Part 28

Origin (Fig. 72, c′, c″) in two parts. (1) By fleshy fibres from the ventral surface of the atlantal transverse process near its caudal border (c′). (2) By a small tendon from the ventral surface of the basioccipital opposite the middle of the length of the bulla tympani (c″) in common with the longus capitis (a). The two heads unite to form a flat band which passes caudoventrad and is inserted (Fig. 76, f) by a flat short tendon, about eight millimeters broad, into the outer surface of the metacromion along its ventral border, and into the infraspinatus fossa for one or two millimeters distad of that border.

Relations.--Near the insertion the dorsal border overlaps the border of the acromiotrapezius (Fig. 68, h), and the ventral border is attached by strong fascia to the border of the clavotrapezius (Fig. 68, d). The outer surface is in relation with the longissimus capitis (Fig. 73, g), clavotrapezius (Fig. 68, d), cleidomastoid (Fig. 73, d), platysma, and the skin. The inner surface is in relation caudad with the supraspinatus and with a mass of fat and a lymphatic gland which occupy the hollow of the shoulder. Craniad it is in relation with the cleidomastoid (Fig. 73, d), the scalenus (Fig. 73, f), the longus capitis (Fig. 73, e), and the obliquus superior (Fig. 71, e).

Action.--Pulls the scapula craniad.

=M. latissimus dorsi= (Fig. 68, m; Fig. 65, q; Fig. 77, e).--A large triangular sheet covering rather more than the dorsocranial half of the abdomen and thorax. It arises from the middorsal line, from the pelvis to the fifth thoracic spine, and is inserted into the humerus.

Origin from the tips of the neural spines of the vertebræ from the fourth or fifth thoracic to about the sixth lumbar. As far as the tenth or eleventh thoracic vertebra the origin is directly by muscle-fibres. Caudad of this is a broad triangular tendon which may be so closely united to the tendons of the underlying muscles that it cannot be separated. The muscle passes cranioventrad, the fibres converging to the axilla. In the axilla a part of the cutaneus maximus and the epitrochlearis (Fig. 65, r) take origin from the outer surface.

Insertion.--The muscle then ends in a flat tendon, to the cranial surface of which the fibres of the teres major (Fig. 77, c) are attached. Five to ten millimeters from the humerus the muscle-fibres of the teres give place to the tendon-fibres, and the conjoined tendon of the teres and latissimus which is thus formed (Fig. 79, d′) is inserted into a rough elongated area (Fig. 82, f) which lies parallel with the pectoral ridge on the medial surface of the shaft of the humerus (Fig. 48, g). The teres tendon forms the proximal part of the conjoined tendon. A part of the tendon of the latissimus may join the pectoralis minor at its distal or caudal border (Fig. 65), and thus it may contribute to the formation of both pillars of the bicipital arch (Fig. 65, t′).

Relations.--Its outer surface is covered caudad by the cutaneus maximus, and craniad by the spinotrapezius (Fig. 68, j) and the long head of the triceps (Fig. 68, k). The cranial border comes into relation with the spinotrapezius (j) and teres major (Fig. 77, c). The caudal border is in contact at the ventral end with the xiphihumeralis (Fig. 65, p) and pectoralis minor (Fig. 65, o). The inner surface of the latissimus covers near its cranioventral end parts of the xiphihumeralis (Fig. 68, n), pectoralis minor (Fig. 68, o), teres major (Fig. 77, c), and serratus anterior (Fig. 73, i). The caudal border covers a portion of the obliquus externus abdominis (Fig. 68, p); the dorsal and middle portions cover parts of the longissimus dorsi (Fig. 69, f″), the spinalis dorsi (Fig. 69, g), the serratus posterior superior (Fig. 73, l), and serratus posterior inferior (Fig. 73, n).

Action.--Pulls the arm caudodorsad.

=M. serratus anterior= and =levator scapulæ= (Fig. 73, i and h) in the cat are continuous at both origin and insertion, and it is barely possible to separate their contiguous borders without cutting the fibres. The muscle extends from the transverse processes of the last five cervical vertebræ and from the first nine or ten ribs to the medial surface of the scapula near its vertebral margin (Fig. 78, c and d).

=M. serratus anterior= (or serratus magnus) (Fig. 73, i).

Origin.--From the first nine or ten ribs in as many subdivisions. The first subdivision arises from the whole or nearly the whole of the caudal border of the first rib. The next four or five arise either from the ribs or from the costal cartilages near or at their junction with the ribs. The last four subdivisions arise from the ribs at increasing distances from the cartilages. The fibres converge and the subdivisions join one another.

Insertion (Fig. 78, c) into a narrow area on the medial surface of the scapula near the vertebral border. The glenoid half of the insertion is by a short tendon, the remainder by muscle-fibres.

Relations.--Outer (lateral) surface with the latissimus dorsi (Fig. 68, m), teres major (Fig. 77, c), subscapularis (Fig. 77, a), and near the origin with the rectus abdominis (Fig. 73, k), the scalenus (Fig. 73, f-f‴), and the obliquus externus (Fig. 68, p). Inner (medial) surface with the serratus posterior superior (Fig. 73, l) and its tendon, and with the intercostales externi (Fig. 73, m). At its insertion the muscle touches the rhomboideus. The cranial edge is united to the caudal edge of the levator scapulæ (Fig. 73, h).

Action.--Depressor of the scapula.

=M. levator scapulæ= (Fig. 73, h).

Origin.--From the dorsal tubercles of the transverse processes of the last five cervical vertebræ and from the ligaments between the tubercles. A slip sometimes arises from the atlas.

Insertion (Fig. 78, d) into a triangular area on the medial surface of the scapula near its vertebral border. It is continuous with the insertion of the serratus anterior.

Relations.--Outer (lateral) surface with a mass of fat separating it from the clavotrapezius (Fig. 68, d) and cleidomastoid (Fig. 65, h), and with the subscapularis (Fig. 77, a). Inner (medial) surface with the splenius (Fig. 73, b), the tendon of the serratus posterior superior (Fig. 73, l), and the cervical portion of the longissimus dorsi (Fig. 69, f″). Near the insertion the muscle touches the occipitoscapularis (Fig. 73, a) and rhomboideus. Caudal border united with the serratus anterior (Fig. 73, i).

Action.--Draws the scapula cranioventrad.

B. MUSCLES OF THE BACK (interconnecting parts of the vertebral column) (Fig. 69).--The muscles connected with the vertebral column form a mass which is less markedly differentiated into distinct muscles than is the case in the limbs. A great longitudinal mass of fibres begins in the sacral region and extends along the vertebral column to the head. This mass contains fibres running in various directions and attached at one or both ends to the sacrum, the innominate bones, the spinous, transverse, and articular processes, and to the arches, of the vertebræ; to the ribs and to the head, and having in general the function of moving the vertebræ in various ways, or of moving parts connected with the vertebræ. In some regions, especially the cervical, it is possible to distinguish clearly distinct muscles which have been differentiated from this mass. In the greater part of its extent, however, it is possible to distinguish only more or less closely interconnected bundles,--the more clearly marked of which have received special names.

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, 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.