OF THE PELVIC OUTLET 



441 



Erector spinae. The Quadratus lumborum extends, however, beyond the outer border of the 

 Erector spinae. 



Nerve-supply. The anterior branches of 'he last thoracic and the first lumbar nerves; 

 sometimes also a branch from the second lumbar nerve. 



Actions. The Quadratus lumborum draws down the last rib. It acts as a muscle of inspi- 

 ratian 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. 



IV. MUSCLES AND FASCIA OF THE PELVIC OUTLET. 



The muscles and fasciae of the pelvic outlet are those structures which form the 

 floor of the pelvis. Perineum is a term used to designate that segment of the 

 structure lying dorsal to the pubic arch and subpubic ligament and ventral to 

 a line drawn transversely, in front of the anus, between the anterior part of the 

 tuberosity of the ischium of each side. The ischiorectal space is the name given 

 to the segment dorsal to the line above mentioned and ventral to the tip of the 

 coccyx. This space is bounded laterally by the Gluteus maximus muscle. 



FIG. 333. The perineum. The integument and superficial layer of superficial fascia reflected. 



The fascia of the pelvic outlet is most easily understood if it is considered in 

 the order of its dissection. Considered thus, it resolves itself into three strata 

 the superficial fascia, the triangular ligament, and the pelvic fascia. The super- 

 ficial fascia is made up, as in other regions of the body, of a superficial and a deep 

 layer. The fascia over the ischiorectal region is arranged in fatty layers, which 

 fill the ischiorectal fossa on each side of the rectum and anus. In the region 

 of the perineum the fascia requires fuller consideration; here it is definitely 

 arranged into two layers superficial and deep. The superficial layer is thick, ' 

 loose, areolar in texture, and, except toward the scrotum, contains in its meshes 



