THORACIC SPINAL NERVES. 633 



ber on each side. They arise by filaments from the ante- 

 rior and posterior root of the spinal marrow, and pass out 

 through the intervertebral foramina ; each nerve then 

 divides into an anterior and posterior branch. The ante- 

 rior branches occupy the intercostal spaces, pursue the course 

 of the intercostal arteries, and are called the intercostal 

 nerves. Each receives two branches from the thoracic 

 ganglia of the sympathetic, and then runs along the groove 

 on the under margin of each rib, between the two laminae 

 of the intercostal muscles. The five or six upper intercos- 

 tal nerves pass round to the sternum, and when near the 

 latter, emerge from between the intercostal muscles, and 

 are distributed upon the pectoral muscles and integuments. 

 The five or six lower nerves supply the abdominal muscles 

 and their integuments. 



The first anterior thoracic nerve joins the last cervical, 

 and sends a branch on the inner face of the first rib, which 

 goes to supply the intercostal muscles. The second ante- 

 rior thoracic, or dorsal nerve, in addition to the ordinary 

 distribution, sends a branch between the ribs, which passes 

 outward to the axilla, there joins the internal cutane- 

 ous of the upper extremity, and then descends the arm, 

 distributing filaments upon the integuments as far as the 

 elbow. The third anterior dorsal also sends a branch to 

 the axilla, which supplies the integuments on the inner side 

 of the arm. These nerves are called intercosto-humeral, or 

 the nerves of Wrisberg, and are supposed to explain the 

 numbness of the arm in angina pectoris. 



The lower intercostal nerves, about the middle of the ribs, 

 send off branches, called external pectoral, which are spent 

 upon the muscles and integuments upon the side of the 

 chest. The continued nerve, which is the intercostal proper, 

 emerges, as already stated, near the sternum, and supplies 

 the pectoral muscles, mamma, and integuments on the 

 front of the chest. 



The posterior branches of the dorsal spinal nerves are 

 smaller than the anterior. They pass backward between 

 the corresponding transverse processes of the vertebrae, 



