THE SUPRA-HYOID REGION. 413 



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 comruunicans 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 the act of sucking, and is also a tensor 

 of the cervical fascia. 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. 1 The Sterno-thyroid acts as a depressor of the thyroid 

 cartilage. 



3. Supra-hyoid Region (Figs. 276. 277). 

 ELEVATORS OF THE Os HYOIDES DEPRESSORS OF THE LOWER JAW. 



/ Digastric. Mylo-hyoid. 



Stylo-hyoid. < renio-hyoid. 



Dissection. To dissect these muscles a Jalock 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 syrnphysis 

 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 belli/ arises from a depression 

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

 - 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-hvoid 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 anterior belly of the Digastric, the lower or 

 superior <:<ir<Ai<l ///<///</? being bounded above by the posterior belly of the Digas- 

 tric, behind by the Sterno-mastoid, below by the Omo-hyoid. 



Relations. By it- /'<// surface, with the Platysma, Sterno-mastoid. part 



of the Splenius. Trachelo-mastoid, and Stylo-hyoid muscles, and the parotid gland. 

 By its d>:-> /' turfa^e. the anterior belly lies on the Mylo-hyoid : the posterior belly 

 on the Stylo-glossus. Stylo-pharyngeus, and Hvo-glossus muscles, the external 

 carotid artery and its lingual and facial branches, the internal carotid artery, 

 internal jugular vein, and hypoglossal nerve. 



The Stylo-hyoid is a small, slender muscle, lying in front of, and above, the 

 posterior belly of the Digastric. It arises from the back and outer surface of the 

 styloid process, near the base; and. passing downward and forward, is inserted 

 into the body of the hyoid bone, just at its junction with the greater cornu. and 



1 It is this action of the Thyro-hyoid muscle which, as Dr. Buchanan has pointed out, " causes or 

 permits the folding back of the epiglottis over the upper orifice of the larynx." (Joui-n. of Anat. and 

 Physs. 2d series, No. III. p. 255;. 



