258 MUSCLES AND FASCIAE. 



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

 Sterno-mastoid. By its posterior surface, from below upwards, 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; an important relation to be remembered in 

 the operation of tracheotomy. 



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

 of the Sterno-thyroid. It arises from the oblique line on the side of the thyroid 

 cartilage, and passes vertically upwards to be inserted into the lower border of 

 the body and greater cornu of the hyoid bone. 



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

 muscles. By its internal surface, with the thyroid cartilage, the thyro-hyoid 

 membrane, and the superior laryngeal vessels and nerve. 



The Omo-hyoid passes across the side of the neck, from the scapula to the 

 hyoid bone. It consists of two fleshy bellies, united by a central tendon. It 

 arises from the upper border of the scapula, and occasionally from the transverse 

 ligament which crosses the suprascapular notch ; its extent of attachment to the 

 scapula varying from a few lines to an inch. From this origin, the posterior belly 

 forms a flat, narrow fasciculus, which inclines forwards across the lower part of 

 the neck; behind the Sterno-mastoid muscle, where it becomes tendinous, it changes 

 its direction, forming an obtuse angle, and ascends almost vertically upwards, close 

 to the outer border of the Sterno-hyoid, to be inserted into the lower border of 

 the body of the os hyoides, just external to the insertion of the Sterno-hyoid. 

 The tendon of this muscle, which varies much in its length and form in different 

 subjects, is held in its position by a process of the deep cervical fascia, which 

 includes it in a sheath, and is prolonged down, to be attached to the cartilage 

 of the first rib. It is by this means that the angular form of the muscle is 

 maintained. 



This muscle subdivides each of the two large triangles at the side of the neck 

 into two smaller triangles. The two posterior ones are the posterior superior 

 or suboccipital, and the posterior inferior or subclavian; the two anterior, the 

 anterior superior or superior carotid, and the anterior inferior or inferior carotid 

 triangle. 



Relations. By its superficial surface, with the Trapezius, Subclavius, the 

 clavicle, the Sterno-mastoid, deep cervical fascia, Platysma, and integument. By 

 its deep surface, with the Scaleni, brachial plexus, sheath of the comm'on carotid 

 artery, and internal jugular vein, the descendens noni nerve, Sterno-thyroid and 

 Thyro-hyoid muscles. 



Nerves. The Thyro-hyoid is supplied by the hypoglossal ; the other muscles 

 of this group by branches from the loop of communication between the descendens 

 noni and communicans noni. 



Actions. These muscles depress the larynx and hyoid bone, after they have 

 been drawn up with the pharynx in the act of deglutition. The Omo-hyoid 

 muscles not only depress the hyoid bone, but carry it backwards, and to one or 

 the other side. These muscles are also tensors of the cervical fascia. The Thyro- 

 hyoid may act as an elevator of the thyroid cartilage, when the hyoid bone ascends, 

 drawing upwards the thyroid cartilage behind the os hyoides. 



3. SUPRA-HYOID KEGION (figs. 153 and 154). 



ELEVATORS OF THE Os HYOIDES ; DEPRESSORS OF THE LOWER JAW. 



Digastric. Mylo-hyoid. 



Stylo-hyoid. Genio-hyoid. 



Dissection. To dissect these muscles, a block should be placed beneath the back of the neck, 

 find the head drawn backwards, and retained in that position. On the removal of the deep fascia, 

 '-ho muscles are at once exposed. 



