NERVES. 



first to third, and seventh to twelfth are peculiar as follows: 

 First thoracic (the largest): This divides at once into two 

 parts; the upper and larger part ascends over the neck of the 

 first rib to join the brachial plexus; the lower part runs in 

 the first intercostal space, but rarely has either a lateral or an 

 anterior cutaneous branch. Second thoracic: The lateral 

 branch (intercosto-humeral nerve) does not divide into trunks 

 but crosses the axilla, piercing the deep fascia just beyond its 

 posterior fold, and extends down the arm. It ends between the 

 internal condyle and olecranon, and supplies the skin of the 

 armpit and back of the arm, internally, to the elbow. Third 

 thoracic : The posterior trunk of the lateral branch communi- 

 cates with the intercosto-humeral and supplies the skin on the 

 posterior half of the inner side of the arm high up. [635] 



Seventh to eleventh thoracic : These are typical except that, 

 at the anterior end of the intercostal space, each pierces the 

 attachment of the Diaphragm and Transversalis and runs be- 

 tween the Transversalis and Internal oblique; then passing 

 between the Rectus and the posterior layer of the sheath, it 

 pierces the Rectus and anterior layer of the sheath as the an- 

 terior branch. As usual, muscular branches supply the adja- 

 cent muscles, and the lateral branches divide into trunks, but 

 the lateral branch of the eleventh descends over the iliac crest. 

 Twelfth thoracic: This emerges below the last rib and runs 

 outward under the Psoas, between the external arcuate liga- 

 ment and Quadratus lumborum ; then piercing the Transver- 

 salis it runs between this and the Internal oblique, and 

 through the Rectus like the preceding. Its branches are 

 typical, except that the lateral branch descends through the 

 lateral abdominal muscles, becomes superficial over the iliac 

 crest two inches behind the anterior superior spine, and 

 supplies the skin of the buttock as far as in front of the great 

 trochanter. [638] 



Posterior Divisions of Thoracic Nerves. Each divides 

 into an internal and an external branch. Of the upper six 



[123] 



