l6o ATLAS AND TEXT-BOOK OF HUMAN ANATOMY. 



Fig. 250. — The deeper layers of the abdominal muscles. 



On the left side the anterior layer of the sheath of the rectus abdominis and the obliquus abdominis externus 

 have been removed; on the right side, in addition, the rectus abdominis, the pyramidalis, and the obliquus abdominis 

 internus. The external intercostal ligaments have been removed on the left side. 



The obliquus abdominis internus (Figs. 245, 248 to 250), like the externus, is a decidedly 

 flat muscle. With the exception of the small area in the lumbar triangle it is completely covered 

 by the obliquus externus. It arises from almost the entire length of the middle lip of the crest 

 of the ilium, extending anteriorly as far as the anterior superior spine, from the junction of the 

 two layers of the lumbodorsal fascia (see page 156), and from the outer two-thirds of the inguinal 

 ligament. The direction of the fibers of the upper portion of the muscle is similar to that of the 

 intercostales interni (from without inward and from below upward); the middle fibers are less 

 oblique, and the lower ones are horizontal, those of the lower third even passing slightly from 

 without inward and from above downward, like those of the obliquus externus. 



The posterior fibers coming from the lumbar fascia have fleshy insertions into the lower 

 borders of the three lower ribs. The long fibers coming from the iliac crest, as well as the hori- 

 zontal and descending fasciculi from the inguinal ligament, pass into the sheath of the rectus, 

 the two layers of which are formed by the aponeurosis cf the obliquus internus (Fig. 248). 



The muscular portion of the obliquus internus is broader than that of the externus and 

 consequently approaches much more closely to the sheath of the rectus in the anterior abdominal 

 wall, especially in its lower portion (Fig. 249). A variable number of the inferior fibers of the 

 obliquus internus accompany the spermatic cord, as the cremaster (Figs. 245 and 250), as far as 

 the testicle, and consequently pass through the external abdominal ring as flat isolated fasciculi, 

 and in the female a few fibers of the muscle are similarly continued upon the round ligament of 

 the uterus. 



The transversus abdominis (Figs. 248 to 250) is a flat, rather thin, and largely aponeurotic 

 muscle which is completely covered by the obliquus internus. It arises by flat muscular serrations 

 from the inner surface of the six lower ribs and from the cartilages of the seventh to the tenth, 

 interdigitating with the serrations of origin of the diaphragm (see page 164). It also arises by an 

 aponeurosis from the entire length of the junction of the two layers of the lumbodorsal fascia, 

 from the internal lip of the crest of the ilium, and from the outer third of the inguinal ligament. The 

 fibers pass almost transversely and are attached to the aponeurotic insertion in a curved line, 

 the semilunar line (line oj Spigelius) (Fig. 250), in such a manner that the uppermost fibers 

 coming from the ribs almost approach each other in the median line, the middle ones become 

 aponeurotic at quite a distance from this location, and the inferior fasciculi remain muscular 

 for a somewhat greater distance. The upper two-thirds of the aponeurosis of the transversus, 

 together with that of the internus, form the posterior layer of the sheath of the rectus (Fig. 248) ; 

 the lower third together with the aponeuroses of the obliquus internus and externus, forms the 

 anterior layer of the sheath (Fig. 249). 



