INTERCOSTAL NERVES. 467 



to the tenth, and then increase to the twelfth. Each nerve, as soon 

 as it has escaped from the intervertebral foramen, divides into two 

 branches ; a dorsal branch and the true intercostal nerve. 



The Dorsal branches pass directly backwards between the trans- 

 verse processes of the vertebrae, lying internally to the anterior costo- 

 transverse ligament, where each nerve divides into an anterior or mus- 

 cular and a posterior or musculo-cutaneous branch. The muscular 

 branch enters the substance of the muscles in the direction of a line 

 corresponding with the interval of separation between the longissimus 

 dorsi and sacro-lumbalis, and is distributed to the muscles of the back, 

 its terminal filaments reaching to the integument. The musculo- 

 cutaneous branch passes inwards, crossing the semispinalis dorsi to the 

 spinous processes of the dorsal vertebrae, giving off muscular branches 

 in its course ; it then pierces the aponeurosis of origin of the trapezius 

 and latissimus dorsi, and divides into branches which are inclined out- 

 wards beneath the integument to which they are distributed. 



The dorsal branch of the first dorsal nerve resembles in its mode of 

 distribution the dorsal branches of the last cervical. The dorsal branches 

 of the last four dorsal nerves pass obliquely downwards and outwards 

 into the substance of the erector spinae in the situation of the inter- 

 space between the sacro-lumbalis and longissimus dorsi. After supply- 

 ing the erector spinae and 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, ser- 

 ratus posticus inferior, and latissimus dorsi, and divide into internal 

 branches which supply the integument in the lumbar region upon the 

 middle line, and external branches which are distributed to the integu- 

 ment upon the side of the lumbar and in the gluteal region. 



INTERCOSTAL NERVES. The Intercostal nerves receive one or two 

 filaments from the adjoining ganglia of the sympathetic, and pass for- 

 wards in the intercostal space with the intercostal vessels, lying below 

 the veins and artery, and supplying the intercostal muscles in their 

 course. At the termination of the intercostal spaces near the sternum, 

 the nerves pierce the intercostal 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 trans- 

 versalis 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 ex- 

 tremity of the first intercostal space ; and a larger, which crosses ob- 

 liquely 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 communica- 

 tion 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 



