DEPRESSORS OF OS HYOIDES. 73 



is said by Theile to make tense tbe deep fascia of the neck with which it 

 is connected. 



The STKKXO-HYOID MUSCLE (fig. 15, 2 ) is aflat thin band nearer the 

 middle line than the preceding. It arises from the posterior surface of the 

 sternum and the cartilage of the first rib. From this spot the fibres ascend, 

 and are inserted into the lower border of the body of the os hyoides, inter- 

 nal to the preceding muscle. 



One surface is in contact with the fascia, and is often marked by a ten- 

 dinous intersection near the clavicle. When the muscle is divided and 

 turned aside, the deep surface will be found to touch the sterno-thyroideus, 

 and the superior thyroid vessels. The muscles of opposite sides are sepa- 

 rated by an interval which is largest below. 



Action. It draws the os hyoides downwards after swallowing ; and in 

 laborious respiration it will act as an elevator of the sternum. 



The STERNO-TIIYROID MUSCLE is wider and shorter than the sterno- 

 hyoid, beneath which it lies. Like the other hyoid muscle, it arises from 

 the posterior surface of the sternum, and the cartilage of the first rib below 

 the former ; and it is inserted into the oblique line on the side of the thy- 

 roid cartilage, where it is continuous with the thyro-hyoid muscle. 



The inner border touches its fellow for about an inch, along the middle 

 line of the neck, whilst the outer reaches over the carotid artery. The 

 superficial surface is concealed by the preceding hyoid muscles ; and the 

 opposite surface is in contact with the lower part of the common carotid 

 artery, the trachea, and the larynx and thyroid body. A transverse, ten- 

 dinous line crosses the muscle near the sternum. 



Action. Its chief use is to aid the preceding muscle in lowering rapidly 

 the hyoid bone after deglutition ; but it can draw down and forwards the 

 thyroid cartilage, and assist in rendering tight the vocal cords. 



Like the sterno-hyoid it participates in the movement of the chest in 

 laborious breathing. 



The THYRO-HYOIDEUS (fig. 15, 4 ) is a continuation in direction of the 

 last muscle. Beginning on the side of the thyroid cartilage, the fibres 

 ascend to the inner half of the great cornu of the os hyoides, and to the 

 outer part of the body of the bone. 



On the muscle lies the orno-hyoideus ; and beneath it are the superior 

 laryngeal nerve and vessels. It is sometimes considered one of the special 

 muscles of the larynx. 



Action. Raising the thyroid cartilage towards the os hyoides, it renders 

 lax the vocal cords, and assists in placing the cartilage under the tongue 

 preparatory to swallowing. 



Directions The remaining parts included in this SECTION are the 



scaleni muscles and the subclavian bloodvessels, with the cervical nerves 

 and the carotid bloodvessels. The student may examine them in the order 

 here given. 



Dissection (fig. 16). Supposing the sterno-mastoid cut, the fat and 

 fascia are to be taken away from the lower part of the neck, so as to pre- 

 pare the scaleni muscles with the subclavian vessels and their branches. 

 By means of a little dissection the anterior scalenus muscle will be seen 

 ascending from the first rib to the neck, having the phrenic nerve and sub- 

 clavian vein in front of it, the latter crossing it near the rib. 



The part of the subclavian artery on the inner side of the scalenus is 

 then to be cleaned, care being taken not only of its branches, but of the 

 branches of the sympathetic nerve which course over and along it from the 



