THE BROAD MUSCLES AND THE ABDOMINAL APONEUROSES
The Broad Muscles
The abdominals found on the sides of the body are called the broad muscles (as opposed to the elongated form of the rectus abdominis in the front). There are three brad muscles on each side, superimposed in three layers and covering each other almost completely.
From the deepest to the most superficial, they are:
- The transversus abdominis
- The internal oblique
- The external oblique
This triple layer of muscles can be quite thin in slight people. Conversely, it can be more developed and extend beyond the bony limits of the pelvis in very muscular people. (Of course, it can also increase in size with weight gain, but that has nothing to do with muscle.)
The Aponeuroses of the Abdominals
Look at any muscle and you will see areas of red and white. This is true for the abdominals as well. Like all muscles, they are composed of two types of fibers:
- Contractile fibers (the red areas), the “active” part of the muscle
- Aponeuroses (the white areas), which do not contract
Aponeuroses can take two forms, each with its own function:
- They can envelop the muscle like a sheath.
- They can form flat, fibrous sheets of connective tissue that extend the contractile reach of muscles. In the abdominals, these extensions are found at the front of the belly and are called the anterior aponeuroses.
The external obliques and their aponeuroses
The aponeuroses of the broad muscles envelop the rectus abdominis and join at the linea alba.
The Anterior Aponeuroses of the Broad Muscles
Each of the three broad muscles is enveloped in two aponeuroses, one deep and one superficial. There are therefore six aponeuroses in all, and they overlap one another, separating and fusing to envelop the rectus abdominis before rejoining at the midline of the body to form the linea alba. This rather complex arrangement varies according to the level:
- In the upper two-thirds of the abdomen, the aponeuroses of the transversus abdominis and the deep aponeurosis of the internal oblique pass behind the rectus abdominis. Simultaneously, the aponeuroses of the external oblique and the superficial aponeurosis of the internal oblique pass in front of the rectus abdominis.
- In the lower one-third of the abdomen, all the aponeuroses of the broad muscles pass in front of the rectus abdominis. This area is visible in the lower abdomen where it forms a horizontal line, below which the belly looks a bit more pulled in.
The Linea Alba
All of the connective tissues coming from the broad muscles on the sides of the abdomen (the aponeuroses) come together and commingle their fibers to form what is called the linea alba or “white line.” This intersection of fibers makes for an area of strength. Yet the linea alba also has several points of weakness where hernias can develop. (See “Hernia of the Linea Alba”.)