Part 13

The great dorsal mass running along each side of the vertebral column, occupying the space between the spinous processes and the transverse processes, receives the general name of =M. extensor dorsi communis=; the separate muscles are to be considered as parts or differentiations of this. The largest part of the mass is in the lumbar region (first portion of the =longissimus dorsi=, Fig. 69, _f_). At the caudal end of the thorax a lateral mass, parts of which become connected with the ribs, is separated off from the main part of the muscle; this extends craniad into the cervical region as =M. iliocostalis= (_h_). The main portion of the longissimus dorsi (_f′_) continues craniad on the medial side of the iliocostal into the neck region. At about the level of the eighth or ninth thoracic vertebra a strip begins to become separated off on the medial side of the longissimus; further forward it becomes clearly distinct, forming the =spinalis dorsi= (_g_). The separate muscles connecting the cervical vertebræ with each other and with the head are differentiations of the extensor dorsi communis.

A still deeper set of fibres, interconnecting the vertebræ, forms the =multifidus spinæ= and a number of other small muscles.

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.

(_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.

_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″_).

_Origin_ by five slips which are attached by strong tendons to the anterior articular processes of the last four cervical vertebræ. The tendons are common to this muscle and the complexus. The five slips unite to form a flat belly which has its

_Insertion_ by a strong rounded tendon into the mastoid process of the temporal bone. The tendon may be closely united to that of the splenius.

_Relations._--Lateral surface with the sternomastoid (Fig. 68, _c_), the levator scapulæ (Fig. 73, _h_), and longissimus dorsi (Fig. 69, _f″_). Medial surface with the splenius (Fig. 73, _b_) (to which it is partly united), the complexus (Fig. 69, _b_), and the longus atlantis (Fig. 69, _d_).

_Action._--Lateral flexor of the head.

=M. biventer cervicis= (Fig. 69, _a_) (=medial portion of M. semispinalis cervicis et capitis=).--The biventer cervicis is a flat muscle which lies beneath the splenius (Fig. 73, _b_), next the median line of the neck, with its fibres longitudinal.

_Origin_ in three or four slips from the surface of the three or four aponeurotic arches which take origin from the tips of the spinous processes of the last cervical and the first three thoracic vertebræ and pass laterad and caudad to the anterior processes of the second, third, fourth, and fifth thoracic vertebræ. These arches are deviated from their straight course by four aponeurotic bands which pass from them obliquely caudad and dorsad to the sides of the spinous processes of the vertebræ into which the arches are inserted. The arches form the beginning of the sheet of fascia which passes outside of the longitudinal supraspinous muscles, and the anchoring bands form the beginning of a similar sheet which passes within the longitudinal supraspinous muscles. An additional slip may take origin from the cervical ligament between the last cervical and the first thoracic vertebræ. The fibres form a flat band divided by two or three oblique tendinous intersections. It adheres closely to the cervical supraspinous ligament.

_Insertion_ by a strong tendon into the medial part of the lambdoidal crest.

_Relations._--Outer surface with the splenius (Fig. 73, _b_) and at the caudal end with the longissimus dorsi (Fig. 69, _f″_). Inner surface with the spinalis dorsi (Fig. 69, _g_), the semispinalis cervicis (Fig. 71, _c_), and the rectus capitis posterior major (Fig. 71, _a_). Medial edge with the muscle of the opposite side; lateral edge with the complexus (Fig. 69, _b_).

_Action._--Raises the snout.

=M. complexus= (Fig. 69, _b_) (=lateral portion of M. semispinalis cervicis et capitis=).--The complexus is closely associated with the biventer cervicis (_a_) and lies on its lateral side beneath the splenius (Fig. 73, _b_).

_Origin_ by tendinous bands from the anterior articular processes of the last five or six cervical and the first one, two, or three thoracic vertebræ. The same bands give origin externally to the fibres of the longissimus capitis (Fig. 69, _e_), and internally to those of deeper muscles of the neck. The six or more muscular slips continued from these tendons unite to form a flat band, the

_Insertion_ of which is by a flat tendon into the medial third of the lambdoidal crest some distance ventrad of the free edge of the crest. Near the insertion the lateral border of the muscle is connected by an aponeurotic band to the lateral border of the transverse process of the atlas.

_Relations._--Outer surface at the dorsal border with the biventer cervicis (Fig. 69, _a_); at the middle with the splenius (Fig. 73, _b_); at the ventral border with the longissimus capitis (Fig. 69, _e_) and the longissimus dorsi (Fig. 69, _f″_). Inner surface with the cervical portion of the spinalis dorsi, with the semispinalis cervicis (Fig. 71, _c_), the obliquus capitis inferior (Fig. 71, _b_), the rectus capitis posterior major (Fig. 71, _a_), and the obliquus superior (Fig. 71, _e_).

_Action._--Raises the head.

=M. spinalis dorsi= extends into the cervical region; it has been described.

=M. semispinalis cervicis= (Fig. 71, _c_).--This represents that portion of the multifidus spinæ which extends into the cervical region. It is not composed of distinct bundles, like the multifidus of the thoracic region, but all the fibres are united into a fairly well-defined muscle.

_Origin_ from the articular processes of the last five cervical vertebræ.

_Insertion_ into the spinous processes of the cervical vertebræ up to the second, the largest part of the muscle being inserted into the caudal end of the spine of the atlas.

_Relations._--Outer surface with the spinalis dorsi (Fig. 69, _g_) and the complexus (Fig. 69, _b_). Inner surface with the vertebræ.

_Action._--Extends the neck.

=M. longissimus dorsi= extends into the cervical region, where it is often distinguished as the =longissimus cervicis= (Fig. 69, _f″_). It has been described.

=M. longus atlantis= (Fig. 71, _f_; Fig. 69, _d_).--This represents a differentiated portion of the longissimus dorsi.

_Origin_ from the transverse process and the side of the vertebral arch of the third cervical vertebra.

_Insertion_ into the caudolateral angle of the wing of the atlas.

_Relations._--Dorsal surface with the complexus (Fig. 69, _b_), the longissimus capitis (Fig. 69, _e_), and the obliquus inferior (Fig. 71, _b_). Ventral surface with the scalenus (Fig. 71, _g_), longissimus dorsi (Fig. 69, _f″_), and levator scapulæ (Fig. 71, _h_).

_Action._--Extends the neck and turns the head sideways.

=M. rectus capitis posterior major= (Fig. 71, _a_).

_Origin._--The whole length of the spinous process (or crest) of the axis, or its caudal part only. It is united by a raphe to the opposite muscle. The muscle passes craniad and laterad to its

_Insertion_ into an area ventrad of the medial part of the lambdoidal crest beneath the insertion of the complexus (Fig. 69, _b_) and the biventer cervicis (Fig. 69, _a_).

_Relations._--Dorsal surface with the biventer cervicis (Fig. 69, _a_). Ventral surface with the rectus capitis posterior medius (Fig. 71, _d_) and the obliquus capitis inferior.

_Action._--Raises the snout.

=M. rectus capitis posterior medius= (Fig. 71, _d_). (This is in man a part of the rectus capitis posterior major.)--It lies beneath the rectus capitis posterior major (Fig. 71, _a_).

_Origin_ from the cranial end of the axial spine. It forms a triangular prismatic mass which passes craniad and has its

_Insertion_ into an elongated area on the occipital bone, ventrad of the median half of the lambdoidal crest.

_Relations._--Dorsal surface with the rectus capitis posterior major (Fig. 71, _a_). Ventral surface with the atlas and the rectus capitis posterior minor.

_Action._--Assists the rectus capitis posterior major.

=M. rectus capitis posterior minor.=--A small triangular muscle beneath the rectus capitis posterior medius (Fig. 71, _d_).

_Origin_ by the apex of the triangle from the cranial border of the dorsal arch of the atlas for about two millimeters laterad of the median line. The muscle passes over the ligament which connects the atlas to the occiput and adheres to it.

_Insertion_ into an elongated area on the occipital bone, ventrad of the insertion of the rectus capitis posterior medius for about five millimeters next to the median line.

_Relations._--Dorsal surface with the rectus capitis posterior medius. Ventral surface with the posterior atlanto-occipital membrane.

_Action._--Assists the rectus capitis posterior major.

=M. obliquus inferior= (Fig. 71, _b_).

_Origin_ from the whole lateral surface of the spine of the axis. The parallel fibres form a thick mass which passes laterad and craniad.

_Insertion_ into the dorsal surface of the transverse process of the atlas along a narrow area near its lateral margin.

_Relations._--Outer surface with the rectus capitis posterior major (Fig. 71, _a_), the complexus (Fig. 69, _b_), and the obliquus superior (Fig. 71, _e_). Inner surface with the axis.

_Action._--Rotates the head.

=M. obliquus capitis superior= (Fig. 71, _e_; Fig. 72, _e_).--This is a triangular muscle passing from the outer border of the atlas laterad of the recti capitis posteriores to the occiput.