422 THE MUSCLES 



Avliich runs upwards and inwards, and tlien breaks up again into sliort tendons of 

 insertion. Each muscle bridges over six or seven vertel)ne. 



Nerve-supply. — The internal divisions of the posterior l)ranches of the thoracic 

 spinal nerves. 



Action. — To extend and laterally Hex the lower cervical and upper thoracic 

 portions of the spinal cohnnn. The upjjerniost tendons will also assist in rotating 

 the lower ])art of the neck to the opposite side. 



Relations. — .Superficially, the spinalis and longissinms dorsi; deeply, the 

 multitidus spinse. 



3. SEMISPINALS COLLI 



The semispinalis colli — named from the attachment of one-half the muscle, 

 i.e. the insertion, to the spines of the cervical vertebrae — is a somewhat triangular 

 sheet, with a serrated base placed vertically along the transverse processes, and the 

 apex at the spine of the second cervical vertebra. 



Origin. — The back of the transverse processes of the five or six upper thoracic 

 vertebne. 



Insertion. — The spines of the second to the fifth cervical vertebrae. 



Structure. — Arising by short tendons, the fleshy fibres pass upwards and 

 inwards, l>ridging over in their course five or six vertebrae, and are inserted by still 

 shorter tendons into the spines of the cervical vertebrae. The highest is by far the 

 largest. 



Nerve-supply. — The internal divisions of the posterior branches of the lower 

 cervical nerves, the upper ones entering the muscle on its superficial, and the lower 

 on its deep aspect. 



Action. — (1) To extend; (2) to flex laterally; (3) to rotate to the opi^osite 

 side, the second and following cervical vertebrae. 



Relations. — Superficially, the complexus, from which it is separated by 

 branches of the posterior cervical nerves and the anastomosis of the arteria jiro- 

 funda cervicis with the arteria princeps cervicis; deeply, the multifidus spinae. 



4. MULTIFIDUS SPIN^ 



The multifidus spinae — named from its many divisions (mtdhis, many; findo, 

 to cleave) — is a compound muscle, fleshy and thick in the sacral and lumbar 

 regions, but becoming thin and more aponeurotic in the thoracic and cervical 

 regions. 



Origin. — (1) The groove in the back of the sacrum, between the spines and the 

 elevations which represent articular j^rocesses, the posterior sacro-iliac ligaments, 

 and the deep surface of the tendon of the erector spinae; (2) the manmiillary 

 processes of the lumbar vertebrae; (3) the liack of the transverse processes of 

 all the thoracic vertebrae; (4) the articular processes of the fourth to the sixth 

 cervical vertebrae, and the back of the transverse process of the seventh cervical 

 vertebra. 



Insertion. — The lower borders of the spines of the vertebrae from the last 

 lumltar to the second cervical. 



Structure. — Arising by tendinous fibres superficially, but by fleshy fil)res 

 deeply, the bundles of each element of the muscle diverge, the more superficial 

 ones running obliquely upwards and inwards so as to In-idge three vertebne, while 

 the deeper ones pass more transversely to the third, second, and in the neck, the 

 next vertel)ra above. Consequently, the more superficial fil)res from one vertebra 

 overlap the (lee])er ones from some of the vertebrae above. 



Nerve-supply. — The internal divisions of the posterior branches of tlie spinal 

 nerves from the second cervical to the third sacral. 



Action. — To extend, laterally flex, and to rotate to the opposite side, the 

 various ]iarts of the spinal column. 



Relations. — Superficially, the erector spinae, longissimus and spmalis dorsi, 

 the semispinalis dorsi and colli; deeply, the rotatores spina-. 



