7 
598 THE NERVOUS SYSTEM. 
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 the first sacral. The plexus, 
for the most part, forms the nerves destined for the supply of the lower limb. In 
addition, however, nerves arise at its upper limit which are distributed to the trunk 
above the level of the limb, and at the lower 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 or sciatic, and pudendal 
plexuses. There is, however, no strict line of demarcation between these parts. 
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. furealis), which also enters into the composition of the 
sciatic or 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 in- 
nominate bone with the sacrum. 
The sacral or sciatic plexus is formed by the fourth and fifth lumbar, and 
first two or three sacral nerves. It is generally limited below by the third sacral 
nerve (n. bigeminus), which also assists 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. 
The pudendal plexus is formed by the second, third, fourth, and fifth sacral 
nerves, and the minute coccygeal nerve. It is placed on the back 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 cord of the sympathetic in the abdomen and pelvis. 
Gray Rami Communicantes.— From the lumbar and sacral ganglia long slender 
gray rami communicantes are directed backwards and outwards over the bodies of the 
vertebra, and (in the lumbar region) beneath the origins of the psoas muscle, to reach the 
anterior primary divisions of the 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 also the third and fourth lumbar nerves, white rami communi- 
cantes are directed forwards, either independently or incorporated with the corresponding 
gray rami, to join the upper part of the lumbar gangliated cord. The fifth lumbar nerve | 
and the first sacral nerves are unprovided with white rami communicantes. From the 
third, and sometimes also the second and fourth sacral nerves, white rami (visceral 
branches) pass inwards, and, crossing over (without joining) the gangliated cord, join the 
pelvic plexus of the sympathetic. The fifth sacral and coceygeal nerves possess no white 
rami communicantes. 
THE LUMBAR PLEXUS. 
The lumbar plexus is formed by the anterior primary divisions 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 
downwards (Fig. 440). 
Position and Constitution—The plexus is placed deeply in the substance of 
the psoas muscle, in front of the transverse processes of the lumbar vertebra. The 
nerves, on emerging from the intervertebral foramina, are connected as above de- 
scribed with the sympathetic system, and then divide in the following manner in 
the substance of the psoas muscle :—The first and second nerves divide into upper 
and lower branches. The upper branch of the first nerve (which may be joined by 
the branch from the twelfth thoracic nerve) forms two nerves, ilio-hypogastric and 
er 
