THOEACIC NERVES. 717 



the side of the chest, the, second (intercosto-brachial) and the third, in part, being 

 drawn out to the arm. The fourth supplies the nipple (Fig. 623). 



Communications. Each of these intercostal nerves communicates with the sympathetic 

 trunk and ganglia by two branches a white ramus communicans to the corresponding sym- 

 pathetic ganglion or the adjacent part of the sympathetic trunk ; and a gray ramus com- 

 municans, which passes to each nerve from the corresponding ganglion. 



The anterior rami of the seventh, eighth, ninth, tenth, and eleventh thoracic 

 nerves differ from the preceding nerves only 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 distribu- 

 tion in the abdominal wall. Each nerve traverses an intercostal space in the way 

 described. At the anterior end of the space, the nerve pierces the attachment of 

 the diaphragm and the transversus abdominis muscle to the costal cartilages, and 

 courses forwards in the abdominal wall between the transversus and obliquus 

 internus muscles. The nerve then passes between the rectus abdominis muscle 

 and the posterior layer of its sheath, and eventually reaches the anterior abdominal 

 wall and becomes cutaneous by piercing the rectus abdominis itself and the anterior 

 layer of its sheath. 



Muscular Branches. The lower intercostal nerves supply the intercostal 

 muscles of the spaces in which they lie ; and in the abdominal wall they innervate 

 the transversus, obliquus externus and internus, 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 anterior 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. 625). 



The anterior branches are small. That of the seventh nerve innervates the skin 

 at the level of the xiphoid process. The eighth and ninth appear between the 

 xiphoid process and the umbilicus; the tenth nerve supplies the region of the 

 umbilicus ; and the eleventh, the area immediately below the umbilicus. 



The cutaneous branches of these nerves, including those of the posterior rami, thus supply 

 continuous belts of skin, which can be mapped out on the body from the vertebral column 

 behind to the median plane in front. These areas are not placed horizontally, but tend to be 

 drawn more downwards anteriorly as the series is followed from the upper to the lower nerves. 



The anterior ramus of the twelfth thoracic nerve is peculiar in its course 

 and distribution. It emerges below the last rib (Fig. 625), and passes laterally 

 and downwards in the posterior abdominal wall under cover of the psoas muscle, 

 and between the lateral lumbo-costal arch and the quadratus lumborum muscle ; 

 it pierces the transversus abdominis muscle, and courses forwards in the interval 

 between it and the obliquus internus as far as 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 os pubis. The branches of the nerve 

 are muscular to the transversus, 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 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 

 anterior to the greater trochanter of the femur (Fig. 623, p. 715). 



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 



