UPPER EXTREMITY AND THORAX, POSTERIOR. 375 



Insertion. Into the posterior tubercles of the transverse 

 processes of the cervical vertebrae from the second to the 

 sixth inclusive. 



Nerve Supply. External posterior branches of the dorsal 

 nerves. 



Action. See below. 



Trachelomastoid. (Middle column.) 



Origin. From the four or five upper transverse pro- 

 cesses of the dorsal vertebrae, from the articular processes 

 of the lower four cervical vertebrae. 



Insertion. Into the posterior border of the mastoid 

 process. 



Nerve Supply. The posterior branches of the cervical 

 nerves. 



Action of the Middle Column. To extend the spine in 

 the cervical, dorsal, and lumbar regions ; to flex the spine 

 laterally ; to extend and flex the head laterally and rotate 

 the chin to the same side. 



Iliocostalis (Sacrolumbalis). (External column.) 



Origin. The outer portion of the erector spinae. 



Insertion. Into the angles of the six lower ribs. 



Nerve Supply. External posterior branches of the lum- 

 bar nerves. 



Action. See below. 



Accessorms. (External column.) 



Origin. From the angles of the lower six ribs. 



Insertion. Into the angles of the six upper ribs and the 

 transverse process of the seventh cervical vertebra. 



Nerve Supply. The posterior division of the dorsal 

 nerves. 



Action. See below. 



