410 THE MUSCLES 



ol)li(iue imiscles, in passing from the crest of the ilium to the lower ribs, a much 

 more e(^nii>lete approximation of the bones is effected. 



Relations. — Superiicially, the ol)li(|Uus internus. the cartilages of the lower 

 ribs, some small vessels, and the lower intercostal nerves; above, it is continuous 

 with the lower border of the triangularis sterni; below, it lies above the spermatic 

 cord or round ligament, and its conjoined tendon lies for a short distance beneath 

 these structures; deeply, the transversalis fascia which separates it from the peri- 

 toneum. 



The sheath of the rectus is formed by the aponeuroses of the three preceding 

 muscles. That of the obliquus internus divides into an anterior and posterior 

 layer. The anterior layer l>lcnded with the aponeurosis of the obliquus externus 

 forms the front portion of the sheath. Above, it is attached to the cartilages of 

 the fifth, sixth, and seventh ribs; and internally it l)lends with the posterior por- 

 tion of the sheath along the linea alba. The posterior portion of the sheath is 

 formed by the posterior layer of the aponeurosis of the ol^liquus internus, which 

 is blended with the aponeurosis of the transversalis. Above, it is attached to the 

 lower edge of the cartilages of the seventh and eighth ribs, and to the ensiform 

 cartilage. Internally it blends with the front portion of the sheath along the 

 linea alba. Rather nearer the umbilicus than the pubes, the posterior portion of 

 the sheath becomes suddenly much thinner and weaker at the fold of Douglas, 

 Avhich is a transverse line indicating that the filn'es of the abdominal aponeuroses 

 which have down to this level formed the posterior portion of the sheath, now pass 

 to its front wall. Below this level the posterior portion of the sheath is formed 

 by a thickening of the transversalis fascia (page 411). 



POSTERIOR VERTICAL MUSCLE— QUADRATUS LUMBORUM 



The quadratus lumborum (fig. 286) — named from its approximately square 

 shape and its position in the region of the loins — is a thick quadrilateral sheet. 



Origin. — (1) About two inches (5 cm.) of the inner lip of the crest of the 

 ilium at the junction of its middle and posterior thirds; (2) the ilio-lumbar liga- 

 ment; (3) the tips of the transverse processes of the three or four lower lumbar 

 vertebra?. 



Insertion. — (1) The inner half of the lower border of the last rib; (2) the 

 tips of the transverse processes of the upper three or four lumbar verteljrse; and 

 (3) the fibrous continuations which pass out from these transverse processes in the 

 substance of the middle portion of the lumbar fascia, and which represent the 

 abdominal ribs. 



Structure. — This muscle varies considerably in its structure and arrangement, 

 the origins from, and insertions into the transverse processes differing much in 

 different subjects. Its origin is usually by short tendinous intermixed with fleshy 

 fibres, and extends at its lower part all along the ilio-lumbar ligament, and behind 

 the attachment of this ligament from the inner lip of the crest of the ilium. The 

 fibres converge somewhat as they ascend; the outermost, passing upwards and 

 inwards to the middle point of the lower border of the last rib, while the inner 

 fibres pass vertically upwards along the tips of the transverse processes of the 

 lumbar vertebrae, from which they receive short tendons, and they sometimes give 

 tendinous slips to these processes. On the anterior and posterior surfaces of the 

 muscle are often seen ascending fibres from the transverse processes, which diverge 

 slightly l)efoi-e they are inserted into the lower border of the last rib. 



Nerve-supply. — From the twelfth thoracic and upper lumbar nerves, by fila- 

 ments which enter the muscle upon its anterior surface near its inner border. 



Action. — (1) It will draw downwards the last rib, and will therefore act as a 

 lateral flexor of the thorax as well as of the loAver thoracic and the lumbar spine; 

 (2) it will assist in expiration by drawing* down the last rib; (3) taking its fixed 

 point from the last ril), it will draAv upwards the crest of the ilium, and so act as 

 a lateral flexor of the pelvis upon the thorax. 



Relations. — In front lie the kidney, the colon, the psoas, the lumbar arteries 

 and nerves, separated from it by the anterior layer of the luml)ar fascia. Behind, 



