Part 3: Abdominal Wall

We will move on to the dissection of the abdominal wall. First, identify the linea alba on the ventral midline of the belly. The linea alba is the raphe, or seam, along the ventral midline that extends from the xiphoid process of the sternum to the symphysis pelvis. This is where the two sides of aponeuroses of abdominal muscles and fascia meet and ‘fuse’ on the midline; it is an important landmark used for making surgical incisions.

• Dissection Note: Before proceeding with the dissection of the abdominal wall muscles in the cat, take note that these muscles are VERY thin, so be very careful while you are dissecting.

On the side of the animal with the dissected epaxial muscles, identify the external abdominal oblique m. on the lateral aspect of the abdominal wall. This muscle has a costal part arising from the last several ribs as well as a lumbar part arising from the lumbar region of the thoracolumbar fascia. It inserts on the ventral midline, via an aponeurosis along the linea alba (and also the prepubic tendon). (See DG Figs. 2-76, A & B, 2-77 and 2-79)

• Identify the superficial inguinal ring, which is an ‘opening’, or ‘slit’, in the aponeurosis of the external abdominal oblique m. in the inguinal region. This is the external opening of the inguinal canal, which is a short, natural passageway through the abdominal wall. (The actual ‘ring’ may be obscured by fascia and will often be easier to identify in intact males due to the passage of the spermatic cord through the opening.) (See DG Figs. 2-79, 2-80 and 4-2)

• In males, identify the spermatic cord as it passes out of the superficial inguinal ring. The spermatic cord includes the ductus deferens (along with a small deferent artery and vein) and the testicular artery and vein. These structures are enveloped within a visceral layer of vaginal tunic and a parietal layer of vaginal tunic. (See DG Figs. 2-79, 2-80, 4-4 and 4-5)

  • • Comment: The vaginal tunics are formed by an extension of the lining of the abdominal cavity (peritoneum); these structures will be discussed again in a later lab.

• In females, attempt to identify the vaginal process passing out through the superficial inguinal ring. The vaginal process in the female envelops the round ligament of the uterus and a varying amount of fat. It may vary in appearance in each specimen (short, long, a lot of fat, only a little fat, etc.) and, in some, may be difficult to find. (See DG Figs. 4-2 and 4-4)

  • • Comment: The vaginal process is formed by an extension of the lining of the abdominal cavity (peritoneum); this structure will be discussed again in a later lab.

• Using blunt dissection with the tip of a small scissors, identify the external pudendal artery and vein which also emerge from the superficial inguinal ring region.

  • • Dissection Note: If you are not able to dissect the inguinal region on this side of the animal (if structures were destroyed during previous dissection of this area), you may want to try the other side to see some of the structures described.

    • Comment: You may notice superficial inguinal lymph nodes in this region as well, but you need not identify them.

• Transect the external abdominal oblique m. close to its origin (along the ribs and lumbar region) and reflect it ventrally. Only reflect so far as the fusion of the aponeurosis of the external abdominal oblique and underlying internal abdominal oblique mm. (DO NOT cut through the aponeurosis of insertion of the external abdominal oblique m.)

  • • Comment: You should be aware that the caudal edge/border of the aponeurosis of the external abdominal oblique m. is called the inguinal ligament. (See DG Fig. 2-80)

After reflection of the external abdominal oblique m., identify the underlying internal abdominal oblique m. This muscle runs cranioventrally from the lumbar region to the costal arch and linea alba (and prepubic tendon), and inserts via a wide aponeurosis. (See DG Figs. 2-77, 2-79 and 2-80)

• Transect the internal abdominal oblique m. near its origin, detach it from the ribs, and reflect it ventrally to the level of the rectus abdominis m.

• In males (dog; usually not present in cat), identify the cremaster m. The cremaster m. is a slip of muscle fibers from the caudal edge of the internal abdominal oblique which accompanies the vaginal tunics through the superficial inguinal ring. (See DG Fig. 2-80)

  • • Comment: In the cat, the internal abdominal oblique m. may be extremely thin and very difficult to separate from the underlying transversus abdominis m. Do the best you can as you carefully dissect these layers.

Deep to the internal abdominal oblique m., identify the transversus abdominis m. This muscle arises from the ribs and lumbar region and runs transversely toward the linea alba (and prepubic tendon) where it attaches via an aponeurosis. (See DG Figs 2-77 and 2-80)

Running along the ventral midline of the belly, identify the rectus abdominis m. This muscle extends from the pubis (as the prepubic tendon) to the sternum. It inserts cranially via an aponeurosis on the first few ribs and sternum. The rectus abdominis has distinct, transverse tendinous intersections along its length. (When someone has “six-pack abs”, this is the muscle you are noticing – left and right sides, running together along the ventral midline- see DG Fig 2-78 as an illustration.) (See DG Figs. 2-76, A & B, 2-77, 2-78 and 2-80)

Identify the area/region of the deep inguinal ring that would be found on the inside of the abdominal wall (this is formed by the annular reflection of the transversalis fascia onto the vaginal tunic or process). You should think of this as the internal ‘opening’ of the inguinal canal. (See DG Fig. 4-5)

• Note that this ring is a boundary, NOT a distinct anatomical structure.

  Related Learning Objective

  • D3.5

   

Before moving on, with your dissection partner, be sure you can visualize the concept of the inguinal canal. The inguinal canal is a ‘slit’ between the abdominal muscles that forms a very short, natural passageway through the abdominal wall. It extends from the deep inguinal ring (inside the abdomen, which we can’t see very well) to the superficial inguinal ring (outside the abdomen) and allows passage of the structures mentioned previously (the vaginal tunics and spermatic cord (male) or vaginal process (female)) . (See DG Figs. 2-80 and 4-5)

To continue this dissection, and prepare for opening the thorax in the next part of this chapter, move to the side of the animal that did not have the forelimb dissection and complete the following.

• Transect the pectoral muscles (superficial and deep) parallel to the sternum and reflect them toward the forelimb (away from the body wall).

• Transect the latissimus dorsi m. just caudal to the forelimb, detach the latissimus dorsi m. from the ribs and reflect it dorsally to the mid-dorsal line of the back.

• Transect the cranial portion of the external abdominal oblique m. close to its origin along the ribs and reflect it caudally and ventrally. Only reflect so far as the fusion of the aponeurosis of the external abdominal oblique and underlying internal abdominal oblique mm.

On both left and right sides of the animal, free/transect the aponeurotic origin (the most cranial thoracic attachment) of the rectus abdominis m. on both left and right sides and reflect the muscles caudally. (See DG Fig. 2-76, A) Do this slowly so you can identify the cranial epigastric a. emerging from the caudal end of the sternum/thoracic cavity.