Part 27

=M. acromiotrapezius=, or trapezius superior (Fig. 68, h).--The acromiotrapezius (middle part of the human trapezius) is a flat four-sided muscle just craniad of the spinotrapezius, from the median dorsal line to the scapular spine.

Origin along the median dorsal line from the spinous process of the axis to a point anywhere between the spinous processes of the first and fourth thoracic vertebræ. Its caudal two-thirds is from a flat tendon which is directly continuous with the muscle of the opposite side and thus bridges the depression between the vertebral borders of the scapulæ. This tendon narrows craniad, and its cranial fourth is attached by a fascia to the spinous processes of the cervical vertebræ. At the cranial border of the muscle the fibres sometimes reach the middle line. The fibres of the muscle run nearly transversely.

Insertion (Fig. 76, h).--Into the outer surface of the metacromion, the glenoid border of the scapular spine from the metacromion to the tuberosity, and along a line continued for about one centimeter from the tuberosity onto the surface of the spinotrapezius (j) at the junction of its muscular and tendinous portions.

a, M. frontoscutularis; b, M. masseter; c, M. sternomastoideus; d, M. clavotrapezius; e, M. clavobrachialis; f, M. levator scapulæ ventralis; g, M. acromiodeltoideus; h, M. acromiotrapezius; i, M. spinodeltoideus; j, M. spinotrapezius; k, caput longum of M. triceps brachii; l, caput laterale of M. triceps brachii; m, M. latissimus dorsi; n, M. xiphihumeralis; o, M. pectoralis minor; p, M. obliquus abdominis externus; q, M. sartorius; r, M. tensor fasciæ latæ; s, M. caudofemoralis; t, M. biceps femoris; u, M. semitendinosus; v, M. extensor caudæ lateralis; w, M. abductor caudæ externus; x, M. flexor caudæ longus; y, lumbodorsal fascia, superficial layer; z, fascia lata. 1, parotid gland; 2, submaxillary gland.]

Relations.--Outer surface with the cutaneus maximus, the integument and platysma. Inner surface with a mass of fat partially covering the rhomboideus; with the occipitoscapularis, spinotrapezius, supraspinatus, levator scapulæ, and splenius. Near its origin its caudal border touches the outer surface of the spinotrapezius (j). Its cranial border is overlaid near its insertion by the levator scapulæ ventralis (f) and is closely related to the caudal border of the clavotrapezius (d) near its origin.

Action.--The two muscles hold the scapulæ together.

=M. clavotrapezius= (Fig. 68, d).--The clavotrapezius (cranial part of the human trapezius) is a flat muscle passing from the lambdoidal crest and middorsal line to the clavicle and covering the side of the neck.

Origin.--The medial half of the lambdoidal crest and the middorsal line between the crest and the caudal end of the spine of the axis. The fibres are parallel and pass caudoventrad so that the muscle covers the ventral surface of the neck caudally and fills the concavity at the front of the shoulder.

Insertion into the clavicle and into a raphe between the clavotrapezius and the clavobrachial (e) muscles. This raphe extends along the clavicle and for some distance laterad of it. The muscle is partly continuous with the clavobrachial (e).

Relations.--Outer surface with the platysma, and at its craniomedial angle with the levator auris longus. Inner surface with the rhomboideus, splenius, occipitoscapularis, levator scapulæ, cleidomastoid, and supraspinatus. The cranial edge touches the sternomastoid. The caudal border touches the acromiotrapezius near the origin.

Action.--Draws the scapula craniodorsad.

This muscle and the clavobrachial (e) are sometimes considered as forming a single muscle, the cephalohumeral, or cephalobrachial.

=M. occipitoscapularis= (or levator scapulæ dorsalis or rhomboideus capitis) (Fig. 73, a).--The occipitoscapularis (equivalent to a part of the human rhomboideus) is a slender flat muscle from the lambdoidal ridge to the coracovertebral angle of the scapula.

Origin, the medial half of the lambdoidal ridge beneath the clavotrapezius (Fig. 68, d). The origin does not extend quite to the middle line. The muscle passes almost directly caudad. Its caudal end is thicker and narrower than the cranial end and is wedged between the rhomboideus dorsally and the levator scapulæ ventrally.

Insertion either into the inner surface of the levator scapulæ near its dorsocranial border at its insertion or into the scapula near the coracovertebral angle (Fig. 78, f) or into both. Its fibres are very closely related to those of the rhomboideus.

Relations.--By its outer surface with the clavotrapezius (Fig. 68, d) and acromiotrapezius (h). At its insertion it is often overlaid by either the rhomboideus or the levator scapulæ or both. By its inner surface with the splenius (Fig. 73, b). Its caudal third is wedged between the rhomboideus and the levator scapulæ.

Action.--Draws forward and rotates the scapula.

=M. rhomboideus= (probably equivalent to the human rhomboideus major and rhomboideus minor).--A rather thick trapezoid muscle connecting the vertebral border of the scapula with the spinous processes of the adjacent vertebræ.

Origin from the caudal two or three fifths of the cervical supraspinous ligament and from the sides and tips of the first four thoracic vertebral spines and the interspinous ligaments caudad of these four spines. It passes lateroventrad, and the fibres converge somewhat so that the line of insertion is shorter than the line of origin.

Insertion by a short tendon (1 millimeter long) into the vertebral border of the scapula, the line of insertion passing gradually from the inner (Fig. 78, e) to the outer (Fig. 76, i) surface; and by fleshy fibres into the outer surface of the glenovertebral angle of the scapula (Fig. 76, i) in close connection with the origin of the teres major (j). A bundle of fibres at the cranial border may be inserted into the cranial border of the levator scapulæ at its insertion.

Relations.--By its outer surface with the spinotrapezius (Fig. 68, j), acromiotrapezius (h), latissimus dorsi (m), and clavotrapezius (d); it is usually partially separated from these muscles by a mass of fat. By its inner surface near its insertion with the serratus anterior (Fig. 73, i) and levator scapulæ (Fig. 73, h), and craniad with the splenius (Fig. 73, b), and the tendon of the serratus posterior superior (Fig. 73, l). At its lateral end it touches the occipitoscapularis (Fig. 73, a), serratus anterior (i), levator scapulæ (h), supraspinatus (Fig. 75, a), and infraspinatus (Fig. 75, c), and the teres major (Fig. 75, d).

Action.--Draws the scapula toward the vertebral column.

=M. cleidomastoideus= (clavicular portion of the human sternocleidomastoid) (Fig. 65, h).--A narrow flat muscle between the mastoid process and the clavicle.

Origin from the apex and caudal margin of the mastoid process of the temporal bone, by tendon and muscle fibres. The muscle passes caudad, becoming broader and thinner.

Insertion into the lateral four-fifths of the clavicle within the insertion of the clavotrapezius, and into the raphe which continues laterad of the clavicle.

Relations.--Outer or lateral surface at the caudal end with the clavotrapezius (Fig. 65, j); further craniad with the depressor conchæ (Fig. 64, b), the platysma, and the sternomastoid (Fig. 65, g). Inner or medial surface with the supraspinatus (Fig. 75, a) caudad, with the mass of fat in the axilla at its middle, with the levator claviculæ ventralis (Fig. 65, i) and obliquus capitis superior (Fig. 71, e) craniad.

Action.--Pulls the clavicle craniad when the head is fixed. Turns the head and depresses the snout when the clavicle is fixed.

=M. levator scapulæ ventralis=, or levator claviculæ (Fig. 68, f; Fig. 65, i; Fig. 72, c, c′, c″) (not usually found in man).