POSrERIOR DIVISIONS OF SPINAL NERVES 781 



THORACIC NERVES. — The pos^tcrior i)riiimrv divit^ioii.s of the thoracic nerves 

 I)ass backwards between the transverse processes of the thoracic vertebrae, and 

 divide into internal and external branches. The internal branches become })ro- 

 gressively smaller from the second to the last of the series. The external branches 

 show a reciprocal increase in size. 



The internal branches pass to the inner side of the longissimus dorsi, giving 

 branches to that muscle and to the sj)inalis dorsi, semis})inalis, multifidus spina;, 

 rotatorcs spinte, intertransversales, and interspinales. Large branches from the 

 upper six or seven nerves reach the surface l)y piercing the origin of the trapezius 

 close to the spinous processes of the vertebrae They run outwards in the superfi- 

 cial fascia, and supply the integument of the back, including the skin over the 

 imier half of the infraspinous part of the dorsum scapula) and the integument over 

 th(! supraspinous fossa. The cutaneous branch of the second nerve is the largest of 

 tlie series. Frtun the lower five or six nerves small twigs usually reach the surface 

 and become cutaneous. 



Tlie external branches of the ui)per six or seven nerves end in the accessorius 

 and longissimus dorsi muscles. The lower five or six, after supplying the adjacent 

 muscles, pierce the outer series of insertions of the longissimus dorsi, and appear 

 in the interval between that muscle and the accessorius. They then pierce the 

 latissimus dorsi, and are distributed to the integument on the lower and outer part 

 of the back. 



LUMBAR NERVES. — The internal branches of all the lumbar nerves end in 

 the multifidus spin;e. 



The external branches of the upper three nerves, after supplying twigs to 

 the adjacent muscles, pierce the posterior layer of the lumbar aponeurosis at the 

 outer border of the erector spinjB. The three nerves cross the crest of the ilium 

 and occupy different planes in the thick superficial fascia which covers the upper 

 part of the gluteus medius. The branch from the first nerve is comparatively 

 small, and occupies the most superficial plane. The second occupies an interme- 

 diate j)Osition. The branch from the third nerve is the largest of the three, and 

 occupies the deepest position; it distributes branches over the gluteus maximus as 

 far as the great trochanter. The three nerves communicate Avith one another and 

 also with the cutaneous branches from the i)osterior divisions of the two upper 

 sacral nerves. The external branch of the fourth lumbar nerve is of small 

 size and ends in the lower part of the erector sjjina'. The external branch of the 

 fifth lumbar is distributed to the erector spinas and communicates with the first 

 sacral nerve. 



SACRAL AND COCCYGEAL NERVES. — The posterior divisions of the upjier 

 four sacral nerves csca]>e from the neural canal by passing through the posterior 

 sacral foramina; the fifth sacral nerve passes between the sacrum and coccyx. The 

 coccygeal nerve escapes through the termination of the neural canal. The upper 

 three sacral nerves divide in the ordinary manner into internal and external 

 branches, the lower two sacral and the coccygeal nerves remain undivided. 



The internal branches of the U])per three sacral nerves are of small size, and 

 are distri])Uted to the multifidus sj^ina*. The external branches unite with one 

 another and with the external branch of the last lumbar nerve to form loops on the 

 ])osterior surface of the sacrum. From these loops branches proceed to the poste- 

 rior surface of the great sacro-sciatic ligament, where they communicate to form 

 a second series of loo{)s, whence two or three branches are given ofl^. These 

 branches pierce the gluteus maximus and come to the surface of that muscle in a 

 line between the posterior superior spine of the ilium and the tip of the coccyx. 

 Tiiey are distributed to the integument over the inner part of the gluteus maximus, 

 and connnunicate, in their course through the superficial fascia, with the posterior 

 branches of the linnbar nerves. 



The posterior divisions of the lower two sacral and of the coccygeal nerve 

 unite with one another and with the posterior l)ranch of the third sacral, and form 

 loops whence twigs pass to the integument over the lower end of the cocc^'x. 



