THE LUMBAR PLEXUS 163 



forms a horizontal strip round the body. In the girdle pains 

 of tabes dorsalis, the pain is experienced in the same hori- 

 zontal strips, but it is not uncommon for several adjoining 

 areas to be affected at the same time. 



Areas of hyperaesthesia are frequently found in the skin of 

 the abdominal wall, and, like the areas of localised muscular 

 contraction, they are due to the presence of a " focus of 

 irritation" (p. 195) in the spinal medulla. The level at which 

 they occur may be of help in determining the diagnosis. It 

 is useful, for this purpose, to remember that the umbilicus lies 

 in the zone supplied by the tenth thoracic nerve, and that the 

 first lumbar nerve is restricted to a very small area in the lowest 

 part of the abdominal wall. 



THE LUMBAR PLEXUS 



The Anterior Kami of the upper four Lumbar Nerves take 

 part in the formation of the Lumbar Plexus. After leaving 

 the vertebral canal, they enter the substance of the psoas 

 major muscle, where the plexus is formed. Cases of psoas 

 abscess in which the muscle is infiltrated by pus may be ac- 

 companied by muscular paralysis or by pain which is referred 

 to the peripheral distribution of the sensory branches of the 

 plexus. 



The Ilio-hypogastric and the Ilio-inguinal Nerves arise 

 from the first lumbar nerve, very often by a common trunk. 

 They appear at the lateral border of the psoas major and run 

 laterally, at first posterior to the kidney. In this part of their 

 course they may be compressed by tumours of the kidney, and 

 the pain is referred to their distribution. 



The ilio-hypogastric nerve gives off an iliac branch, which 

 crosses the iliac crest to supply the skin over the lateral part of 

 the buttock, and it then runs forwards, terminating by supply- 

 ing the skin over the lower part of the rectus muscle (Fig. 69). 

 In addition to its sensory branches, the nerve usually helps 

 to supply the lateral muscles of the abdominal wall. 



