supply._—External spermatic or cremasteric artery.
_Nerve-supply._—External spermatic nerve.
=The Inguinal Canal.=—This term (Canalis inguinalis) is applied to an
oblique passage through the posterior part of the abdominal wall.[53] It
begins at the internal inguinal or abdominal ring, and extends obliquely
downward, inward, and somewhat forward, to end at the external inguinal
or subcutaneous ring. Its anterior wall is formed by the fleshy
posterior part of the internal oblique muscle, and the posterior wall by
the strong tendinous inguinal (Poupart’s) ligament. The average length
of the canal, measured along the spermatic cord, is about four inches
(ca. 10 cm.). The =internal inguinal ring= (Annulus inguinalis
abdominalis) is bounded in front by the thin margin of the internal
oblique muscle, and behind by the inguinal ligament. It is directed
approximately from the edge of the prepubic tendon toward the external
angle of the ilium. Its length is about four or five inches (ca. 10 to
12 cm.). The edge of the muscle is attached to the surface of the
ligament here by delicate connective tissue, except where structures
intervene between the walls of the canal. Consequently the limits of the
ring are not very clearly defined. The =external inguinal ring= (Annulus
inguinalis subcutaneus) is a well defined slit in the aponeurosis of the
external oblique muscle, situated lateral to the prepubic tendon. Its
long axis is directed from the edge of the prepubic tendon outward and
forward, and its average length is about four inches (ca. 10 cm.). The
canal contains in the male the spermatic cord, the tunica vaginalis, the
external cremaster muscle, the external pudic artery and a small
satellite vein, and the inguinal lymph vessels and nerves. In the female
it contains the mammary vessels and nerves; in the bitch it also lodges
the round ligament of the uterus, inclosed in a tubular process of
peritoneum.
The two rings do not correspond in direction, so that the length of
the canal varies greatly when measured at different points. The inner
angle of the internal ring lies almost immediately above that of the
external ring, but the outer angle is situated five to six inches (ca.
12 to 15 cm.) from that of the external ring. The inner angles of the
external rings are well defined and distinctly palpable in the living
subject; they are about three to four inches (ca. 8 to 10 cm.) apart.
=The Prepubic Tendon.=—The =prepubic tendon= is essentially the tendon
of insertion of the two recti abdominis, but also furnishes attachment
to the obliqui, the graciles, and the pectinei. It is attached to the
anterior borders of the pubic bones, including the ilio-pectineal
eminences. It has the form of a very strong thick band, with concave
lateral borders which form the inner boundaries of the external inguinal
rings. Its direction is oblique upward and backward.[54] Its structure
is somewhat complex. Most of the fibers of the posterior part extend
from one ilio-pectineal eminence to the other. The fibers which belong
to the recti curve in to the median line. The aponeuroses of the
internal oblique muscles are inserted into its abdominal surface, and
the inguinal ligaments are attached to and continue across it in
arciform fashion. The anterior part of the tendon of origin of the
gracilis is fused with it ventrally, and many of the fibers of the
pectineus arise from it. It gives off on either side a strong round
band, the so-called =pubo-femoral= or =accessory ligament=, which is
inserted into the fossa of the head of the femur with the round ligament
(_vide_ hip joint).