- f v. 

 414 INTERCOSTAL NERVES. 



dorsi to the spinous processes of the dorsal vertebrae ; it then pierces 

 the aponeurosis of origin of the trapezius and latissimus dorsi, and 

 divides into branches which are inclined outwards beneath the in- 

 tegument to which they are distributed. The posterior branch of 

 the first dorsal nerve resembles in its mode of distribution the pos- 

 terior branches of the last cervical. The posterior branches of the 

 four last dorsal nerves pass obliquely downwards and outwards into 

 the substance of the erector spinse in the situation of the interspace 

 between the sacro-lumbalis and longissimus dorsi. After supplying 

 the erector spinse arid communicating freely with each other they 

 approach the surface along the outer border of the sacro-lumbalis, 

 where they pierce the aponeuroses of the transversalis, internal 

 oblique, serratus posticus inferior, and latissimus dorsi muscle, and 

 divide into internal branches which supply the integument upon the 

 middle line in the lumbar region, and external branches which are 

 distributed to the integument upon the side of the lumbar and in the 

 gluteal region. 



The Intercostal nerves receive one or two filaments from the ad- 

 joining ganglia of the sympathetic, and pass forwards in the inter- 

 costal space with the intercostal vessels, lying below the veins and 

 artery. At the termination of the intercostal spaces near to the 

 sternum, the nerves pierce the intercostal and pectoral muscles, and 

 incline downwards and outwards to be distributed to the integu- 

 ment of the mamma and front of the chest. Those which are situ- 

 ated between the false ribs pass behind the costal cartilages, and 

 between the transversalis and obliquus internus muscles ; and sup- 

 ply the rectus and the integument on the front of the abdomen. 

 The first and last dorsal nerves are exceptions to this distribution. 

 The anterior branch of the first dorsal nerve divides into two 

 branches ; a smaller, which takes its course along the under surface 

 of the first rib to the sternal extremity of the first intercostal space ; 

 and a larger, which crosses obliquely the neck of the first rib, to 

 join the brachial plexus. The last dorsal nerve, next in size to the 

 first, sends a branch of communication to the first lumbar nerve, to 

 assist in forming the lumbar plexus. 



The Branches of each intercostal nerve are a muscular twig to 

 the intercostal and neighbouring muscles, and a cutaneous branch 

 which is given off at about the middle of the arch of the rib. The 

 first dorsal nerve has no cutaneous branch. The cutaneous branches 

 of the second and third intercostal nerves are named, from their 

 origin and distribution, intercosto-humeral. 



The First intercosto-humeral nerve is of large size ; it pierces the 

 external intercostal muscle of the second intercostal space, and 

 divides into an internal and an external branch. The internal 

 branch is distributed to the integument of the inner side of the arm. 

 The external branch communicates with the nerve of Wrisberg, 

 and divides into filaments which supply the integument upon the 

 inner and posterior aspect of the arm as far as the elbow. This 

 nerve sometimes takes the place of the nerve of Wrisberg. 



