=M. spinalis dorsi= (Fig. 69, _g_).--A muscle interconnecting the spinous processes of vertebræ some distance apart, in the thoracic and cervical regions. It lies on the medial side of the longissimus dorsi (_f′_).
_Origin_ by strong tendons from the tips of the spinous processes of the tenth to the thirteenth thoracic vertebræ. These tendons represent the cranial portion of the deep layer of the lumbodorsal fascia, and give origin also to many fibres of the longissimus dorsi, so that the two muscles are completely united at the origin of the spinalis. They become separated at about the level of the sixth thoracic vertebra, the spinalis forming a strong separate bundle passing into the neck region close against the sides of the spinous processes. The insertion is by fleshy bundles into the sides of the spinous processes of the first nine or ten thoracic vertebræ and of the cervical vertebræ as far forward as the second. Some of the fibres of this muscle pass craniad into the biventer cervicis (Fig. 69, _a_).
_Relations._--Outer surface with the tendons of the serratus posterior inferior (Fig. 73, _n_) and serratus posterior superior (Fig. 73, _l_), and with the splenius (Fig. 73, _b_), biventer cervicis (Fig. 69, _a_), and complexus (Fig. 69, _b_). Lateral and ventral surface with the longissimus dorsi (_f′_); medial surface with the spinous processes and the multifidus spinæ.
_Action._--Extensor of the vertebral column.
=M. multifidus spinæ=.--This consists of bundles of fibres which have origin on the transverse processes or neighboring parts, pass craniodorsad across one or more vertebræ, and are inserted into the spinous processes of vertebræ lying some distance craniad of the origin. They lie deeper than the muscles previously described. The muscle is most strongly developed in the lumbar region (Fig. 70, _d_), where it forms a thick interwoven mass in which it is difficult to distinguish separate bundles. The fibres in this region have origin on the accessory or mammillary processes and usually pass over more than one vertebra between origin and insertion; their insertions reach the dorsal ends of the spinous processes, so that part of the muscle lies immediately beneath the lumbar fascia. In other regions the multifidus is covered by other muscles. In the thoracic region the separate bands are more distinct, and usually pass in their course over but one intervening vertebra. In the cervical region the bands are interconnected, forming a fairly distinct single muscle, which is described separately below as the =semispinalis cervicis= (Fig. 71, _c_). The portion of this muscle attached to the head (=semispinalis capitis=) forms the biventer cervicis (Fig. 69, _a_) and complexus (_b_). Caudad this muscle passes onto the tail as the =extensor caudæ medialis= (Fig. 70, _c_).
