596 THE NERVOUS SYSTEM. | 
Communications. — Each of these intercostal nerves communicates with the sym- 
pathetic cord and ganglia by two branches —a white ramus communicans to the 
corresponding sympathetic ganglion or the adjacent part of the sympathetic cord ; and a 
gray ramus communicans, which passes to each nerve from the corresponding ganglion. 
The seventh, eighth, ninth, tenth, and eleventh thoracic nerves only differ 
from the preceding nerves in regard to a part of their course and distribution. 
Each has the same course and communications as the preceding nerves in 
the thoracic wall. In addition, these nerves have a further course and dis- 
tribution in the abdominal wall. Each nerve traverses its intercostal space in 
the way described. At the anterior end of the space, the nerve pierces the attach- 
ment of the diaphragm and the transversalis abdominis muscles to the costal 
cartilages, and courses forwards in the abdominal wall between the transversalis 
and obliquus internus muscles. The nerve then passes between the rectus muscle 
and the posterior layer of its sheath, and eventually reaches the anterior abdominal 
wall and becomes cutaneous by piercing the rectus itself and the anterior layer 
of its sheath. 
Muscular Branches.—The lower intercostal nerves supply the intercostal 
muscles of the spaces in which they he; and in the abdominal wall they innervate 
the transversalis, obliqui, and rectus abdominis. The branches arise from the 
main trunk as well as from its lateral and anterior branches. (The ninth, tenth, 
and eleventh nerves are described as assisting in the innervation of the diaphragm 
by communications with the phrenic nerve.) 
Cutaneous Branches.—These are lateral and anterior. The lateral branches 
divide into anterior and posterior parts, and, becoming superficial along the line of 
inter-digitation of the obliquus externus muscle with the serratus magnus and 
latissimus dorsi, they are directed more obliquely downwards than the lateral 
branches of the higher intercostal nerves, and are distributed to the skin of the 
loin as low down as the buttock. The lateral branch of the eleventh nerve can be 
traced over the iliac crest (Fig. 438). 
The anterior branches are small. That of the seventh nerve innervates the 
skin at the level of the ensiform cartilage. The eighth and ninth appear between 
the ensiform cartilage and the umbilicus; the tenth nerve supphes the region of the 
umbilicus; and the eleventh, the area immediately below the umbilicus. 
The cutaneous branches of these nerves, including the posterior primary divisions, thus supply 
continuous belts of skin, which can be mapped out on the body from the vertebral column 
behind to the middle line in front. These areas are not placed horizontally, but tend to be 
drawn downwards as the series is followed from the upper to the lower nerves. 
The twelfth thoracic nerve is peculiar in its course and distribution. It 
emerges below the last rib (Fig. 459), and passes outwards and downwards in the 
posterior abdominal wall under cover of the psoas muscle, and between the external 
arcuate ligament and the quadratus lamborum muscle; it pierces the transversalis 
muscle, and courses forwards in the interval between it and the obliquus internus 
to the sheath of the rectus muscle. After piercing the posterior layer of the 
sheath, the rectus muscle, and the anterior layer of the sheath, it terminates by 
supplying the skin of the anterior abdominal wall midway between the umbilicus 
and the pubis. The branches of the nerve are muscular, to the transversalis, 
obliqui, rectus, and pyramidalis muscles of the abdominal wall, and cutaneous 
branches, two in number—an anterior terminal branch, which supplies the skin of 
the anterior abdominal wall midway between the umbilicus and the pubis, and a 
large lateral cutaneous (iliac) branch, which, passing obliquely downwards through 
the lateral muscles of the abdominal wall, becomes superficial above the iliac crest, 
a couple of inches behind the anterior superior spine. It supplies the skin of the 
buttock as far down as a point below and in front of the great trochanter of the 
femur (Fig. 442, p. 603). 
The twelfth thoracic nerve, in many cases, receives a communicating branch from the eleventh, 
near its origin, and still more frequently sends a fine branch to join the origin of the first 
lumbar nerve in the psoas muscle. It may communicate also with the ilio-hypogastric nerve, as 
taey lie together in the abdominal wall. 
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