THE THORACIC XKltVl'.X 933 



turn abruptly ventral a short distance from the sternum, pierce the internal inter- 

 costals, the anterior intercostal membrane, and the pectoralis major, and give off three 

 sets of terminal branches. One set supplies the transverse thoracic muscle and the 

 back of the .sternum. A second set, cutaneous, runs mesially. The third set passes 

 laterally over the pectoralis major, supplying the skin in that region, and, in the 

 female, the mammary gland through the medial mammary branches. The anterior 

 branches in their course supply the intercostal and subcostal muscles and give fila- 

 ments that supply the ribs, the periosteum, and the pleura. 



The thoraco-abdominal nerves. The relations of the posterior portions of 

 the seventh, eighth, ninth, tenth, and eleventh thoracic nerves to the thoracic wall 

 are, similar to those of the thoracic intercostal nerves. Each divides in a similar 

 manner into a lateral and an anterior branch, but these branches are distributed 

 partly to the abdominal and partly to the thoracic wall, and the smaller muscular 

 branches have also different distributions. 



The lateral branches, lateral cutaneous nerves of the abdomen, pierce the external 

 intercostal muscles and pass through or between the digitations of the external 

 oblique into the subcutaneous tissue, where they divide in the typical way into 

 anterior and posterior branches. The posterior branches pass backwards over the 

 latissimus dorsi. The anterior branches give filaments to the digitations of the 

 external oblique and extend forwards, inwards, and downwards to the outer border 

 of the sheath of the rectus. 



The anterior branches pass forwards between the external and internal inter- 

 costal muscles, to the anterior ends of the intercostal spaces; there they insinuate 

 themselves between the interdigitating slips of the diaphragm and the transver- 

 sua 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 forwards between the transversus abdo- 

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

 where they pierce the posterior lamella of the internal oblique aponeurosisand enter 

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

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

 and become anterior cutaneous nerves of the abdomen. 



The muscular branches. Muscular branches are distributed to the levatores 

 costarum, the intercostal muscles, the transversus abdominis, the internal oblique, 

 and to the rectus abdominis from all the thoraco-abdominal nerves, and the 

 ninth, tenth, and eleventh nerves give branches 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 communicating 

 branch, 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 

 behind the kidney and the ascending or the descending colon according to the side 

 in which it lies. At the outer border of the quadratus lumborum it pierces the apo- 

 neurosis of origin of the transversus abdominis muscle and divides, between the 

 transversus and the internal oblique muscle, into a lateral and an anterior branqh. 

 It gives branches to the transversus abdominis, the quadratus lumborum, and the 

 internal oblique muscles. 



The anterior branch passes forwards, between the internal oblique and the 

 transversus abdominis, to which it supplies twigs. It enters the sheath of the rectus, 

 turns forwards through that muscle, and terminates in branches which become cuta- 

 neous midway between the umbilicus and the symphysis. Before it becomes cuta- 

 neous it supplies twigs to the transversus abdominis, the internal oblique, the rectus 

 abdominis, and the pyramidalis muscles. 



The lateral branch pierces the internal oblique; it supplies the lowest digita- 

 tion 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 an- 



