LUMBAR PLEXUS. 719 



a fine communicating branch of the twelfth thoracic nerve joins the first lumbar 

 nerve near its origin. 



Of the nerves in question the first sacral is generally the largest in size, the 

 nerves diminishing gradually above and rapidly below this nerve. The plexus, 

 for the most part, forms the nerves destined for the supply of the lower limb. In 

 addition, however, nerves arise at its superior limit which are distributed to the 

 trunk above the level of the -limb, and at the inferior end of the plexus nerves 

 arise for the supply of the perineum. 



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

 in position and course of some of the nerves emanating from it, the plexus is sub- 

 divided into three subordinate parts lumbar, sacral, and pudendal plexuses. 

 There is, however, no strict line of demarcation between the three parts. 



Plexus Lumbalis. The lumbar plexus is formed by the first four lumbar 

 nerves, and is often joined by a branch from the twelfth thoracic nerve as well. 

 It is limited below by the fourth lumbar nerve (n. furcalis), which enters also 

 into the composition of the sacral plexus. The nerves of the lumbar plexus are 

 formed in the loin, and supply that region as well as part of the lower limb. 

 They are separated from the nerves of the sacral portion of the plexus by the 

 articulation of the hip bone with the sacrum. 



Plexus Sacralis. The sacral plexus is formed by the fourth and fifth lumbar, 

 and the first two or three sacral nerves. It is generally limited below by the third 

 sacral nerve (n. bigeminus), which assists also in forming the pudendal plexus. The 

 nerves of the sacral plexus are placed on the posterior wall of the pelvis, and are 

 destined almost entirely for the lower limb. 



Plexus Pudendus. The pudendal plexus is formed by the second, third, 

 fourth, and fifth sacral nerves, and the minute coccygeal nerve. It is placed on 

 the posterior wall of the pelvis and supplies branches mainly to the perineum. 



Communications with the Sympathetic. Each of these nerves has communica- 

 tions with the gangliated trunk of the sympathetic in the abdomen and pelvis. 



Gray Kami Communicantes. From the lumbar and sacral ganglia long slender gray 

 rami Communicantes are directed backwards and laterally over the bodies of the vertebrae, 

 and (in the lumbar region) beneath the origins of the psoas muscle, to reach the spinal 

 nerves. These branches are irregular in their arrangement. A given nerve may receive 

 branches from two ganglia, or one ganglion may send branches to two nerves. The rami 

 are longer in the loin than in the pelvis, owing to the projection of the lumbar portion of 

 the vertebral column. 



White Rami Communicantes. Certain lumbar and sacral nerves are also connected 

 with the abdominal and pelvic sympathetic by means of white rami Communicantes. From 

 the first two, and possibly in some cases also the third and fourth lumbar nerves, white 

 rami Communicantes are directed forwards, either independently or incorporated with the 

 corresponding gray rami, to join the upper part of the lumbar sympathetic trunk. The fifth 

 lumbar nerve and the first sacral nerves are unprovided with white rami Communicantes. 

 From the anterior rami of the second and third, or third and fourth sacral nerves, white 

 rami (visceral or splanchnic branches) pass medially, and, crossing over (without joining) 

 the sympathetic trunk, enter the pelvic plexus of the sympathetic. The fifth sacral and 

 coccygeal nerves possess no white rami Communicantes. 





PLEXUS LUMBALIS. 



The lumbar plexus is formed by the anterior rami of the first three and a part 

 of the fourth lumbar nerves, with the addition, in some cases, of a small branch 

 from the twelfth thoracic nerve. The nerves increase in size from above down- 

 wards (Fig. 624). 



Position and Constitution. The plexus is formed in the substance of the psoas 

 muscle, in front of the transverse processes of the lumbar vertebrae. The nerves, 

 on emerging from the intervertebral foramina, are connected as above described 

 with the sympathetic system, and then divide in the following manner in the sub- 

 stance of the psoas major muscle. The first and second nerves divide into superior 



