The muscle is largest in the lumbar region (Fig. 69, _f_; Fig. 70), where it is divided into a narrow medial (Fig. 70, _a_) and a thick lateral portion (Fig. 70, _b_), the latter being again partially subdivided by the fascia. The two parts unite farther craniad.
The =medial division= (Fig. 70, _a_) consists of muscular bundles connecting the spinous processes of the vertebræ with the accessory and mammillary processes of other vertebræ; it is continuous caudad with the extensor caudæ lateralis (Fig. 70, _f_). The muscle-fibres take origin in the sacral region by small round tendons from the spinous processes of the last two sacral and the caudal vertebræ. They curve cranioventrad, forming a large belly, and are inserted into the accessory and mammillary processes of the lumbar vertebræ. In the thoracic region this division unites with that portion of the lateral division in which the fibres coming from the fascia of origin are inserted into the laminæ and transverse processes of the thoracic vertebræ.
The =lateral division= (Fig. 70, _b_) is much larger than the medial one, forming in the lumbar region a nearly cylindrical mass.
[Illustration: FIG. 70.--MUSCLES ON THE DORSAL SIDE OF THE VERTEBRAL COLUMN IN THE LUMBAR, SACRAL, AND CAUDAL REGIONS.
Both sheets of the lumbodorsal fascia have been removed, the deep layer (_c_) being cut where it passes into the longissimus dorsi. 1, crest of ilium; 4-7, tips of spinous processes of the fourth to seventh lumbar vertebræ. _I_, _II_, tips of spinous processes of first two sacral vertebræ. _a_, _b_, M. longissimi dorsi (_a_, medial portion; _b_, lateral portion; _b′_, portion taking origin from the lumbodorsal fascia); _c_, cut edge of deep layer of lumbodorsal fascia; _d_, M. multifidus spinæ; _e_, M. extensor caudæ medialis; _f_, M. extensor caudæ lateralis, _g_, M. abductor caudæ externus.]
_Origin_ from the crest of the ilium (Fig. 70, 1) and the medial surface of the ilium as far caudad as the auricular impression; also from the deep layer of the lumbodorsal fascia (_c_). This fascia is connected with the crest of the ilium and with the tips of the spinous processes of the vertebræ in the lumbar and thoracic regions, and from it a large proportion of the fibres of the longissimus take origin. In the lumbar region it dips into the muscle as an intermediate longitudinal sheet (_c_), partially dividing it lengthwise into two parts. Fibres taking origin from the lateral surface of this intermediate sheet curve cranioventrad, and are inserted on the transverse processes of the lumbar vertebræ. Fibres taking origin from the medial surface of the sheet pass mediocraniad and are inserted into the accessory processes and the surfaces of the vertebral arches. In the thoracic region (Fig. 69, _f′_) most of the fibres are inserted in separate bundles by small tendons on the transverse processes of the vertebræ, while some of the medial fibres unite with tendinous strands which become attached to the laminæ and articular processes of the vertebræ. At the region of the eighth or ninth thoracic vertebra the =spinalis dorsi= (Fig. 69, _g_) begins to be separated off on the medial side, the separation becoming complete only some distance farther craniad; the longissimus dorsi then continues into the cervical region (_f″_). Bundles of fibres become attached in the manner above described to transverse processes of all the thoracic vertebræ. In the cervical region (_f″_) the muscle spreads out and becomes thinner, and bundles of fibres become attached to the transverse processes of the cervical vertebræ as far forward as the second. In the more cranial portion of its extent the muscle receives fibres having origin on the articular processes and laminæ of the cervical and a few of the more cranial thoracic vertebræ.