1282 HUMAN ANATOMY. 



In addition to assisting in the formation of the posterior cervical plexus it communicates 

 with the great occipital nerve. 



The fourth, fifth, sixth, seventh and eighth cervical nerves have quite 

 small posterior primary divisions (rr. posteriores). The fourth, fifth and sixth 

 divide into the usual external and internal branches (rr. laterales et mediales), which 

 supply respectively the adjacent muscles and the dorsal integument. The seventh 

 and eighth usually have no cutaneous branches and are distributed solely to the 

 deeper muscles of the back. 



A communicating filament from the fourth may aid in the formation of the posterior 

 cervical plexus. 



Variations. — The cutaneous branches of- the fifth and sixth may be very small or absent 

 entirely. 



THE THORACIC NERVES. 



The posterior primary divisions (rr. posteriores) of the thoracic or dorsal nerves 

 (nn. thoracales) follow the general arrangement of dividing into external and internal 

 branches.  Of these the internal branches of the upper six are mainly cutaneous 

 and the external entirely muscular. In the lower six, on the contrary, the external 

 branches are principally cutaneous and the internal entirely muscular. 



The external branches (rr. laterales) gradually increase in size from above 

 downward. They pierce or pass under the longissimus dorsi to reach the interval 

 between that muscle and the ilio-costalis, eventually reaching and supplying the 

 erector spinae. Those from the lower half of the thoracic nerves distribute sensory 

 fibres for the supply of the skin overlying the angles of the ribs (Fig. 1083). 



The internal branches (rr. mediales) of the upper six or seven pass dorsally 

 between the multifidus spinee and semispinalis muscles. After innervating the trans- 

 verso-spinales they become superficial close to the median dorsal line and supply the 

 skin of the back, sometimes extending laterally beyond the vertebral border of the 

 scapula. The internal branches of the lower nerves traverse the interval between 

 the longissimus dorsi and the multifidus spinae and supply the latter muscle. 



Variations. — The sixth, seventh and eighth thoracic nerves may give off cutaneous twigs 

 from both external and internal branches. The first thoracic nerve may have no cutaneous 

 branch. 



THE LUMBAR NERVES. 



The posterior primary divisions (rr. posteriores) of the lumbar nerves (nn. lum- 

 bales) divide into the usual external and internal branches. 



The external branches (rr. laterales) of all five lumbar nerves enter and sup- 

 ply the erector spinae, those of the lower two terminating there. From the external 

 branches of the first, second and third arise cutaneous ^offshoots (nn. clunium supe- 

 riores) of considerable size (Fig. 1083). These pierce the ilio-costalis and the 

 aponeurosis of the latissimus dorsi above the crest of the ilium and supply the skin 

 of the gluteal region as far forward as the great trochanter. From the fifth a branch 

 passes downward to inosculate with a similar branch of the first sacral nerve to aid in 

 the formation of the posterior sacral plexus. 



The internal branches (rr. mediales) turn directly backward and supply the 

 multifidus spinae muscle. 



THE SACRAL NERVES. 



The posterior primary divisions (rr. posteriores) of the sacral nerves (nn, 

 sacrales), with the exception of that of the fifth, emerge from the vertebral canal 

 through the posterior sacral foramina. The first, second and third pass outward 

 under cover of the multifidus spinre and divide into external and internal branches. 



The external branches (rr. laterales) of the first, second and third sacral nerves 

 unite over the upper part of the sacrum with a similar branch of the fifth lumbar and 

 with the fourth sacral nerve to form a series of loops, the posterior sacral plexus 



