THE DORSAL NERVES. 779 



confined to the parietes of the thorax, and are named upper or pectoral intercostal 

 nerves. They pass forward in the intercostal spaces with the intercostal vessels, 

 being situated below them. At the back of the chest they lie between the pleura 

 and the External intercostal muscle, but are soon placed 'between the two planes 

 of Intercostal muscles as far as the middle of the rib. They then enter the 

 substance of the Internal intercostal muscles, and, running amidst their fibres as far 

 as the costal cartilages, they gain the inner surface of the muscles and lie between 

 them and the pleura. Near the sternum, they cross in front of the internal mam- 

 mary artery and Triangularis sterrii muscle, pierce the Internal intercostal muscles, 

 the anterior intercostal membrane, and Pectoralis major muscle, and supply the 

 integument of the front of the chest and over the mammary gland, forming the 

 anterior cutaneous nerves of the thorax ; the branch from the second nerve is 

 joined with the supraclavicular nerves of the cervical plexus. 



Branches. Numerous slender muscular filaments supply the Intercostals, the 

 Infracostales, the Levatores costarum, Serratus posticus superior, and Triangularis 

 sterni muscles. Some of these branches, at the front of the chest, cross the costal 

 cartilages from one to another intercostal space. 



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 marnma ; those of the fifth and 

 sixth nerves supply 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 inter costo-humeral nerve (Fig. 413). 

 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. v lt 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, and the twelfth dorsal is continued throughout its 

 whole course in the abdominal wall, since it is placed below the last rib ; hence 

 these nerves are named lower or abdominal intercostal nerves. They have (with 

 the exception of 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, and, about the middle of their 

 course, 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 supply the 

 digitations of the External oblique muscle and extend downward and forward 

 nearly as far as the margin of the Rectus ; the posterior branches pass backward 

 to supply the skin over the Latissimus dorsi. 



