THE LUMBOSACRAL PLEXUS. 875 



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 psoas parvus 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 sacro-sciatic foramen, corresponds, inwardly, to the subsacral vessels ; out- 

 wardly, 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 muscles, and particularly to the iliacus — these 

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

 nates in two large branches— the crural and obturator nerves. The posterior por- 

 tion is continued by two important trunks— the great and small femoro-popliteal 

 n&rves. 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-sacral plexus.— After removing the skin and abdominal viscera, the 

 hind quarters are isolated by sawing through the vertebral column behind the last rib ; then, by- 

 means of a section almost in the middle of the pelvis, one of the limbs is cut off, and the pieces, 

 disposed as in Fig. 473, should be maintained iu the first position— that is, with the croup 

 resting on the dissecting-table near one of the bars, and the limb suspended vertically, the foot 

 upwards, by a cord attached to the ring of the bar. 



Afterwards, the preparation is executed in two stages. In the first, after the excision of 

 the pelvic organs and the psoas parvus muscle, the whole of the plexus and its formative 

 branches are dissected, taking Fig. 373 as a guide. In the second, the posterior part of the 

 plexus, with the nerves it gives off, are exposed on the external side, by excising the greater 

 portion of the principal gluteal muscle and the anterior portion of the biceps femoris, as in 

 Fig. 474. 



To follow the various divisions of the nerves emanating from the plexus, to their termma- 

 tioas, it is well to use the other limb, which, not being fixed, can be laid on a table, and in this 

 way is uiore convenient than the first for this part of the operation. 



A. Anteeior Portion. 

 1. Iliaco-muscular Nerves. 



These nerves are of little importance. The principal one accompanies the 

 iliaco-muscular artery across the iliacus muscle. 



2. Crural or Anterior Femoral Nerve (Fig. 473, 2). 



This is the largest of the branches arising from the anterior portion of the 

 plexus. It descends between the psoas magnus and parvus, to the common conical 

 extremity of the latter muscle and the iliacus, where it is covered by the sartorius ; 

 there it terminates in a wide tuft of branches, which, pass to the rectus femoris 

 and vastus internus. 



Below the sartorius, it successively emits two long branches, which deserve a 

 particular description. 



The first represents the fasciculus which, in Man, includes the crural musculo- 

 cutaneous branch. We have named it the accessory branch of the internal saphenic 

 nerve. It reaches the interstice between the sartorius and gracilis muscles, in 

 crossing the crural vessels very obhquely forward. Leaving this space, it becomes 

 subcutaneous, in forming numerous divisions which surround the saphena artery 

 and vein. 



The second, or internal saphenic nerve, passes at first between the sartorius 

 and vastus internus, and parallel to the first, which is situated more inwardly and 



