1194 SURFACE AND SURGICAL ANATOMY. 
point of view the costo-ilace space may be said to be limited imternally by the outer 
edge of the erector spine, and, more deeply, by the tips of the transverse processes 
of the lumbar vertebrae, while externally it is bounded by the posterior free border 
of the external oblique, and, more deeply, by the line of reflexion of the peritoneum 
from the colon on to the lateral wall of the abdomen. The space is roofed over by — 
the latissimus dorsi, except below, where a narrow triangular interval is left between 
its outer border and the posterior border of the external oblique, the base of the 
triangle being formed by the crest of the ium, a little behind its centre. This 
triangle (Fig. 250, p. 339), known as the “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 posticus inferior, another triangle will be exposed, 
which constitutes a second weak area in the loin: Tihs bounded above by the last 
rib, internally by the outer border of the erector spine, and externally by the 
posterior muscular fibres of the internal oblique; the floor of the triangle is formed 
by the aponeurosis of origin of the transversalis abdominis muscle. At the outer 
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 erector spine. 
Kidneys.—By retracting inwards the erector spine and dividing the posterior 
fibres of the three abdominal muscles, along with the outer fibres of the quadratus, 
the kidney may be exposed, with the colon lying vertically in front of its outer border. 
The upper limit of the kidney i is indicated by a line drawn transv ersely across the lom 
opposite the eleventh dorsal spine, the lower limit by a line on a level with the 
third lumbar spine. The upper extremity reaches to the eleventh rib; the lower, 
which lies immediately external to the tip of the transverse process of the third 
lumbar vertebra, reaches to within 14 to 2 in. of the crest of the ihum. About 
a third of the kidney lies above the lower margin of the twelfth rib. The left 
kidney usually lies about 4 in. higher than the right. The most external poimt of 
the outer border les 4 in. from the middle line, w ‘hile the hilus lies 14 in. external 
to it in front of the interval between the tips of the transverse processes of the 
first and second lumbar vertebre. 
The psoas muscle intervenes between the postero-internal surface of the kidney 
and the transverse processes, and protects the organ from injury by a blow directed 
from the front. Between the upper end of the kidney and the eleventh and twelfth 
ribs is the diaphragm and the posterior costo- diaphragmatic reflexion of the pleura. 
The relations of the pleura to the last rib have already been considered (p. 1188). 
Posteriorly the course of the upper part of the ureter may be indicated by a 
line drawn vertically upwards from the posterior superior 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 vertebre, covered by the psoas muscle. 
A needle passed through the inner extremity of the eleventh intercostal space 
will transfix the suprarenal body. 
The pus of a perinephritic abscess occuples the extraperitoneal fatty layer (peri- 
renal fat), and lies, therefore, within the fascial envelope of the abdomen ; the pus in 
a psoas abscess, on the other hand, hes external to the fascia. In opening a psoas 
abscess from behind, a vertical incision is made in the angle formed by the outer 
border of the erector spine and the crest of the ilium; in the deeper part of the 
dissection the surgeon should keep close to the front of the transverse process of 
the fourth lumbar vertebra. 
Diaphragm, Liver, Stomach, and Large Intestine.—Posteriorly the right 
arch of the diaphragm ond the right lobe of the liver extend upwards to the level 
of the angle of the scapula (eighth rib), while the left arch and the fundus of the 
stomach lie 1 in. lower ( eighth interspace); the central tendon reaches up to 
the eighth dorsal spine. The right lobe of the liver is covered posteriorly by the 
eighth to the twelfth ribs, and is overlapped by the base of the right lung as far as 
a line drawn horizontally outwards from the tenth dorsal spine ; hence, posteriorly, 
the upper limit of the liver cannot be defined by percussion, and its lower limit 
merges into the dulness of the lom muscles and kidney. 
