160 DEPRESSORS OF THE OS IIYOIDES A3MD LARYNX. 



old persons as would be the case if the extension of the skin were 

 the mere result of elasticity. It draws also upon the angle of the 

 mouth, and is one of the depressors of the lower jaw. The trans- 

 verse fibres draw the angle of the mouth outwards and slightly 

 upwards. The sterno-mastoid muscles are the great anterior mus- 

 cles of connexion between the thorax and the head. Both mus- 

 cles acting together will bow the head directly forwards, The 

 clavicular portions, acting more forcibly than the sternal, give 

 stability and steadiness to the head in supporting great weights. 

 Either muscle acting single would draw the head towards the 

 shoulder of the same side, and carry the face towards the opposite 

 side. 



Second Group. Depressors of the Os Hyoidcs and Larynx. 



Sterno-hyoid, 



Sterno-thyroid, 



Thyro-hyoid, 



Omo-hyoid. 



Dissection. These muscles are brought into view by removing 

 the deep fascia from off the front of the neck between the two 

 sterno-mastoid muscles. The omo-hyoid to be seen in its whole 

 extent requires that the sterno-mastoid muscle be divided from its 

 origin and turned aside. 



The sterno-hyoideus is a narrow riband-like muscle, arising from 

 the posterior surface of the first bone of the sternum and inner ex- 

 tremity of the clavicle. It is inserted into the lower border of the 

 body of the os hyoides. The sterno-hyoidei are separated by a con- 

 siderable interval at the root of the neck, but approach each other 

 as they ascend ; they are frequently traversed by a tendinous inter- 

 section. 



Relations. By its external surface with the deep cervical fascia, 

 the platysma myoides and sterno-mastoid muscle; by its internal 

 surface with the sterno-thyroid and thyro-hyoid muscle, the thyroid 

 gland, and the superior thyroid artery. 



The sterno-thyroideus, broader than the preceding beneath which 

 it lies, arises from the posterior surface of the upper bone of the 

 sternum, and from the cartilage of the first rib ; and is inserted into 

 the oblique line, on the great ala of the thyroid cartilage. The inner 

 borders of these muscles lie in contact along the middle line, and 

 they are generally marked by a tendinous intersection at their lower 

 part. 



Relations. By its external surface, with the sterno-hyoid, omo- 

 hyoid, and sterno-mastoid muscle ; by its internal surface, with the 

 trachea and inferior thyroid veins, with the thyroid gland, the lower 

 part of the larynx, the sheath of the common carotid artery and 

 internal jugular vein, with the subclavian vein and vena innominata, 

 and on the right side with the arteria innominata. The middle thy- 

 roid vein lies along its inner border. 



The t/iyro-hyoideus is the continuation upwards of the sterno-thy- 

 roid muscle. It arises from the oblique line on the thyroid cartilage, 



