770 THE NERVES. 



LUMBO-SACEAL PLEXUS. 



The last two lumbar pairs, and the three first sacral, in becoming fused 

 together, form the lumbo-sacral plexus, which corresponds in every respect, 

 by its constitution, as well as by its mode of distribution, to the plexus of 

 the thoracic limb. 



It is usual, in human anatomy, to describe a lumbar and a sacral plexus, 

 each formed by the inferior branches of all the spinal pairs, whose names 

 they bear. In our opinion, this proceeding has two inconveniences : at first, 

 it separates into two fasciculi the nerves of the abdominal member, and, 

 besides, in including in the description of these nerves the first lumbar pair 

 and the last sacral, elements are introduced in this description which are 

 altogether foreign to it. It may be remarked, that the four first lumbar 

 pairs, when they anastomose with each other, only do so by very slender 

 filaments ; that they only send some subcutaneous filaments to the posterior 

 limb ; that the two last sacral branches, principally destined to the genito- 

 urinary organs and the posterior extremity of the digestive tube, are 

 ordinarily without any direct communication with the others ; that the two 

 last lumbar pairs and the three first sacral are alone fused in the same manner 

 <is the brachial plexus, and comport themselves like that plexus in the distribution 

 of their branches. 



It is with some reason, then, that we have described, in a special manner, 

 the inferior branches of the four lumbar pairs and the two last sacral, 

 reserving the fasciculus formed by the five intermediate pairs for a special 

 description, under the name of the lumbo-sacral plexus. 



Mode of constitution. In glancing at this plexus, we may perceive that 

 it is divided into two portions an anterior and posterior, each having a 

 thick trunk in the centre. 



The first of these trunks is formed by the two above-named lumbar pairs, 

 which join each other after a short course, and after receiving an accessory 

 branch from the fourth pair. The second, wider and thinner than the 

 preceding, comprises the fibres of the three sacral pairs which escape from 

 beneath the subsacral vessels, and unite in a single fasciculus. These two 

 trunks are connected with each other by one or two branches proceeding 

 from the first sacral pair to the obturator nerve one of the distributive 

 branches of the first. 



Relations. The anterior portion of the lumbo-sacral plexus is concealed 

 beneath the small psoas muscle, and separated by the internal iliac artery 

 from the posterior portion. The latter, placed above and on the side of the 

 pelvis, at the great sciatic opening, corresponds, inwardly, to the subsacral 

 vessels ; outwardly, and in front, to the gluteal vessels. 



Mode of distribution. The anterior portion of the plexus at first gives off 

 several small branches to the psoas muscle, and particularly to the iliacus : 

 these branches were designated by Girard the iliaco-muscular nerves; it 

 then terminates in two large branches the crural and obturator nerves. The 

 posterior portion is continued by two important trunks, the great and small 

 femoro-popliteal nerves. At the base of the latter, it emits the anterior and 

 posterior gluteal nerves. These branches and their ramifications will be 

 successively studied. 



Preparation of the lumbo-mcral plexus. After removing the skin and abdominal 

 visce a, the hind quarters are isolated by sawing through the vertebral column behind 

 the last rib ; then, by means of a section almost in tne middle of the pelvis, one of the 

 limbs is cut off. and the pieces, disposed as in figure 355, should be maintained in th.j 



