[Illustration: FIG. 69.--DEEP MUSCLES OF BODY, EXPOSED AFTER REMOVAL OF THE MUSCLES SHOWN IN FIG. 73. _a_, M. biventer cervicis; _b_, M. complexus; _c_, M. obliquus capitis superior; _d_, M. longus atlantis; _e_, cut end of M. longissimus capitis; _f_, _f′_, _f″_, M. longissimus dorsi; _g_, M. spinalis dorsi; _h_, M. iliocostalis; _i_, Mm. intercostales externi; _k_, Mm. intercostales interni; _l_, M. transversus abdominis.] (_a_) _Muscles of the Lumbar and Thoracic Regions._--The muscles of the lumbar and thoracic regions are mostly covered by a strong fascia, known as the =lumbodorsal= fascia (Fig. 68, _y_). This consists of two sheets, the superficial sheet being applied directly to the outer surface of the inner sheet, or separated from it by a mass of fat. The superficial sheet (Fig. 68, _y_) overlies the lumbar region and the caudal half of the thoracic region. On the medial side it is attached to the spinous processes of the vertebræ and is united closely to the deeper layer. Laterally this fascia is continuous with the latissimus dorsi (_m_) and obliquus abdominis externus (_p_). Caudad it is attached to the spine of the ilium and becomes continuous with the fascia covering the gluteus muscles. The deeper sheet (Fig. 70, _c_) is of a tendinous character, forming the external tendinous layer of the longissimus dorsi, many of whose fibres take origin from its under surface. It is described more fully in the account of this muscle. =M. longissimus dorsi= (Fig. 69, _f_, _f′_, _f″_; Fig. 70, _a_ and _b_).--A very large muscle, filling most of the region between the spinous processes and transverse processes of the lumbar and thoracic vertebræ, and extending into the cervical region. 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.