Part 33

Action.--Bends the vertebral column sideways.

B. MUSCLES ON THE VENTRAL SIDE OF THE NECK (Fig. 65, page 109).--=M. sternomastoideus= (Fig. 65, g; Fig. 68, c).--The sternomastoid (sternal portion of the human sternocleidomastoid) is a flat band one to three centimeters wide extending from the cranial end of the manubrium and the midventral line craniad of it to the lambdoidal ridge.

The origin extends from the cranial end of the manubrium along the midventral line as far as the caudal border of the cricoid cartilage, and is in two parts. The =caudal= portion arises from the lateral surface of the manubrium along the dorsal half of its cranial end, and from the median raphe for about one centimeter craniad of the manubrium. Its fibres are parallel and tend to diverge into two layers. The =cranial= portion arises by means of fibres which cover the median line and interdigitate with the fibres of the opposite muscle between the cricoid cartilage and a point a few millimeters craniad of the manubrium. Its caudal border is thus overlaid by the caudal portion. Its fibres converge and join those of the caudal portion. The muscle passes dorsocraniad and is

Inserted by means of a flat tendon into the lateral half of the lambdoidal ridge and into a continuation of the ridge onto the mastoid portion of the temporal bone as far as the mastoid process. The thickest part of the tendon is inserted into the mastoid process.

Relations.--Outer surface at the caudal end with the pectoralis major (Fig. 65, l); in the middle part with the platysma, the depressor conchæ (Fig. 64, b), and with the external jugular vein (Fig. 65, 5), which crosses it obliquely; at the cranial end with the submaxillary (Fig. 65, 2) and parotid (1) glands. Inner surface with the sternohyoid (Fig. 65, e), sternothyroid (g′), internal jugular vein, longus capitis muscle, the cleidomastoid (h), levator scapulæ ventralis (i), a large lymphatic gland ventrad of the ear, and the splenius (Fig. 73, b).

Action.--One muscle turns the head and depresses the snout. Both together depress the snout.

=M. sternohyoideus= (Fig. 65, e).--A slender muscle on the midventral line of the neck close to the opposite muscle.

Origin from the cranial border of the first costal cartilage. The muscle passes craniad closely united near its caudal end to the sternothyroid (g′).

Insertion (Fig. 67, m) into the outer half of the ventral surface of the body of the hyoid bone caudad of the origin of the geniohyoid (Fig. 67, g).

Relations.--Outer surface with the sternomastoid (Fig. 65, g). Inner surface with the thyrohyoid (Fig. 65, f; Fig. 67, p), the trachea (Fig. 67, 4), and the larynx.

Action.--Draws the hyoid caudad. Raises the ribs and sternum when the hyoid is fixed.

=M. sternothyreoideus= (Fig. 65, g′).--The sternothyroid lies beneath the sternohyoid and is connected with it at its caudal end.

Origin from the first costal cartilage beneath the sternohyoid.

Insertion (Fig. 67, o) into the lateral part of the caudal border of the thyroid cartilage of the larynx.

Relations.--Outer surface with the sternohyoid (Fig. 65, e) and laterally with the sternomastoid (Fig. 65, g). Inner surface with the trachea (Fig. 67, 4), and at the lateral edge with the thyroid gland (Fig. 67, 6).

Action.--Pulls the larynx caudad.

=M. scalenus= (Fig. 73, f-f‴).--This is a large and complex muscle lying on the ventral side of the neck and the lateral surface of the thorax. It might equally well be classified with the muscles of the thorax. It is divisible into a considerable number of interconnected bundles which are sometimes described as separate muscles.

The largest, middle portion (=scalenus medius=) (f′) takes origin by thin tendons from the sixth, seventh, eighth, and ninth ribs, just dorsad of their junction with the cartilages; these tendons unite near their origins to form a flat band which becomes fleshy at about the fifth rib, passes craniad, and unites just craniad of the first rib with the other parts of the muscle. The dorsal portion (=scalenus posterior=) (f″) has origin by a very slender tendon from about the middle of the outer surface of the third or fourth rib; it extends craniad as a narrow band which passes laterad between the heads of the serratus anterior (i) that are attached to the second and third (or third and fourth) ribs, and unites with the other heads. The ventral head (part of =scalenus anterior=) (f‴) arises by one or two minute tendons from the cartilages of the second and third ribs; they are partly united with the transversus costarum (j). This ventral head passes craniad and unites with the other heads.

These three divisions unite just craniad of the first rib with each other and with the cervical portion of the muscle (f) (part of =scalenus anterior=). This consists of a number of small bundles of fibres which arise from the first rib and the transverse processes of the first thoracic and the last six cervical vertebræ.

The insertion is onto the transverse processes of all the cervical vertebræ, including the axis and atlas. According to Strauss-Durckheim the fibres from any given transverse process are inserted into the transverse processes of all the vertebræ craniad of it.

The muscle is partly continuous craniad with the longus capitis (e). The cervical nerves pass out between the bundles of the cervical portion.

Relations.--Outer (lateral) surface in the thoracic region with the external oblique (Fig. 68, p), the pectoralis minor (Fig. 65, o), and in part with the serratus anterior (Fig. 73, i); in the cervical region with the sternomastoid (Fig. 65, g). Inner (medial) surface with the serratus anterior (Fig. 73, i), the levator scapulæ (Fig. 73, h), the longus atlantis, the transverse processes of the cervical vertebræ, and the longus capitis (Fig. 73, e).

Action.--Flexes the neck and draws the ribs craniad.

=M. longus capitis= (or =rectus capitis anterior major=) (Fig. 72, a; Fig. 73, e).--This is a long muscle lying on the ventral aspect of the cervical vertebræ. With the muscle of the opposite side and the underlying longus colli (Fig. 72, g′) it forms a trough in which lie the œsophagus, pharynx, and trachea.

Origin by five (or six) heads from the ventral margins of the transverse processes of the cervical vertebræ from the second to the sixth inclusive. The heads unite into a common belly, the lateral border of which is united with the levator scapulæ ventralis (Fig. 72, c) near its insertion.

Insertion into the body of the sphenoid bone between the bulla and the middle line. The insertion extends craniad onto the basisphenoid.

Relations.--Ventral surface with the sternomastoid (Fig. 65, g) and the large lymphatic gland ventrad of the external ear. Lateral surface with the scalenus (Fig. 72, h) and craniad with the levator scapulæ ventralis (Fig. 72, c-c″). Medial surface with the carotid artery, the trachea, œsophagus, and pharynx. Dorsal edge with the vertebræ, the longus colli (Fig. 72, g′), and the rectus capitis anterior minor (Fig. 72, b).

Action.--Lowers the snout.