42 EXERCISE VII 



Note the lumbo-dorsal aponeurosis (PI. Ill, fig. 1) covering the back 

 muscles ; from it slopes headward and ventrally the latissimus dorsi and 

 posteriorly and ventrally the obliquus externus of the abdomen. The 

 latter muscle is partly overlapped by latissimus dorsi. Through the external 

 oblique emerge in parallel series 3 nerves with accompanying blood-vessels. 

 The most anterior of these is a branch of the last thoracic nerve. 



Along the line followed by the longitudinal skin incision cut through 

 latissimus dorsi and the spinal aponeurosis, and reflect these to either side 

 (PI. Ill, fig. 2). The longissimus dorsi is thus exposed. 



With the handle of the scalpel follow out the interval between the 

 external oblique and longissimus dorsi ; the main trunks of the nerves and 

 vessels whose branches were previously seen will thus be followed ; the most 

 anterior of these nerves is the last (13th) thoracic (13, PI. Ill, fig. 2) ; in the 

 line of incision it lies a full centimetre behind the last rib. Follow it up in 

 the direction of the vertebral column and sever it as far proximal as may be. 

 Deep to this nerve a thin sheet of muscle is seen, its fibres slanting headward 

 and ventral (upward and to right). Cut it free from its attachment to the 

 last rib and reflect it towards the spine. 



Posterior to this muscle the aponeurosis of the muscles of the abdominal 

 wall is thin for a short area, but becomes thicker farther back. The 

 splanchnic nerve lies deeply under this aponeurosis. Feel along the lateral 

 edge of longissimus dorsi for the edges of the transverse processes (PI. Ill, 

 fig. 3) of the 1st, 2nd, 3rd, and 4th (PL III, fig. 1) lumbar vertebrae. The 

 ends of these processes form an interrupted ridge (in the position shown in 

 PI. Ill, fig. 1). The aponeurosis of origin of the muscles of the abdominal wall 

 takes its dorsal attachment from this ridge. 



Retract the longissimus dorsi as far dorsally as practicable, and detach 

 the abdominal aponeurosis from the ridge from 1st transverse process in front 

 to 4th behind. Reflect the abdominal aponeurosis ventrally (to animal's left). 

 A large lengthwise-running muscle overhung by the bony transverse 

 processes is now seen; this is the psoas. Through the thin transparent 

 membrane of the peritoneum ventral to psoas certain of the contents of the 

 abdomen are now visible (PI. Ill, fig. 3). 



Note the somewhat pale kidney with some veins radiating over it ; 

 posterior and dorsal to this some fatty tissue. Forward, emerging from 

 below the last rib, is the posterior edge of the dark-tinted liver, moving freely 

 forward and backward with the respiratory movements. 



Push carefully with the scalpel handle the peritoneum and anterior end of 

 kidney laterally away from the psoas and explore the ventral face of that 

 muscle. Make out deep in the anterior end of the wound a strong muscular 



