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.

[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.

The portion of the muscle which is inserted on the cervical transverse processes (f″) is sometimes distinguished as the longissimus cervicis; it is not well separated from the rest of the muscle in the cat. The longissimus capitis ([Fig. 69], c; [Fig. 73], g) is to be considered a differentiated cranial portion of this muscle.

Relations.—Outer surface with the following, beginning with the caudal end: the sartorius ([Fig. 68], q), the latissimus dorsi ([Fig. 68], m), the lumbodorsal fascia ([Fig. 68], y), the serratus posterior inferior ([Fig. 73], n) and superior ([Fig. 73], l), and the levator scapulæ ([Fig. 73], h). Lateral margin with the abdominal muscles, the iliocostal ([Fig. 69], h), and the levator scapulæ ([Fig. 73], h). Medial side with the multifidus spinæ ([Fig. 70], d), the spinous processes of the thoracic vertebræ, the spinalis dorsi ([Fig. 69], g), the complexus ([Fig. 69], b), the biventer cervicis ([Fig. 69], a), and the longissimus capitis ([Fig. 69], e).

Action.—Extends the vertebral column.

M. iliocostalis ([Fig. 69], h).—This is a muscle about 1½ to 2 centimeters wide, lying on the dorsal portion of the ribs, laterad of the longissimus dorsi (f′). It is composed of many partly separated bundles, each with an oblique tendon. The muscle begins at the last or next to the last rib, where it is connected with the longissimus by a rather small bundle of fibres. The rest of the muscle-fibres take origin in bundles from the lateral surface of the ribs, at about the angles, or from thin tendons connecting the angles, of the ribs; they pass obliquely craniad and are inserted, usually by tendons, on the lateral surface of the third or fourth rib craniad of the one on which the given bundle has origin. The insertions on any given rib lie ventrad of the origins on the same rib. In the cat this muscle is confined to the thoracic region.