THE LUMBAR PLEXUS. 



1319 



result from this disease, and an abdominal lesion may be suspected ; this has 

 occurred more particularly in children. A feeling of tightness is sometimes observed 

 about the abdomen, corresponding to the course of one or more pairs of these 

 nerves, and may be due to impaired sensation in them. Since the abdominal 

 muscles are supplied chiefly by the seven lower intercostal nerves, they are 

 concerned in respiration. When they are contracted as in general peritonitis, the 

 lower ribs become immobile, and breathing takes place chiefly in the upper portion 

 of the chest. 



FIG. i i 06. 



THE LUMBAR PLEXUS. 



The lumbar plexus (plexus lumbalis) lies in the substance of the psoas magnus 

 muscle, anterior to the transverse processes of the lumbar vertebrae, and consists of 

 a series of loops formed by the anterior primary divisions of the first, second and 

 third lumbar nerves, the smaller subdivision of the fourth lumbar and sometimes a 

 branch from the twelfth thoracic nerve. The remainder and major portion of the 

 fourth lumbar nerve unites with the entire anterior primary division of the fifth to 

 form a conjoint trunk, the lumbo-sacral cord (truncus lumbosacralis), which 

 passes into the pelvis to become a constituent of the sacral plexus (Fig. 1106). 

 The lumbar nerves increase in thickness from above downward, the first being only 

 2.5 mm., while the fifth attains a diameter of 7 mm. The length of the nerves from 

 their exit at the intervertebral foramina to their point of division varies considerably, 

 in the case of the first being i mm. or less, of the second 10 mm. and of the third 

 from 20-25 mm. 



Constitution and Plan. In forming the plexus (Fig. 1106), the first lumbar 

 nerve divides almost immediately after its exit from the vertebral column into an 

 upper and a lower branch. The upper, 

 which may receive a contribution from 

 the twelfth thoracic nerve, becomes the 

 ilio-hypogastric'd3\& ilio-inguinal nerves. 

 The lower branch, near the body of the 

 second lumbar vertebra joins the upper 

 part of the second lumbar nerve, which, 

 like the first, divides into an upper and 

 a lower branch. The union of the lower 

 branch of the first and the upper branch 

 of the second results in the formation of 

 the genito-crural nerve. Sometimes 

 fibres from the first aid in the formation 

 of the anterior crural and obturator 

 nerx r es. The lower branch of the second, 

 all of the third and that part of the 

 fourth which enters the lumbar plexus 

 divide into smaller anterior and larger 

 posterior trunks. From the union of 

 the anterior branches of these three the 

 obturator nerve is formed, and from the 

 union of the posterior results the an- 

 terior crural nerve. The posterior por- 

 tions of the second and third nerves 

 give off from their dorsal aspect small 



Diagram illustrating plan of lumbar plexus. 



branches which unite into the external 



cutaneous nerve. The accessory obturator 



nerve, when it exists, arises from the third and fourth lumbar between the roots of 



the anterior crural and obturator nerves. 



Communications. All of the lumbar nerves receive gray rami communicantes 

 from the gangliated cord of the sympathetic ; and from the first and second, and 

 possibly the third and fourth, white rami communicantes pass to the lumbar portion 

 of the gangliated cord. 



