848 THE NERVOUS SYSTEM. 



Lateral Cutaneous Nerves. These are derived from the intercostal nerves, 

 midway between the vertebrae and sternum : they pierce the External intercostal 

 and Serratus magnus muscles, and divide into two branches, anterior and posterior. 



The anterior branches are reflected forward to the side and the fore part of the 

 chest, supplying the integument of the chest and mamma and the upper digitations 

 of the External oblique. 



The posterior branches are reflected backward to supply the integument over 

 the scapula and over the Latissimus dorsi. 



The lateral cutaneous branch of the second intercostal nerve is of large size, 

 and does not divide, like the other nerves, into an anterior and posterior branch. 

 It is named, from its origin and distribution, the intercosto-humeral nerve (Fig. 500). 

 It pierces the External intercostal muscle, crosses the axilla to the inner side of 

 the arm, and joins with a filament from the nerve of Wrisberg. It then pierces 

 the fascia, and supplies the skin of the upper half of the inner and back part of 

 the arm, communicating with the internal cutaneous branch of the musculo-spiral 

 nerve. The size of this nerve is in inverse proportion to the size of the other 

 cutaneous nerves, especially the nerve of Wrisberg. A second intercosto-humeral 

 nerve is frequently given off from the third intercostal. It supplies filaments to 

 the armpit and inner side of the arm. 



The Lower Dorsal Nerves. The anterior divisions of the seventh, eighth, ninth, 

 tenth, and eleventh dorsal nerves are continued anteriorly from the intercostal 

 spaces into the abdominal wall, hence these nerves are named lower or abdominal 

 intercostal nerves ; the twelfth dorsal is continued throughout its whole course in 

 the abdominal wall, since it is placed below the last rib (subcostal nerve). They 

 have (except the last) the same arrangement as the upper ones as far as the 

 anterior extremities of the intercostal spaces, where they pass behind the costal 

 cartilages, and between the Internal oblique and Transversalis muscles, to the 

 sheath of the Rectus, which they perforate. They supply the Rectus muscle, and 

 terminate in branches which become subcutaneous near the linea alba. These 

 branches are named the anterior cutaneous nerves of the abdomen. They are 

 directed outward as far as the lateral cutaneous nerves, supplying the integument 

 of the front of the belly. The lower intercostal nerves supply the Intercostals, 

 Serratus posticus inferior, and Abdominal muscles. Filaments have been traced 

 to the costal part of the Diaphragm. About the middle of their course they 

 give off lateral cutaneous branches, which pierce the External intercostal and 

 External oblique muscles, in the same line as the lateral cutaneous nerves of the 

 thorax, and divide into anterior and posterior branches, which are distributed 

 to the integument of the abdomen and back, the anterior branches passing nearly 

 as far forward as the margin of the Rectus, the posterior branches passing 

 backward to supply the skin over the Latissimus dorsi, where they join the dorsal 

 cutaneous nerves. 



The last dorsal is larger than the other dorsal nerves. Its anterior division 

 runs along the lower border of the last rib in front of the Quadratus lumborum, 

 perforates the Transversalis, and passes forward between it and the Internal 

 oblique to be distributed in the same manner as the lower intercostal nerves. It 

 communicates with the ilio-hypogastric branch of the lumbar plexus, and is 

 frequently connected with the first lumbar nerve by a slender branch, the dorso- 

 lumbar nerve, which descends in the substance of the Quadratus lumborum. 



The lateral cutaneous branch of the last dorsal is remarkable for its large size : 

 it perforates the Internal and External oblique muscles, passes downward over 

 the crest of the ilium in front of the iliac branch of the ilio-hypogastric (Fig. 509), 

 and is distributed to the integument of the front of the hip, some of its filaments 

 extending as low down as the trochanter major. It does not divide into an anterior 

 and posterior branch like the other lateral cutaneous branches of the intercostal 

 nerves. 



Surgical Anatomy. The lower seven intercostal nerves and the ilio-hypogastric from the 

 first lumbar nerve supply the skin of the abdominal wall. They run downward and inward 



