Rectus sheath : easy and brief description with memorization tips

Rectus sheath : is formed by connective tissue which enclosing the rectus abdominis muscle (and occationally  pyramidalis muscle )

It is formed by aponeurosis of three flat muscles of anterior abdominal wall .

The entire anterior surface of rectus abdominis muscle is covered by anterior part of rectus sheath but only middle 2/3 of posterior surface of rectus abdominis muscle is covered by posterior part of rectus abdominis muscle. The lower limit of posterior part of rectus sheath is known as arcuate line . This line is situated in midpoint  of the umbilicus and upper surface of pubis symphysis . so posterior surface of rectus abdominis muscle are devoid of rectus sheath at two places , above the costal margin and below the arcuate line. Above the costal margin it is related with 5^th , 6^th and 7^th costal cartilage and below the arcuate line it is related with fascia transversalis.

Internal oblique abdominis muscle of anterior abdominal wall is an interesting muscle. Because its aponeurosis divide into two part at the lateral surface of each rectus abdominis muscle and hold the rectus abdominis muscles as testtube holder . So the aponeurosis of internal oblique abdominis muscles, form both anterior and posterior part of rectus sheath .

Arrangement of anterior part of rectus sheath: arrangement are different in three places ,

1) above costal margin : this part is formed by one aponeurosis: the aponeurosis  of external oblique abdominis

2) from costal margin to arcuate line : this part is formed by two aponeurosis : the aponeurosis of external oblique abdominis and anterior layer of aponeurosis of internal oblique abdominis

3) below the arcuate line : this part is formed by three aponeurosis : the aponeurosis of external oblique abdominis, internal oblique abdominis and transversus abdominis

Arrangement of poterior part of rectus sheath: arrangement are different in three places ,

1)    Above costal margin : No rectus sheath . this part is related with 5-7 costal cartilage 

2) From costal margin to arcuate line : this part is formed by two aponeurosis : posterior layer of  aponeurosis of internal oblique abdominis and transversus abdominis muscles

3) Below the arcuate line: no rectus sheath. This part is related with fascia transversalis.

Tips for memorization:

ant part of rectus sheath	rectus abdominis	Pos part of rectus sheath
Above costal margin : 1 aponeurosis  (EOA)		no rectus sheath
Costal margin to arcuate line : 2 aponeurosis (EOA & IOA)		2 aponeurosis (IOA & TA)
Below the arcuate line : 3 aponeurosis (EOA, IOA, TA)		no rectus sheath

SN:

1)    aponeurosis of external oblique abdominis muscles are present in all level of anterior part of rectus sheath

2)    Aponeurosis of internal oblique abdominis muscles are divided and hold the  rectus abdominis muscles like test tube holder and it forms both anterior and posterior part of rectus sheath

3)    Below arcuate line three aponeurosis of flat anterior   abdominal wall muscles are present at the anterior part of rectus sheath

Content of rectus sheath :

Two Muscles : rectus abdominis  and pyramidalis

Two arteries : superior epigastric artery  and inferior epigastric artery

Two veins : superior and inferior epigastric veins

Six nerves: lower five intercostals nerves and subcostal nerve  

Latest view of rectus sheath formation:

Recent observation is all aponeurosis of flat anterior abdominal muscles are divided into two layers . so three muscles have total six aponeurosis , three aponeurosis present  in front and three aponeurosis present behind .

It is also observe aponeurosis of flat muscles are not end in linea alba but they decussate and continued as contralateral aponeurosis . 

The lateral part of rectus abdominis muscle produce a groove in the anterior abdominal wall , is known as linea semilunaris.

This groove is important because medial to this  groove and lateral to this  groove the layers of anterior abdomnal wall is different

Layers of anterior abdominal wall, medial to the linea semilunaris  (from before backward )

1)    Skin

2)    Superficial fascia : fascia camper and fascia scarpa

3)    Aponeurosis of external oblique abdominis

4)    Aponeurosis of anterior layer of internal oblique abdominis

5)    Rectus abdominis muscle

6)    Aponeurosis of posterior layer of internal oblique abdominis

7)    Aponeurosis of transversus abdominis

8)    Fascia transversalis

9)    Extraperitoneal tissue

10)                       Parietal layer of peritoneum

 Layers of anterior abdominal wall, lateral to the linea semilunaris  (from before backward )

1)    Skin

2)    Superficial fascia : fascia camper and fascia scarpa

3)     external oblique abdominis muscle

4)     internal oblique abdominis muscle

5)     transversus abdominis muscle

6)    Fascia transversalis

7)    Extraperitoneal tissue

8)    Parietal layer of peritoneum

SN:

Flat muscles have two part fleshy part and fibrous part (aponeurosis ) . Fleshy part of anterior abdominal wall muscles is present away from the median plan and fibrous / aponeuroses are present at the middle part of abdomen. So the middle part of anterior abdominal wall is made by three aponeurosis of flat abdominal wall muscles.  This is the weakest part of abdomen and it is secured by rectus abdominis muscles which present two side of linea alba ( the median groove of anterior abdominal wall ) and rectus abdominis is secured by aponeurosis of three flat muscles of abdomen .