TUK l.l'MHAR I'Ll'IXUS 935 



THK LUMBO-SACRAL I'LKXl s 



The lumbo-saeral plexus is formed by the union of the anterior primary divisions of 

 the lumbar, sacral, and coccyneal nerves. In about 50 per cent, of eases it rec<v 

 branch from the txvelth thoracic nerve. 



Partly for convenience of description and partly on account of the differences in 

 posit ion and course of MI me of the nerves arising from it t his plexus is subdivided into 

 four parts the lumbar, sacral, pudendal, and coecygeal ple\u>es. These plexuses 

 overlap so that there is no definite line of demarcation between them. However, 

 they will be considered separately. 



THE L UMBAR M-:i! \ 'ES 



The anterior primary divisions of the five lumbar nerves increase in size from 

 the first io the last. Kach lumbar nerve is connected by one or two long, slender 

 branches with a lumbar sympathetic ganglion. The first three and the greater 

 part of the fourth enter into the formation of the lumbar plexus, and the smaller 

 part of the fourth and the fifth nerve commonly unite to form the lumbo-sacral cord 

 which takes part in the formation of the sacral plexus (figs. 687 and 688). 

 When the fourth nerve enters into the formation of both lumbar and sacral plexuses, 

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

 t hat 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 classi- 

 fication is the relation of the nerves of the limb 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 

 nrxii. However, in the more marked degrees of variation the origin of a given per- 

 ipheral 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 limb is placed a segment more proximal 

 than in the ordinary cases, and when the distal condition is present, that the 

 limb 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 

 Bardeen. 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: 



COMPOSITION OF THE NERVES OF THE LUMBAR PLEXUS 



RANGE OF VARIATION 

 NERVE. PROXIMAL. ORDINARY. DISTAL. 



Lateral (external) cutaneous 12 T, 1,2, 3 L. 1,2, 3.4 L. 1, 2, 3, 4 L. 



Femoral (anterior crural) . . 12 T, 1 , 2, 3, 4 L. 1 . 2. 3, 4 L. 1, 2, 3, 4, 5 L. 



Obturator 1.2.3. 4 L. 1,2, 3,4 L. 2, 3, 4, 5 L. 



Furcal 3 or 3, 4 L. 4 L. 4. 5 or 5 L. 



