320 SURGICAL APPLIED ANATOMY. [Chap. xvi. 



(Shaw). The pus, following the muscle, at last reaches 

 the thigh, and usually points, just below the groin, to 

 the outer side of the femoral vessels. 



The abscess, however, often shows much variation. 

 It may avoid the psoas, or leave it when once it has 

 entered it, and make its way into the lumbar region, 

 to find an exit in the loin. Or it may extend into the 

 iliac fossae, and open above the groin, or mount up over 

 the iliac crest, and discharge in the gluteal region. 

 It may pass along the inguinal canal, and be mis- 

 taken for- a hernia. It may sink into the pelvis, and 

 may open into the bladder, or discharge itself through 

 the great sciatic foramen, or through a sinus in the 

 perineum. Some of the latter cases have led to much 

 confusion in diagnosis, since there would appear to be 

 little connection between caries of the spine and a 

 perineal abscess. 



Lumbar region. The muscles that form the 

 lateral and posterior walls of the abdomen,- and that fill 

 in the interval between the iliac crest and the lowest 

 rib, are the external oblique and latissimus dorsi, 

 the internal oblique, the transversalis muscle and 

 fascia lumborum, the erector spime and quadratus 

 lurnborurn. 



The external oblique and latissimus dorsi muscles 

 are separated by a small triangular interval below (the 

 triangle of Petit), but above they overlap. The interval 

 between these two muscles may be represented by a 

 line drawn vertically, upwards from the middle of the 

 crest of the ilium. The outer edge of the quadratus 

 lumborum corresponds to this line, just above the iliac 

 crest ; but as the border of the muscle slopes upwards 

 and backwards, it may be about an inch behind the 

 line, at a point midway between the crest and the last 

 rib. The edge of the muscle is overlapped by the 

 internal oblique, and its inner half or two-thirds is 

 overlapped by the erector spiuae. The subcutaneous 



