SECOND INTERCOSTO-HUMERAL NERVE. 421 



the intercostal space with the intercostal vessels, lying below the veins ana 

 artery, and supplying the intercostal muscles in their course. At the termi- 

 nation of the intercostal spaces near the sternum, the nerves pierce the in- 

 tercostal and pectoral muscles, and incline downwards and outwards to be 

 distributed to the integument of the mamma and front of the chest. Those 

 which are situated between the false ribs pass behind the costal cartilages, 

 and between the transversalis and obliquus internus muscles, and supply 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 fc;e in- 

 tercostal and neighbouring muscles, and a cutaneous branch which is given 

 orF at about the middle of the arch of the rib. The first intercostal nerve 

 has no cutaneous branch. The cutaneous branches of the second and 

 third intercostal nerves are named, from their origin and distribution, in- 

 tercosto-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 distri- 

 buted 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. 



The SECOND INTERCOSTO-HUMERAL NERVE is much smaller than the 

 preceding ; it emerges from the external intercostal muscle of the third 

 intercostal space between the serrations of the serratus magnus muscle, and 

 divides into filaments which are distributed to the integument of the 

 shoulder. One of these filaments may be traced inwards to the integu- 

 ment of the mamma. The two intercosto-humeral nerves not unfrequently 

 communicate previously to their distribution. 



The cutaneous branches of the fourth and fifth intercostal nerve send 

 anterior twigs to the integument of the mammary gland and posterior fila- 

 ments to the scapular region of the back. The cutaneous branches of the 

 remaining intercostal nerves reach the surface between the serrations of 

 the serratus magnus muscle above and the external oblique below, and 

 each nerve divides into an anterior and a posterior branch ; the former 

 being distributed to the integument of the antero-lateral. and the latter to 

 that of the lateral part of the trunk. 



The cutaneous branch of the last dorsal nerve is remarkable for its size 

 (n. clumum superior anticus) ; it pierces the internal and external oblique 

 muscles, crosses the anterior part of the crest of the ilium, and is distri- 

 buted to the integument of the gluteal region as low down as the trochante* 

 major. 



36 



