998 



THE NERVOUS SYSTEM 



it may be called the furcal nerve, but this name is also applied to any of the 

 nerves that enter into the formation of both plexuses, so there may be one or more 

 furcal nerves. 



THE LUMBAR PLEXUS 



Although the lumbar plexus is ordinarily formed by the first three lumbar 

 nerves and a part of the fourth, yet it is subject to considerable variation in the 

 manner of its formation. 



Owing to this variation three general classes of plexuses may be found, proximal or pre- 

 fixed, ordinary, and distal or post-fixed. The basis of classification is the relation of the nerves 

 of the Hmb to the spinal nerves which enter into their formation. The intermediate or slighter 

 degrees of variation may consist only of changes in the size of the portions contributed by the 

 different spinal nerves to a given peripheral nerve, for a given nerve may receive a larger share 

 of its fibres from a more proximal spinal nerve, and a smaller share from a more distal nerve, or 

 vice versa. However, in the more marked degrees of variation the origin of a given peripheral 

 nerve may vary in either direction to the extent of one spinal nerve. The more extreme types 

 of the plexuses are sometimes associated with abnormal conditions of the vertebral column. 

 It has been suggested that when the prefixed or proximal condition occurs, it indicates that the 

 lower hmb is placed a segment more proximal than in the ordinary cases, and when the distal 

 condition is present, that the Jimb is arranged a segment more distal. Three types each of the 

 proximal and the distal classes and one type of the ordinary class have been described by Bar- 

 deen. His statistics are made use of in the compilation of the following tables, in which are 

 shown the range of variation and the common composition of each class of plexus:— 



Composition of the Nerves of the Lumbar Plexus 

 Range of Variation 



Nerve. 



Lateral (external) cutaneous 

 Femoral (anterior crural) . . . 



Obturator 



Furcal 



Proximal. 



12 T, 1, 2, 3 L. 



12 T, 1, 2, 3, 4 L. 



1, 2, 3, 4 L. 



3 or 3, 4 L. 



Ordinary. 



1, 2, 3, 4 L. 



1, 2, 3, 4 L. 



1, 2, 3, 4 L. 



4L. 



Distal. 



1, 2, 3, 4 L. 

 1, 2, 3, 4, 5 L. 



2, 3, 4, 5 L. 

 4, 5 or 5 L. 



Common Composition 

 Proximal. Ordinary. 



Distal. 



Nerve. 



Lateral (external) cutaneous 

 Femoral (anterior crural) . . 



Obturator 



Furcal 



The lumbar plexus lies in the posterior part of the psoas muscle (fig. 765), in 

 front of the transverse processes of the lumbar vertebrae and the medial border 

 of the quadratus lumborum, and its terminal branches are distributed to the 

 lower part of the abdominal wall, the front and medial part of the thigh, the 

 external genital organs, the front of the knee, the medial side of the leg, and the 

 medial side of the foot. 



The first and second of the lumbar nerves give collateral muscular branches to 

 the quadratus lumborum muscle, and the second and third nerves give similar 

 branches to the psoas. The remaining branches of the plexus are terminal 

 branches. The first lumbar nerve, after it has been joined by the branch from the 

 last thoracic nerve, divides into three terminal branches, the ilio-hypogastric 

 nerve, the ilio-inguinal nerve, and a branch which joins the second nerve. The 

 fibres of this latter branch pass mainly into the genito-femoral (genito-crural) 

 nerve, but occasionally some of them enter the femoral (anterior crural) and 

 obturator nerves. The remaining nerves divide into anterior or ventral and 

 posterior or dorsal divisions. The anterior divisions form a portion of the genito- 

 femoral (genito-crural) nerve and the obturator nerve, and the posterior divisions 

 enter the lateral (external) cutaneous and femoral (anterior crural) nerves. 



All the terminal branches of the plexus are formed in the substance of the 

 psoas muscle; four of them, the ilio-hypogastric, the ilio-inguinal, the lateral 

 (external) cutaneous, and tlie fciinoral (anterior crural), leave the muscle at its 

 lateral bord(!r. The genito-femoral (genito-crural) pass(!S through its anterior 

 surface, and 1 he; obturator through its nKnlial l)or(ler. 



Terminal branches. — The ilio-hypogastric nerve springs from the first 

 lumbar nerve, after the latter has been joined Ijy the communicating branch 



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