FLEXOR MUSCLES OF THE XECK. 193 



At its scapular origin the muscle is covered by the trapezius, in the middle of its 

 course by the sterno-mastoid, and at its upper part by the platysma ; it crosses the 

 scaleni muscles, the cervical nerves, the sheath of the common carotid artery and 

 jugular vein, and the sterno-thyroid and thyro-hyoid muscles. 



The muscle occasionally is attached to the clavicle instead of the scapula, arising 

 from the former bone about its middle, and in such a case the posterior belly is 

 absent. One instance has been recorded (R. Quain) in which the posterior belly alone 

 was present, and was connected to the hyoid bone by a band of fascia. The muscle 

 has likewise been observed double, one slip being attached to the clavicle and the 

 other to the normal place of origin on the scapula. 



ACTIONS. While the sterno-hyoid and omo-hyoid muscles act only as depressors of 

 the hyoid bone, and the sterno-thyroid as a depressor of the larynx, the sterno-thyroid 

 muscle, being a direct depressor of the thyroid cartilage, may draw down the hyoid 

 bone when it acts in conjunction with the thyro-hyoid : the latter muscle elevates the 

 larynx when the hyoid bone is fixed. In the act of swallowing, the hyoid bone and 

 thyroid cartilage pass suddenly upwards and forwards, and their natural position is 

 restored by the action of the muscles under consideration, the bone moving in the 

 direction of the omo-h}'oid muscles. In the utterance of low notes the larynx and 

 hyoid bone descend below the natural level, in the direction in which the sternal 

 muscles pull ; while in the utterance of high notes there is little elevation of the hyoid 

 bone, but the larynx is raised by the action of the thyro-hyoid muscles. 



FLEXOR MUSCLES OF THE NECK. 



The sterno-cleido-mastoid or sterno-mastoid muscle is attached inferiorly 

 in two parts to the anterior surface of the sternum and the inner third of 

 the clavicle. The sternal attachment is thick and rounded, tendinous in 

 front and fleshy behind. The clavicular portion, separated from the sternal 

 by a narrow interval, is flat, and is composed of fleshy and tendinous fibres. 

 Those two portions become blended together about the middle of the neck 

 into a thick and prominent muscle, which, extending upwards and back- 

 wards, is attached superiorly to the anterior border and external surface of 

 the mastoid process, and from thence backwards into a rough ridge of the 

 temporal, and by a thin aponeurosis into the outer part of the superior 

 curved line of the occipital bone. 



This muscle is covered for more than the middle three-fifths of its extent by 

 the platysma. It is also crossed by the external jugular vein, and by the ascending 

 superficial branches of the cervical plexus of nerves. It rests on part of the sterno- 

 hyoid and sterno-thyroid muscles, and crosses the omo-hyoid muscle ; in the middle 

 part of the neck it covers the cervical plexus and the great cervical vessels, and in the 

 upper part, the digastric and stylo-hyoid muscles : it is pierced by the spinal accessory 

 nerve. 



The sterno-cleido-mastoid is occasionally described as two muscles, under the names 

 sterno-mastoid and cleido-mastoid. Normally the fibres of the clavicular part run 

 upwards for some distance internally to those of the sternal part before finally 

 blending with them. The muscle varies much in breadth at the lower end, the 

 variation being due altogether to the clavicular part, which is sometimes as 

 narrow as the sternal tendon, while in other instances it extends for three 

 inches along the clavicle. This part of the muscle may likewise, when broader than 

 usual, be divided into several slips separated by intervals near the clavicle. A band 

 of muscular fibres has, in a few instances, been found reaching from the trapezius to 

 this muscle over the subclavian artery ; and the margins of the two muscles have 

 been observed in contact. In animals without a clavicle the cleido-mastoid muscle is 

 continued into the clavicular part of the great pectoral muscle, thus forming a 

 mastoido-humeral muscle. 



The scalenus anticus muscle is attached superiorly to the anterior 



o 



