THE BACK. 1437 



Below the inferior angle of the scapula the last five ribs can readily be felt 

 lateral to the sacro-spinales muscle ; when the twelfth rib does not reach beyond 

 this muscle, the eleventh' rib will be mistaken for it, unless the ribs are counted 

 from above downwards. 



The inferior border of the trapezius is indicated by a line extending upwards and 

 laterally from the twelfth thoracic spine to the root of the spine of the scapula ; the 

 superior border of the latissimus dorsi by a line extending from the sixth thoracic spine 

 transversely laterally across the angle of the scapula. Between these two muscles and 

 the inferior part of the vertebral margin of the scapula is a triangular area, the floor 

 of which is formed by the rhomboideus major muscle and the sixth costal interspace. 



The lateral border of the sacrospinalis is indicated on the surface by drawing a 

 line from a point on the iliac crest 3| in. (four fingers' breadth) from the median line, 

 upwards and slightly laterally to the angles of the ribs. The lateral border of the 

 quadratus lumborum, which passes upwards and slightly medially, lies a little lateral to 

 that of the sacrospinalis at the crest, and a little medial to it at the twelfth rib. 



The anatomy of the muscles and fasciae which complete the abdominal wall 

 between the last rib and the iliac crest is of great importance in connexion with 

 operations in the region of the loin. The space between the last rib and the iliac 

 crest varies greatly according to the length of the rib, and according to the general 

 shape of the chest and slope of the ribs as a whole. As a rule, the tip of the 

 twelfth rib lies about two inches vertically above the centre of the iliac crest. 

 From a surgical point of view the costo-iliac space may be said to be limited medially 

 by the lateral edge of the sacrospinalis, and, more deeply, by the tips of the trans- 

 verse processes of the lumbar vertebrae, while laterally it is bounded by the posterior 

 free border of the external oblique, and, more deeply, by the line of reflection of the 

 peritoneum from the colon on to the side wall of the abdomen. The space is 

 roofed over by the latissimus dorsi, except below, where a narrow triangular interval 

 is left between its lateral border and the posterior border of the external oblique, 

 the base of the triangle being formed by the crest of the ilium, a little behind 

 its centre. This triangle, known as the lumbar triangle of Petit, represents a weak 

 area through which a lumbar abscess may come to the surface, and through which a 

 lumbar hernia occasionally develops. On removing the latissimus dorsi and the lower 

 part of the thin serratus posterior inferior, another triangle will be exposed, which 

 constitutes a second weak area in the loin ; it is bounded above by the last rib, medially 

 by the lateral border of the sacrospinalis, and laterally by the posterior muscular fibres 

 of the internal oblique ; the floor of the triangle is formed by the aponeurosis of 

 origin of the transversus abdominis muscle. At the lateral border of the quadratus 

 lumborum this aponeurosis splits into three layers to form two compartments, the 

 anterior enclosing the quadratus lumborum and the posterior the sacrospinalis. 



Kidneys. The superior limit of the kidney is indicated by a line drawn trans- 

 versely across the loin opposite the eleventh thoracic spine, the inferior limit by a 

 line on a level with the third lumbar spine. The superior extremity reaches to the 

 eleventh rib ; the lower, which lies immediately lateral to the tip of the transverse 

 process of the third lumbar vertebra, reaches to within 1 J to 2 in. of the crest of the 

 ilium. About a third of the kidney lies above the inferior margin of the twelfth 

 rib. The left kidney usually lies about J in. higher than the right. The most 

 lateral point of the lateral border lies 4 in. from the median plane, while the hilum 

 lies 1 \ in. lateral to the median plane in front of the interval between the tips of 

 the transverse processes of the first and second lumbar vertebrae. 



The psoas major muscle intervenes between the postero-medial surface of the 

 kidney and the transverse processes, and protects the organ from injury by a blow 

 directed from the front. Between the superior end of the kidney and the eleventh 

 and twelfth ribs is the diaphragm and the posterior costo-diaphragmatic reflection 

 of the pleura (Fig. 1110). The relations of the pleura to the last rib have already 

 been considered (p. 1401). 



Posteriorly the course of the superior part of the ureter may be indicated by a 

 line drawn vertically upwards from the superior posterior iliac spine to the level of 

 the second lumbar spine ; the deep guides are the tips of the transverse processes 

 of the second, third, and fourth lumbar vertebrae, covered by the psoas major muscle. 



