522 



HUMAN ANATOMY. 



tially the outer border of the rectus, except below, where they He a Httle lateral to 

 that muscle. More medially, however, the layers become intimately associated and 

 can only be separated artificially. 



At the outer border of the rectus muscle the aponeurosis divides into two layers 

 (Fig. 529, A) which pass one in front and the other behind the rectus, thus forming a 

 sheath for it (vagina rausculi recti). The Hne of the division is indicated on the surface 

 of the abdomen by a slight groove, and constitutes what is termed the /mea scmi- 

 hinaris. When they reach the mesial border of the rectus the two layers unite and 

 become continuous in the middle line with the aponeurosis of the opposite side to 

 form a strong iibrous band which extends from the front of the xiphoid process of the 



sternum above to the 

 Fig. 528. symphysis pubis be- 



low, and is termed the 

 linea alba. In its 

 upper part this band 

 is fairly broad, but 

 below the umbilicus, 

 which is situated in 

 the band, it suddenly 

 narrows to a thin line 

 which becomes con- 

 tinuous below with the 

 superior pubic liga- 

 ment, behind the in- 

 sertion of the recti, by 

 a triangular expansion 

 which occasionally 

 contains muscle-fibres 

 and is termed the ad- 

 miniculum lineae albae. 

 Th.Q posterior layer 

 of the aponeurosis, 

 which forms the poste- 

 rior wall of the sheath 

 of the rectus, is fairly 

 thick above, but a lit- 

 tle below the level of 

 the umbilicus it sud- 

 denly becomes very 

 much thinner along an 

 arched line, the con- 

 cavity of which is downward, and may sometimes be represented by a distinct fold. 

 This margin is termed the line ox fold of Douglas (linea semicircularis) (Fig. 523). 



Uncovered, 

 fibres of exter- 

 nal oblique 



Anterior, 

 sheath of rec 

 tus muscle 



Anterior superior 

 iliac spine 

 Intercolumnar 

 fibres 



External abdoini 

 nal ring p 



Spermatic cord 



— ^l.ineae 



transversae 

 Linea alba 



' tL- 



overed ex- 

 rnal oblique 



-Linea 



semilunaris 



— Suspensory 

 ligament 

 of penis 



\ 



V 



Superfjcia! dissection of abdomen, showing ventral aponeurosis. 



Various suggestions have been made in explanation of this sudden change in the thickness 

 of the posterior layer of the sheath of the rectus. It has been supposed that it was connected 

 with the passage of the inferior epigastric artery into the substance of the muscle (Henle), a 

 somewhat inadequate cause even if the jioint of passage of the artery through the sheath cor- 

 responded with the semicircular Hne. The thinness of the portion of the sheath below the line 

 has been e.xplained on the ground that it represents the portion with which the urinary bladder 

 was in contact in_foetal Hfe (Gegenbaur), and also by the view that the strain exerted on this 

 portion of the sheath is less than that placed upon the upper part, since the latter is acted on 

 by fibres of the oblique and transverse muscles which have bony attachments drawn upward 

 during inspiration, while the lower part is in relation to the less active fibres attached to the 

 inguinal h'gament (Solger). 



Finally, it may be stated that the immediate cause for the sudden change in thickness has 

 been assigned to the development of the processus vaginalis peritona;i, the pouch of peritoneum 

 which in the embryo descends into the genital swelling and gives rise in the male to the tunica 

 vaginalis testis. The formation of this peritoneal pouch is held to prevent the lower portions 

 of the posterior layer of the abd(Mnin,al aponeurosis which are derived from the aponeuroses of 

 the internal oblique and transversahs from passing behind the rectus muscle, the posterior wall 

 of its sheath being formed only by the fascia transversahs (Eisler). 



