THE ANTERO-LATERAL MUSCLES OF THE ABDOMEN 



417 



muscle. It arises by tendinous fibers from the front of the pubis and the anterior 

 pubic ligament; the fleshy portion of the muscle passes upward, diminishing 

 in size as it ascends, and ends by a pointed extremity which is inserted into the 

 linea alba, midway between the umbilicus and pubis. This muscle may be wanting 

 on one or both sides; the lower end of the Rectus then becomes proportionately 

 increased in size. Occasionally it is double on one side, and the muscles of the two 

 sides are sometimes of unequal size. It may extend higher than the level stated. 



Peritoneum 



Transversalis fascia 



Obliquus interims 



Transversus 

 FIG. 399. Diagram of sheath of Rectus. 



Besides the Rectus and Pyramidalis, the sheath of the Rectus contains the superior and inferior 

 epigastric arteries, and the lower intercostal nerves. 



Variations. The Rectus may insert as high as the fourth ot third rib or may fail to reach the 

 fifth. Fibers may spring from the lower part of the linea alba. 



Nerves. The abdominal muscles are supplied by the lower intercostal nerves. The Obliquus 

 internus and Transversus also receive filaments from the anterior branch of the iliohypogastric 

 and sometimes from the ilioinguinal. The Cremaster is supplied by the external spermatic branch 

 of the genitofemoral and the Pyramidalis usually by the twelfth thoracic. 



The Linea Alba. The linea alba is a tendinous raphe in the middle line of the abdomen, 

 stretching between the xiphoid process and the symphysis pubis. It is placed between the medial 

 borders of the Recti, and is formed by the blending of the aponeuroses of the Obliqui and Trans- 

 versi. It is narrow below, corresponding to the linear interval existing between the Recti; but 

 broader above, where these muscles diverge from one another. At its lower end the linea alba 

 has a double attachment its superficial fibers passing in front of the medial heads of the Recti 

 to the symphysis pubis, while its deeper fibers form a triangular lamella, attached behind the 

 Recti to the posterior lip of the crest of the pubis, and named the adminiculum lineae albae. 

 It presents apertures for the passage of vessels and nerves; the umbilicus, which in the fetus 

 exists as an aperture and transmits the umbilical vessels, is closed in .the adult. 



Peritoneum 

 Transversalis fascia 



Linea alba 



Obliquus ezternus. 

 Obliquus internus 



Transversus' 

 FIG. 400. Diagram of a transverse section through the anterior abdomina wall, below the linea semicircularis 



The Lineae Semilunares. The lineae semilunares are two curved tendinous lines placed one 

 on either side of the linea alba. Each corresponds with the lateral border of the Rectus, extends 

 from the cartilage of the ninth rib to the pubic tubercle, and is formed by the aponeurosis of the 

 Obliquus internus at its line of division to enclose the Rectus, reinforced in front by that of the 

 Obliquus externus, and behind by that of the Transversus. 



Actions. When the pelvis and thorax are fixed, the abdominal muscles compress the abdominal 

 viscera by constricting the cavity of the abdomen, in which action they are materially assisted 

 by the descent of the diaphragm. By these means assistance is given in expelling the feces 

 from the rectum, the urine from the bladder, the fetus from the uterus, and the contents of the 

 stomach in vomiting. 



If the pelvis and vertebral column be fixed, these muscles compress the lower part of the thorax, 

 materially assisting expiration. If the pelvis alone be fixed, the thorax is bent directly forward, 

 when the muscles of both sides act; when the muscles of only one side contract, the trunk is bent 

 toward that side and rotated toward the opposite side. 

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