THE ABDOMINAL WALLS 251 



Their nerve - supply indicates that the muscles of the 

 abdominal wall have arisen from the fusion of the muscles of 

 six or seven adjoining segments, which were originally quite 

 independent. In consequence of its segmental character the 

 rectus abdominis may be cut across in operations and reunited 

 with much less functional loss than results from section of 

 muscles which have not arisen in this way (e.g. biceps brachii). 

 In addition, one or more adjoining segments may become 

 contracted, while the rest of the muscle is relaxed. Such 

 localised areas of contraction may be due to cramp, but they 

 are frequently found in connection with pathological conditions 

 of the abdominal (p. 354) or even thoracic viscera. These 

 cases are examples of the " viscero-motor reflex " (Mackenzie). 

 The afferent impulses from the viscus at fault to the central 

 nervous system not only set up the viscero-sensory reflex 

 (p. 242) but they may also " overflow " into the motor cells of 

 the anterior column (horn) of the grey matter. The efferent 

 fibres of these cells supply the corresponding muscle segment, 

 and in this way the localised contracted areas may be explained 



(Kg. 73). 



In making incisions through the abdominal wall it is 

 important that the position and course of the nerves should 

 be borne in mind, for although one may be injured without any 

 undesirable after-effects, the abdominal wall never completely 

 recovers its muscular tone after section of the motor nerves. 

 For this reason, when possible, the incisions are planned so as 

 to avoid injuring the nerves, e.g. the median and gridiron 

 incisions. In opening the abdomen through the rectus abdominis 

 the muscle should, when possible, be retracted to the lateral 

 side for the same reason. 



A large rectangular incision is employed by Perthes in 

 order to expose the region of the gall-bladder and the bile-duct. 

 The vertical limb begins a little below the xiphoid process, and 

 descends one finger's breadth to the right of the median plane, 

 almost to the umbilicus. The horizontal limb passes to the 

 right from the lower end of the vertical limb. The anterior wall 

 of the rectus sheath is divided in the line of the vertical part of 

 the incision, and the medial border of the muscle is defined. 

 The finger can then be inserted behind the rectus so as to 

 separate it from the posterior wall of its sheath. 



Two transverse rows of sutures are passed through the 

 anterior wall of the sheath and through the muscle, in the line 



