THE SUPRA-HYOID REGION, 321 



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



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

 into two smaller triangles ; the two posterior ones being the posterior superior or 

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

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



Eelations. By its superficial surface, with the Trapezius, the Sterno-mastoid, 

 deep cervical fascia, Platysma, and integument ; by its deep surface, with the 

 Scaleni muscles, phrenic nerve, lower cervical nerves, Avhich go to form the brachial 

 plexus, the suprascapular vessels and nerve, sheath of the common carotid artery 

 and internal jugular vein, the 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 

 and communicans hypoglossi. 



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 backward and to one or the 

 other side. It is concerned especially in prolonged inspiratory efforts ; for by 

 tensing the lower part of the cervical fascia it lessens the inward suction of the soft 

 parts, which would otherwise compress the great vessels and the apices of the lungs. 

 The Thyro-hyoid may act as an elevator of the thyroid cartilage when the hyoid 

 bone ascends, drawing upward the thyroid cartilage, behind the os hyoides. The 

 Sterrio-thyroid acts as a depressor of the thyroid cartilage. 



3. Supra-hyoid Region (Figs. 202, 203). 

 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, 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 consists of two fleshy bellies united by an intermediate, rounded 

 tendon. It is a small muscle, situated below the side of the body of the lower 

 jaw, and extending, in a curved form, from the side of the head to the symphysis 

 of the jaw. The posterior belly, 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 arises from a depression 

 on the inner side of the lower border of the jaw, close to the symphysis, and 

 passes downward and backward. The two bellies terminate in the central 

 tendon which perforates the Stylo-hyoid, and is held in connection with the side 

 of the body and the greater cornu of the hyoid bone by a fibrous loop, lined by a 

 synovial membrane. A broad aponeurotic layer is given off from the tendon of 

 the Digastric on each side, which is attached to the body and great cornu of the 

 hyoid bone : this is termed the supra-hyoid aponeurosis. It forms a strong layer 

 of fascia between the anterior portion of the two muscles, and a firm investment 

 for the other muscles of the supra-hyoid region which lie deeper. 



The Digastric muscle divides the anterior superior triangle of the neck into 

 two smaller triangles ; the upper, or submaxillary, being bounded, above, by the 

 lower border of the body of the jaw, and a line drawn from its angle to the 

 mastoid process ; below, by the posterior belly of the Digastric and the Stylo- 

 hyoid muscles ; in front, by the middle line of the neck and the anterior belly of the 

 Digastric, the lower or superior carotid triangle being bounded above by the poste- 

 rior belly of the Digastric, behind by the Sterno-mastoid, below by the Omo-hyoid. 

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