_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).
=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.