154 THE MUSCLES 



73, o), the rectus abdominis (Fig. 73, /'), the intercostales ex- 

 terni (Fig. 73, in), a small part of the serratus posterior inferior 

 (Fig. 73, )i), and by its dorsal tendon with the longissimus 

 dorsi (Fig. 69, /). 



Action. — Constrictor of the abdomen. 



M. obliquus abdominis internus (Fig. 73, o). — A thin 

 sheet similar to the preceding but of less extent. Its fibres 

 cross those of the external oblique nearly at right angles and 

 lie beneath them. 



Origin. — (i) Between the fourth and seventh lumbar verte- 

 brc'e froro the lumbar aponeurosis which is common to it and the 

 external oblique. The lumbar aponeurosis takes origin from 

 the lumbar spinous processes and the interspinous ligaments, 

 and is continuous craniad with the aponeurosis of the serratus 

 posterior inferior. Laterad it splits into three sheets, two for 

 the above-named muscles and a third \\hich passes to the 

 vertebral transverse processes and forms the fascia covering the 

 supraspinous muscles of the lumbar region. (2) By a similar 

 aponeurosis from the ventral half of the iliac crest. (3) By 

 fleshy fibres from the three crural arches. These are three 

 delicate ligamentous arches which stretch from the crest of the 

 ilium to the pubic spine. The dorsal one gives exit to the 

 iliopsoas muscle, the middle to the femoral vessels, and the 

 ventral to the spermatic cord. In the female the middle and 

 ventral arches may fuse. The pillar between the dorsal and 

 middle arches is attached to the iliopectineal eminence. 



Insertion. — The fibres pass cranioventrad and end along a 

 longitudinal line in a thin aponeurosis of insertion, which is 

 united in the linea alba to those of the external oblique and 

 transversus. At the caudal end of the aponeurosis all its fibres 

 pass outside of the rectus abdominis (Fig. 73, k). At its 

 cranial end the fibres divide into, two sheets or laminae, one of 

 which passes outside of the rectus and the other inside. The 

 outer lamina unites with the aponeurosis of the external 

 oblique, while the inner lamina unites with that of the trans- 

 versus. There is thus formed a sheath for the cranial part of 

 the rectus muscle. 



Relations. — Outer surface with the obliquus externus (Fig."^ 



