A description of the different bundles of fibres, under the names usually given, follows. Certain general principles are observed in some of the names used. Muscles which interconnect the spinous processes of the vertebræ receive the name =spinalis.= Those interconnecting contiguous spinous processes are called =interspinalis.= Muscles attached at one end to transverse processes, at the other to the spinous processes, receive the name =transversospinalis=. =Semispinalis= has the same signification as transversospinalis, but is a name usually applied to subdivisions of the transversospinalis group. The =intertransversarii= are muscles interconnecting the transverse processes.
[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.