THE IXFRA-HYOID REGION. 411 



into action they serve to depress the head upon the neck and the neck upon the 

 chest. If the head is fixed, they 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 u:ni-ueclc. 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 s'ule. 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. This is performed 

 by introducing a tenotomy knife beneath it, close to its origin, and dividing it from behind for- 

 ward 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. Some of the fibres of the 

 Sterno-mastoid muscle are occasionally torn during birth, especially in breech presentations ; this 

 is accompanied by haemorrhage 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. 



2. Infra-hyoid Region (Figs. 276, 277). 

 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, which arises from the 

 inner extremity of the clavicle 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 considerable interval ; but they approach one another 

 in the middle of their course, and again diverge as they ascend. 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, 

 below the origin of the Sterno-hyoid, and from the edge of the cartilage of the 

 first rib. and is inserted into the oblique line on the side of the ala of the thyroid 

 cartilage. This muscle is in close contact with its fellow at the lower part of the 

 neck, and is occasionally traversed by a transverse or oblique tendinous intersection, 

 like those in the Rectus abdominis. 



Relations. By its anterior surface, with the Sterno-hyoid, Omo-hyoid, and 

 Sterno-mastoid ; by its p^sterinr surface, from below upward, with the trachea, 

 vena innominata. common carotid (and on the right side the arteria innominata), 

 the thyroid gland and its vessels, and the lower part of the larynx. The middle 

 thyroid vein lies along its inner border, a relation which it is important to 

 remember in the operation of tracheotomy. 



The Thyro-hyoid is a small, quadrilateral muscle appearing like a continuation 



