THE INFRA-HYOID REGION. 319 



pierces its upper third ; the cervical plexus, parts of the thyroid and parotid 

 glands, and deep lymphatic glands. 



Nerves. The Platysma myoides is supplied by the facial nerve; the Sterno- 

 cleido-mastoid, by the spinal accessory and deep branches of the cervical plexus. 



Actions. When only one Sterno-mastoid muscle acts, it draws the head 

 toward the shoulder of the same side, assisted by the Splenius and the Obliquus 

 capitis inferior of the opposite side. At the same time it rotates the head so as to 

 carry the face toward the opposite side. If the head is fixed, the two muscles 

 assist in elevating the thorax in forced inspiration. 



Surface Form. The anterior edge of the muscle forms a very prominent ridge beneath 

 the skin, which it is important to notice, as it forms a guide to the surgeon in making the neces- 

 sary incisions for ligature of the common carotid artery and for oesophagotomy. 



Surgical Anatomy. The relations of the sternal and clavicular parts of the Sterno-mastoid 

 should be carefully examined, as the surgeon is sometimes required to divide one or both por- 

 tions of the muscles in wry-neck. One variety of this distortion is produced by spasmodic con- 

 traction or rigidity of the Sterno-mastoid ; the head being carried down toward the shoulder of 

 the same side, and the face turned to the opposite side and fixed in that position. When there 

 is permanent shortening, subcutaneous division of the muscle is resorted to by some surgeons. 

 This is performed by introducing a tenotomy knife beneath it, close to its origin, and dividing 

 it from behind forward whilst the muscle is put well upon the stretch. There is seldom any 

 difficulty in dividing the sternal portion by making a puncture on the inner side of the tendon, 

 and then pushing a blunt tenotome behind it, and cutting forward. In dividing the clavicular 

 portion care must be taken to avoid wounding the external jugular vein, which runs parallel 

 with the posterior border of the muscle in this situation, or the anterior jugular vein, which 

 crosses beneath it. If the external jugular vein lies near the muscle, it is safer to make the 

 first puncture at the outer side of the tendon, and introduce a blunt tenotome from without 

 inward. Many surgeons prefer dividing the muscle by the open method. An incision is made 

 over either origin of the muscle, the tendon is exposed, a director is passed underneath it, 

 and it is then divided. With care and attention to asepsis this plan of treatment is devoid of 

 risk, and in this way the accidental division of the vessels can be avoided. Some of the fibres 

 of the Sterno-mastoid muscle are occasionally torn during birth, especially in breech presenta- 

 tions ; this is accompanied by hemorrhage and formation of a swelling within the substance of 

 the muscle. This by some is believed to be one of the causes of wry-neck, the scar tissue 

 which is formed contracting and shortening the muscle. 



2. Infra-hyoid Region (Figs. 202, 203). 

 DEPRESSORS OF THE Os HYOIDES AND LARYNX. 



Sterno-hyoid. Thyro-hyoid. 



Sterno-thyroid. Omo-hyoid. 



Dissection. The muscles in this region may be exposed by removing the deep fascia from 

 the front of the neck. In order to see the entire extent of the Omo-hyoid it is necessary to 

 divide the Sterno-mastoid at its centre, and turn its ends aside, and to detach the Trapezius 

 from the clavicle and scapula. This, however, should not be done until the Trapezius has been 



dissected. 



The Sterno-hyoid is a thin, narrow, ribbon-like muscle, Avhich arises from the 

 inner extremity of the clavicle, the posterior sterno-clavicular ligament, and the 

 upper and posterior part of the first piece of the sternum ; passing upward and 

 inward, it is inserted, by short, tendinous fibres, into the lower border of the body 

 of the os hyoides. This muscle is separated, below, from its fellow by a consider- 

 able interval ; but the two muscles come into contact with one another in the 

 middle of their course, and from this upward lie side by side. It sometimes 

 presents, immediately above its origin, a transverse tendinous intersection, like 

 those in the Rectus abdominis. 



Relations. By its superficial surface, below, with the sternum, the sternal end 

 of the clavicle, and the Sterno-mastoid ; and above, with the Platysma and deep 

 cervical fascia ; by its deep surface, with the Sterno-thyroid, Crico-thyroid, and 

 Thyro-hyoid muscles, the thyroid gland, the superior thyroid vessels, the thyroid 

 cartilage, the crico-thyroid and thyro-hyoid membranes. 



The Sterno-thyroid is situated beneath the preceding muscle, but is shorter and 

 wider than it. It arises from the posterior surface of the first bone of the sternum, 



