Remove what remains of the dorsal portion of the latissimus dorsi,
serratus posterior superior, and serratus posterior inferior. See that
the skin is removed completely to the dorsal middle line, or even for a
little distance over onto the other side. There is thus exposed the
lumbodorsal fascia (p. 126), covering the muscles of the back.
Cut the superficial layer of the lumbodorsal fascia (that from which the
latissimus dorsi and obliquus externus abdominis have origin) by an
incision parallel to the dorsal median line, and remove it. The muscles
of the back are thus exposed (Fig. 69). Observe in the lumbar region the
longissimus dorsi (_f_, _f′_, _f″_); in the thoracic region the
longissimus dorsi, the spinalis dorsi (_g_), and the iliocostal (_h_);
in the cervical region the splenius (Fig. 73, _b_).
1. The longissimus dorsi (p. 126, and Fig. 69, _f_, _f′_, _f″_). Notice
in the lumbar region the strong deep layer of the lumbodorsal fascia, by
which this muscle is covered, and from the under side of which many of
the fibres of the muscle take origin. Cut this fascia by an incision
parallel to the dorsal median line and about two centimeters from it,
from the level of the crest of the ilium as far craniad as the fascia
can be cut without cutting into the muscle-fibres (usually to about the
level of the last rib). Reflect the medial division of the fascia to the
middle line. Reflect the lateral division of the fascia until it passes
into the muscle and cannot be farther reflected.
Observe then in the lumbar region the medial (Fig. 70, _a_) and lateral
(Fig. 70, _b_) divisions of the longissimus dorsi, and the portion
(_b′_) of the lateral division taking origin from the lumbodorsal fascia
(_c_). Dissect apart some of the muscle-fibres of the longissimus and
observe their origin and insertion.
Trace the longissimus dorsi into the thoracic and cervical regions (Fig.
69, _f′_, _f″_) noting the separation off from it on the dorsal side of
the spinalis dorsi (Fig. 69, _g_); on the ventral side of the iliocostal
(Fig. 69, _h_).
2. The extensor caudæ lateralis (p. 137, and Fig. 70, _f_). Uncover
this by cutting the lumbodorsal fascia over the sacral region.
3. The iliocostalis (p. 128, and Fig. 69, _h_). Uncover this
completely; note its connection with the longissimus dorsi at the caudal
end. Dissect apart some of its muscle-bundles, to see origin and
insertion.
4. The splenius (p. 131, and Fig. 73, _b_). Transect this by an
incision beginning at its lateral border about four centimeters from the
cranial end of the muscle and extending obliquely craniodorsad to the
craniomedial angle of the muscle. Be careful not to injure the
longissimus capitis (Fig. 73, _g_).