442 THE MUSCULAE SYSTEM. 



deep to the splenius capitis muscle. In the neck the muscle is placed between 

 the splenius capitis and semispinalis capitis. 



M. Spinalis Dorsi. The spinalis dorsi forms the medial column of the sacro- 

 spinalis. It lies in the thoracic region, and arises by tendinous fibres from the 

 lower two thoracic and upper two lumbar spinous processes, and also directly from 

 the tendon of the longissimus dorsi. 



It is a narrow muscle which, lying close to the thoracic spinous processes 

 medial to the longissimus dorsi, and it is inserted into the upper (four to eight) 

 thoracic spines. It is not prolonged into the neck. 



The semispinalis capitis (O.T. complexus) closely resembles in position and 

 attachments the longissimus capitis. 



It takes origin from the transverse processes of the upper six thoracic and the 

 articular processes of the lower four cervical vertebrae, medial to the longissimus 

 cervicis and longissimus capitis. It has an additional origin also from the spinous 

 process of the last cervical vertebra. 



It forms a broad muscular sheet which extends upwards in the neck, to be inserted 

 into the medial impression between the superior and inferior nuchal lines of the 

 occipital bone (Fig. 396, p. 444). The medial portion of the muscle is separate, 

 and forms the biventer cervicis, consisting of two fleshy bellies with an intervening 

 tendon, placed vertically in contact with the ligamentum nuchae. The muscle 

 is covered mainly by the splenius and longissimus capitis muscles. It conceals the 

 semispinalis cervicis and the muscles of the suboccipital triangle. 



Nerve-Supply. Posterior rami of spinal nerves. 



Actions. The several parts of the sacrospinalis muscle have a complex action, on the vertebral 

 column, head, ribs, and pelvis. The muscle serves as an extensor of the vertebral column, and 

 assists in lateral movement and rotation. The longissimus capitis and semispinalis capitis assist 

 in extension, lateral movement and rotation of the head. The iliocostales and longissimus are 

 accessory muscles of inspiration. The whole muscle helps in extension and lateral movement of 

 the pelvis in the act of walking. 



Third Group. 



This group comprises the semispinales (dorsi and cervicis) and multifidus. They 

 occupy the vertebral furrow, under cover of the sacrospinalis and semispinalis 

 capitis muscles. They are only incompletely separate from one another. The 

 semispinales, dorsi and cervicis, form a superficial stratum, the multifidus being 

 more deeply placed. The more superficial muscles have the longer fibres ; the 

 fibres of the multifidus pass over fewer vertebrae. Both muscles extend obliquely 

 upwards from transverse to spinous processes. 



M. Semispinalis. The semispinalis muscle extends from the loin to the 

 second cervical vertebra. Its fibres are artificially separated into an inferior 

 part, the semispinalis dorsi, and a superior part, the semispinalis cervicis. 



The semispinalis dorsi arises from the transverse processes of the lower six 

 thoracic vertebrae. 



It is inserted into the spinous processes of the last two cervical and first four 

 thoracic vertebras. 



The semispinalis cervicis arises from the transverse processes of the upper 

 six thoracic, and the articular processes of the lower four cervical vertebrae. 



It is inserted into the spines of the cervical vertebrae from the second to the 

 fifth. 



M. Multifidus. The multifidus (O.T. multifidus spinae) differs from the 

 previous muscle in extending from the sacrum to the second cervical vertebra, 

 and in the shortness of its fasciculi, which pass over fewer vertebrae to reach their 

 insertion. 



It arises from the sacrum, from. the posterior sacro-iliac ligament (Fig. 395, 

 p. 443), from the mamillary processes of the lumbar vertebrae, from the transverse 

 processes of the thoracic vertebrae, and from the articular processes of the lower 

 four cervical vertebrae. 



It is inserted into the spines of the vertebrae up to and including the second 

 cervical. 



