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.