_Relations._--Outer and lateral surface in the lumbar region with the longissimus dorsi (Fig. 69, _f_) and the lumbodorsal fascia (Fig. 68, _y_); in the thoracic region with the longissimus dorsi (Fig. 69, _f′_) and spinalis dorsi (_g_). Inner surface with the arches, articular processes, and spinous processes of the vertebræ.
_Action._--Extends the back when the muscles of both sides work together. Turns the vertebral column obliquely sideways when one set acts alone.
The deepest layer of the multifidus forms what is sometimes distinguished as the =Mm. rotatores=; no separate layer of this sort is to be made out in the cat.
=Mm. interspinales.=--Muscle-bundles passing from the spinous process of one vertebra to that of the vertebra immediately craniad or caudad of it. They are best developed in the lumbar region.
=Mm. intertransversarii.=--Muscle-fibres interconnecting the transverse processes. In the lumbar region the =intertransversarii mediales= connect the accessory and mammillary processes; the =intertransversarii laterales= lying between the transverse processes. In other regions only one set of the intertransversarii is to be distinguished.
(_b_) _Dorsal Muscles of the Cervical Region._--The clavotrapezius (Fig. 68, _d_) and occipitoscapularis (Fig. 73, _a_) have been described in connection with the muscles of the shoulder. The remaining muscles of this region may be considered as differentiations of the general vertebral musculature (M. extensor dorsi communis).
=M. splenius= (Fig. 73, _b_.)--A large sheet of muscle covering the dorsal part of the side of the neck, beneath the trapezii.
_Origin_ from the whole cervical ligament and from the fascia covering the deeper muscles along a line which extends from the first thoracic spinous process caudolaterad for about two centimeters.
_Insertion_ by a thin tendon into the whole lambdoidal ridge. Laterad the tendon may be fused with that of the longissimus capitis (Fig. 73, _g_).
_Relations._--Outer surface with the sternomastoid (Fig. 68, _c_), occipitoscapularis (Fig. 73, _a_), clavotrapezius (Fig. 68, _d_), rhomboideus, tendon of the serratus posterior superior (Fig. 73, _l_), and the levator scapulæ (Fig. 73, _h_). Lateral edge closely united with the longissimus capitis (Fig. 73, _g_). Inner surface with the longissimus capitis, complexus (Fig. 69, _b_), and biventer cervicis (Fig. 69, _a_).