420 MYOLOGY 



to the iliolumbar ligament; above, to the apex and lower border of the last rib. 

 The upper margin of this fascia, which extends from the transverse process of the 

 first lumbar vertebra to the apex and lower border of the last rib, constitutes the 

 lateral lumbocostal arch (page 405). Laterally, it blends with the lumbodorsal 

 fascia, the anterior layer of which intervenes between the Quadratus lumborum 

 and the Sacrospinalis. 



The Quadratus lumborum (Fig. 389, page 398) is irregularly quadrilateral in 

 shape, and broader below than above. It arises by aponeurotic fibers from the 

 iliolumbar ligament and the adjacent portion of the iliac crest for about 5 cm., 

 and is inserted into the lower border of the last rib for about half its length, and 

 by four small tendons into the apices of the transverse processes of the upper four 

 lumbar vertebrae. Occasionally a second portion of this muscle is found in front 

 of the preceding. It arises from the upper borders of the transverse processes of 

 the lower three or four lumbar vertebrae, and is inserted into the lower margin of 

 the last rib. In front of the Quadratus lumborum are the colon, the kidney, the 

 Psoas major and minor, and the diaphragm; between the fascia and the muscle 

 are the twelfth thoracic, ilioinguinal, and iliohypogastric nerves. 



Variations. The number of attachments to the vertebrae and the extent of its attachment to 

 the last rib vary. 



Nerve Supply. The twelfth thoracic and first and second lumbar nerves supply this muscle. 



Actions. The Quadratus lumborum draws down the last rib, and acts as a muscle of inspira- 

 tion by helping to fix the origin of the diaphragm. If the thorax and vertebral column are 

 fixed, it may act upon the pelvis, raising it toward its own side when only one muscle is put in 

 action; and when both muscles act together, either from below or above, they flex the trunk. 



V. THE MUSCLES AND FASCIA OF THE PELVIS. 



Obturator internus. Levator ani. 



Piriformis. Coccygeus. 



The muscles within the pelvis may be divided into two groups : (1) the Obturator 

 internus and the Piriformis, which are muscles of the lower extremity, and will be 

 described with these (pages 476 and 477) ; (2) the Levator ani and the Coccygeus, 

 which together form the pelvic diaphragm and are associated with the pelvic viscera. 

 The classification of the two groups under a common heading is convenient in 

 connection with the fasciae investing the muscles. These fasciae are closely related 

 to one another and to the deep fascia of the perineum, and in addition have special 

 connections with the fibrous coverings of the pelvic viscera; it is customary there- 

 fore to describe them together under the term pelvic fascia. 



Pelvic Fascia. The fascia of the pelvis may be resolved into: (a) the fascial 

 sheaths of the Obturator internus, Piriformis, and pelvic diaphragm; (b) the 

 fascia associated with the pelvic viscera. 



The fascia of the Obturator internus covers the pelvic surface of, and is attached 

 around the margin of the origin of, the muscle. Above, it is loosely connected to 

 the back part of the arcuate line, and here it is continuous with the iliac fascia. 

 In front of this, as it follows the line of origin of the Obturator internus, it gradually 

 separates from the iliac fascia and the continuity between the two is retained only 

 through the periosteum. It arches beneath the obturator vessels and nerve, com- 

 pleting the obturator canal, and at the front of the pelvis is attached to the back 

 of the superior ramus of the pubis. Below, the obturator fascia is attached to the 

 falciform process of the sacrotuberous ligament and to the pubic arch, where it 

 becomes continuous with the superior fascia of the urogenital diaphragm. Behind, 

 it is prolonged into the gluteal region. 



The internal pudendal vessels and pudendal nerve cross the pelvic surface of 



