Part 29

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.

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.

Relations.--Outer surface with the serratus posterior inferior (Fig. 73, n) and superior (Fig. 73, l), and the levator scapulæ (Fig. 73, h). Inner surface with the dorsal surface of the ribs, the external intercostals (Fig. 69, i), and craniad with the levatores costarum. Medial edge in contact with the longissimus dorsi (Fig. 69, f′).

Action.--Draws the ribs together.

=M. spinalis dorsi= (Fig. 69, g).--A muscle interconnecting the spinous processes of vertebræ some distance apart, in the thoracic and cervical regions. It lies on the medial side of the longissimus dorsi (f′).

Origin by strong tendons from the tips of the spinous processes of the tenth to the thirteenth thoracic vertebræ. These tendons represent the cranial portion of the deep layer of the lumbodorsal fascia, and give origin also to many fibres of the longissimus dorsi, so that the two muscles are completely united at the origin of the spinalis. They become separated at about the level of the sixth thoracic vertebra, the spinalis forming a strong separate bundle passing into the neck region close against the sides of the spinous processes. The insertion is by fleshy bundles into the sides of the spinous processes of the first nine or ten thoracic vertebræ and of the cervical vertebræ as far forward as the second. Some of the fibres of this muscle pass craniad into the biventer cervicis (Fig. 69, a).

Relations.--Outer surface with the tendons of the serratus posterior inferior (Fig. 73, n) and serratus posterior superior (Fig. 73, l), and with the splenius (Fig. 73, b), biventer cervicis (Fig. 69, a), and complexus (Fig. 69, b). Lateral and ventral surface with the longissimus dorsi (f′); medial surface with the spinous processes and the multifidus spinæ.

Action.--Extensor of the vertebral column.

=M. multifidus spinæ=.--This consists of bundles of fibres which have origin on the transverse processes or neighboring parts, pass craniodorsad across one or more vertebræ, and are inserted into the spinous processes of vertebræ lying some distance craniad of the origin. They lie deeper than the muscles previously described. The muscle is most strongly developed in the lumbar region (Fig. 70, d), where it forms a thick interwoven mass in which it is difficult to distinguish separate bundles. The fibres in this region have origin on the accessory or mammillary processes and usually pass over more than one vertebra between origin and insertion; their insertions reach the dorsal ends of the spinous processes, so that part of the muscle lies immediately beneath the lumbar fascia. In other regions the multifidus is covered by other muscles. In the thoracic region the separate bands are more distinct, and usually pass in their course over but one intervening vertebra. In the cervical region the bands are interconnected, forming a fairly distinct single muscle, which is described separately below as the =semispinalis cervicis= (Fig. 71, c). The portion of this muscle attached to the head (=semispinalis capitis=) forms the biventer cervicis (Fig. 69, a) and complexus (b). Caudad this muscle passes onto the tail as the =extensor caudæ medialis= (Fig. 70, c).

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).

Action.--Lateral flexor of the head. The two together elevate the head.

=M. longissimus capitis= (=trachelomastoideus=) (Fig. 73, g; Fig. 69, e).--This is a slender muscle lying close against the lateral border of the splenius (Fig. 73, b), and sometimes fused with the splenius at its craniad end. It is a cranial continuation of the longissimus dorsi (Fig. 69, f″).