Anatomy in detail.
The rectus abdominis consist of several “panels” of muscle between strips of connective tissue known as tendinous intersections.
These sections are what give the rectus abdominis the “6-pack” look, but the number of panels can vary between individuals e.g. 4, 8, and 10 packs can occur.
Each rectus abdominis lies within its rectus sheath, a 'tunnel' made from the aponeuroses of the lateral abdominal muscles (the external abdominal oblique, internal abdominal oblique and transversus abdominis) before they merge with the linea alba at the front of the abdomen.
More information on the rectus abdominis:
source Narayanan VK, Peter S, Nair AJ. The rectus abdominis muscle in males and females of Keralaa cadaveric study 2017.
It has a tendinous origin and a muscular insertion.
It arises by two tendinous heads: Medial head arises from the anterior surface of the pubic symphysis and the lateral head arises from the lateral part of the pubic crest and the pubic tubercle.... The origin was seen to be tendinous and narrow and remained tendinous up to 3 cm from its origin. Pyramidalis muscle when present was seen overlapping this part of rectus abdominis.
The muscle is inserted on the anterior thoracic wall by four fleshy slips that are attached in a horizontal manner to the lower part of the anterior thoracic wall - along a horizontal line passing laterally from the xiphoid process and cutting in that order, the 7th, 6th and 5th costal cartilages.
The paired recti muscles are separated in the median plane by the linea alba.
The muscle is three times as wide superiorly as inferiorly. It is broad and thin superiorly and narrow and thick inferiorly.
An increase in length of 1 to 2 cm in the left rectus abdominis was seen in all the 20 cadavers, which was a striking feature
The rectus abdominis is enclosed in the rectus sheath, which is formed by the aponeuroses of the anterolateral muscles of the anterior abdominal wall.
The muscle usually presents tendinous intersections along its length. ... The absence of tendinous intersections in the foetal specimens could imply that they fully develop after birth, probably after the child becomes ambulant.
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