Muscles of Abdomen

Removal of the skin from the ventral and lateral aspects of the thorax and abdomen exposes the cutaneous muscle of the trunk.

Tunica abdominalis

• This is a yellow elastic sheet – the abdominal tunic underneath the cutaneous muscle.
• It is very thick and closely adherent to the aponeurotic part of the external oblique muscle but closely covers its muscular part.
• It is thick towards the common prepubic tendon of the abdominal muscles and the linea alba.
• It is reduced to a very thin layer as it reaches the muscular portion of the external oblique muscle and thereafter it disappears.
• Anteriorly, it extends over some of the intercostals and the posterior digitations of the serratus thoracis. Posteriorly, it is attached to the external angle of the ilium and lumbo-dorsal fascia.
• The suspensory ligament of the prepuce in the male and the mammary gland in the female is derived from the external face of this tunic.
• It forms the elastic bandage and supports the abdominal muscles in bearing the weight of the viscera.
• The tunic is thin in the young animal but increase in size with the distension of the digestive organs.
• The tunic takes the place of deep fascia.

Next to this tunic are the four abdominal muscles on either side of the linea alba, arranged in layers; these from without inwards.

Prepubic tendon

• This is the tendon of insertion of the two-rectus abdominis muscles and is in the form of very strong and thick band. They are directed upwards and backwards and are inserted to the anterior border of the two pubic bones between the ilio-pectineal eminences. The aponeurosis of the two oblique muscles and the inguinal ligament are inserted into this tendon. It gives origin to a number of fibres of the pectineus muscle and fused with the medial face of the anterior part of the origin of the gracilis muscle. This attachment to the gracilis serves to depress the lower part of the abdominal wall.
• Cremaster externus
  • This is narrow flat band of muscle detached from the internal oblique muscle of the abdomen. It passes into the inguinal canal along with the spermatic cord.
  • Origin: Internal oblique muscle and iliac fascia.
  • Insertion: Parietal layer of tunica vaginalis.
  • Action: To raise the tunica vaginalis.
  • Blood supply: External spermatic artery.
  • Nerve supply: External spermatic nerve.

Inguinal canal

• The inguinal canals are paired, slit-like passages through the posterior part of the abdominal floor on either side of the prepubic tendon.
• The anlagen of the canals are present before sex differentiation occurs. They are therefore, equally well developed in both the sexes.
• The term canal is misleading in the large quadrupeds.The canals are potential rather than actual and are between the obliquus abdominis muscles.
• The inguinal rings are slit like rather than circular.
• This canal in male, lodges the spermatic cord, tunica vaginalis, external cremaster muscle, external pudic artery, artery of the cord and veins.
• In the female, it is rudimentary and contains the mammary vessels and nerves.
• The antero-medial wall of the canal is formed by the posterior part of the internal oblique muscle of the abdomen. A portion of the aponeurosis of the external oblique muscle forms the postero-lateral wall. This is the inguinal or poupart’s ligament. It is not a separate distinct structure but is the portion of the aponeurosis of the external oblique muscle related to the canal and lies against the medial surface of thigh.
• It presents two openings, internal and external abdominal opening.
• The upper (peritoneal) opening of this canal is called as the internal abdominal ring. The internal abdominal or internal inguinal ring is formed by the free borders of the internal oblique muscle anteriorly and the upper border of the inguinal ligament posteriorly. Its medial angle is formed by the lateral margin of the prepubic tendon. The lateral angle is formed by the attachment of the obliquus abdominis internus muscle to the inguinal ligament. The long axis of the ring is directed forward, outward and upward from the prepubic tendon towards the tuber coxae and has a length of about 15 cm. The opening of the ring, like that of the canal is potential. The weight of the abdominal viscera keeps the internal oblique muscle in contact with the inguinal ligament except where contents of the canal separate the walls.
• The external inguinal ring is a slit-like opening in the aponeurosis of the external oblique muscle of the abdomen and about 15 cm long. Its direction is forward, upward and little downward from the prepubic tendon. The prepubic tendon forms its rounded medial angle. The lateral angle is acute and quite definite.
• The length of the canal measured between the rings varies. At the medial angle it is equal to the thickness of the prepubic tendon i.e. Less than 1.25 cm, and at the lateral angles about 15 cm.
• Structures passing through the canal in both the sexes, are the external pudic artery, vein, the inguinal nerves and lymph vessels. Additional structures in the male are the spermatic cord, tunica vaginalis and the cremaster externus muscle.

Horse

• The external oblique muscle is extensive and extends as high upto lumbar transverse processes. The internal oblique is less developed.
• The rectus abdominis arises from the cartilage of the fourth or fifth rib to the ninth.
• The transversus abdominis meets the costal attachment of the diaphragm at its origin.

Dog

• The tunica abdominalis muscle is practically absent.
• The external oblique is well developed with clear border at aponeurotic end.
• The rectus abdominis extends from xiphoid cartilage to join the linea alba.
• The inguinal canal in the bitch lodges the round ligament of the uterus enclosed in a tubular process of the peritoneum.