Ans. is 'a' i.e., Rectus sheath Rectus sheath* It is an aponeurotic sheath enclosing rectus abdominis muscle formed by aponeuroses of the three flat abdominal muscles. Rectus sheath has two walls - (i) anterior and (ii) posterior.* Anterior wall is complete and its composition is variable as follows -i) Above costal margin :- Formed by external oblique aponeurosis.ii) Between costal margin and arcuate line Formed by external oblique aponeurosis and anterior lamina of aponeurosis of internal oblique.iii) Below arcuate line:- Aponeurosis of all three muscles (external oblique, internal oblique, transversus abdominis). The oponeurosis of internal oblique and transversus abdominis fuse, but the external oblique neurosis remains separate.* Posterior wall is incomplete and is deficient above the costal margin and below arcuate linei) Above costal margin :- Deficient and rectus abdominis rests directly on 5th, 6th and 7th costal cartilages.ii) Between costal margin and arcute line Formed by posterior lamina of aponeurosis of internal oblique and aponeurosis of transversus abdominis.iii) Below arcuate line :- Deficient and the rectus muscles rests on the fascia transversalis.* Contents of rectus sheathi) Muscles Rectus abdominis, pyramidalis.ii) Arteries:- Superior epigastric artery (a branch of internal thoracic) which anastomoses with inferior epigastric artery (a branch of external iliac).iii) Veins Superior epigastric and inferior epigastric veins.iv) Nerves Terminal parts of lower six thoracic nerves (lower 5 intercostal and subcostal nerves).* Three aponeurotic layers forming rectus sheath of both sides interlace with each other to form a tendinous raphe, Linea alba. It extends from xiphoid process to pubic symphysis. Linea alba is narrow and indistinct below the umblicus, as two recti lie in close contact. Linea alba broaden out above the level of umblicus.