THE SUPRAHYOID REGION 389 



the insertion of the Sternohyoid. The central tendon of this muscle, which varies 

 much in length and form, is held in position by a process of the deep cervical 

 fascia, which includes it in a sheath. T r iis process is prolonged down, to be 

 attached to the clavicle and first rib. It is by this means that the angular form of 

 the muscle is maintained. 



The posterior belly of the Omohyoid divides the posterior triangle of the neck into an upper 

 or occipital, and a lower or subclavian triangle, while its anterior belly divides the anterior 

 triangle of the neck into an upper or carotid, and a lower or muscular triangle. 



Relations. By its superficial surface, with the Trapezius, the Sternomastoid, deep cervical 

 fascia, Platysma, and integument; by its deep surface, with the Scaleni muscles, phrenic nerve, 

 lower cervical nerves which go to form the brachial plexus, the suprascapular vessels and nerve, 

 sheath of the common carotid artery and internal jugular vein, tne Sternothyroid and Thyro- 

 hyoid muscles. 



Nerves. The Depressors of the hyoid bone are supplied by branches from the ansa cer- 

 vicalis formed by the first three cervical nerves. The branch to the Thyrohyoid accompanies 

 the hypoglossal nerve in a common sheath, but leaves the hypoglossal trunk as it approaches 

 the hyoid bone. 



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

 up with the pharynx in the act of deglutition. The Omohvoid muscles not only depress the 

 hyoid bone, but carry it backward and to one side. This bone is concerned especially in pro- 

 longed inspiratory efforts; for by rendering the lower part of the cervical fascia tense it lessens 

 the inward suction of the soft parts, which would otherwise compress the great vessels and the 

 apices of the lungs. This action is synergistic with that of the Platysma. The Thyrohyoid 

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

 the thyroid cartilage, behind this bone. The Sternothyroi<f acts as a depressor of the thyroid 

 cartilage. 



3. The Suprahyoid Region (Figs. 300, 301). 

 ELEVATORS OF THE HYOID BONE DEPRESSORS OF THE MANDIBLE. 



Digastric. Mylohyoid. 



Stylohyoid. Geniohyoid. 



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

 neck, and the head drawn backward and retained in that position. On the removal of the 

 deep fascia the muscles are at once exposed. 



The Digastric (ra. digastricus) consists of two fleshy belli ' united by an inter- 

 mediate, rounded tendon. It is a small muscle, situated below the side of the 

 body of the mandible, and extending, in a curved form, from the side of the 

 head to the symphysis of the mandible. The posterior belly (venter posterior), 

 longer than the anterior, arises from; the digastric groove on the inner side of the 

 mastoid process of the temporal bone, and passes downward, forward, and inward. 

 The anterior belly (venter anterior) arises from a depression on the inner side 

 of the lower border of the mandible, close to the symphysis, and passes downward 

 and backward. The two bellies terminate in the central tendon which per- 

 forates the Stylohyoid, and is held in connection with the side of the body and 

 the greater cornu of the hycyd bone by a fibrous loop, lined with a synovial mem- 

 brane. A ' ' iver ' '" ": : 'ie tendon of the Digastric 

 on each side cornu of the hyoid bone ; 

 this is termed ong layer of fascia between 

 the anterior ; estment for the other muscles 

 of the supra K\ 



The Digastric nic of the neck into two smaller 



triangles; the submaxii) ver border of the body of the 



mandible, and cess; below, by the posterior 



