996 THE NERVOUS SYSTEM 



themselves between the interdigitating slips of the diaphragm and the transversus 

 abdominis and enter the abdominal wall. The seventh, eighth, and ninth nerves, 

 in their transit from the thoracic to the abdominal wall, pass behind the upturned 

 ends of the eighth, ninth, and tenth rib-cartilages respectively. Having entered 

 the abdominal wall the nerves run forward between the transversus abdominis 

 and the internal oblique muscles to the outer border of the rectus abdominis, 

 where they pierce the posterior lamella of the internal oblique aponeurosis and 

 enter the sheath of the rectus. In the sheath they pass through the substance of 

 the rectus. Finally they turn directly forward, pierce the anterior part of the 

 sheath, and become anterior cutaneous nerves of the abdomen. 



The muscular branches. — Muscular branches from all the thoraco-abdominal 

 nerv^es are distributed to the levatores costarum, the intercostal muscles, the 

 transversus abdominis, the internal oblique, and to the rectus abdominis, and the 

 ninth, tenth, and eleventh nerves gives branches also to the serratus posterior 

 inferior. Branches are also distributed from a variable number of the lower nerves 

 to the costal portions of the diaphragm. 



The last thoracic nerve. — The anterior primary division of the last thoracic 

 nerve is distributed to the wall of the abdomen and to the skin of the upper and 

 front part of the buttock. It appears in the thoracic wall immediately below the 

 last rib, where it communicates with the sympathetic cord and gives off a com- 

 municating branch to the first lumbar nerve. It passes from the thorax into the 

 abdomen beneath the lateral lumbo-costal arch (external arcuate ligament), 

 accompanied by the subcostal artery, and it runs across the upper part of the 

 quadratus lumborum dorsal to the kidney and to the ascending or the descending 

 colon according to the side considered. At the lateral border of the quadratus 

 lumborum it pierces the aponeurosis of attachment of the transversus abdominis 

 muscle and divides, between the transversus and the internal oblique muscle, into 

 a lateral and an anterior branch. It gives branches to the transversus abdominis, 

 the quadratus lumborum, and the internal oblique muscles. 



The anterior branch passes forward, between the internal obUque and the transversus 

 abdominis, to which it supphes twigs. It enters the sheath of the rectus, turns forward through 

 that muscle, and terminates in branches which become cutaneous midway between the umbilicus 

 and the symphysis. Before it becomes cutaneous it supphes twigs to the transversus abdominis, 

 the internal oblique, the rectus abdominis, and the pyramidalis muscles. 



The lateral branch pierces the internal oblique; it supphes the lowest digitation of the 

 external oblique, and then pierces the latter muscle from 2.5 to 8 cm. (1 to 3 in.) above the iliac 

 crest, and descends in the superficial fascia of the anterior part of the gluteal region, crossing the 

 iliac crest about 2.5 cm. (1 in.) behind its anterior extremity and reaching as far down as the 

 level of the great trochanter. Occasionally this branch is absent and its place is taken by the 

 ihac branch of the ilio-hypogastric. In such cases, however, the branch from the last thoracic 

 to the first lumbar nerve is larger than usual. 



THE LUMBO-SACRAL PLEXUS 



The lumbo-sacral plexus is formed by the union of the anterior primary 

 divisions of the lumbar, sacral, and coccygeal nerves. In about 50 per cent, of 

 cases it receives a l^ranch from the twelfth thoracic nerve. Its components are 

 distributed to the lower extremity in a manner homologous and similar to the 

 distribution of the parts of the brachial plexus to the upper extremity; the 

 lumbar nerves are distributed similarly to the nerves formed from the anterior 

 (medial and lateral) cords of the brachial plexus, and the sacral nerves are 

 distributed in a manner similar to the distribution of the nerves from the posterior 

 cord of the brachial plexus. 



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

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

 dividcfl into four i)arts — tiie lumbar, sacral, pudendal, and coccygeal plexuses. 

 These pk^xuses overlap so that there is no definite line of demarcation between 

 them. liowever, they will lie considered separately. 



3. THE LUMBAR NERVES 



1'he anterior primary divisions of the five lumbar nerves increase in size from 

 the first to the last. Each lumbar nerve is connected by one or two long, slender 



