184 ELEVATORS OF THE OS HYOIDES. 



brachial plexus, the scaleni muscles, the phrenic nerve, the sheath of 

 the common carotid artery and jugular vein, the descendens noni nerve, 

 the sterno-thyroid, and thyro-hyoid muscle, and the sterno-hyoid at 

 its insertion. The scapular portion of the muscle divides the great 

 posterior triangle into a superior or occipital triangle ; and an inferior 

 or subdavian triangle, which contains the subclavian artery and bra- 

 chial plexus of nerves ; the other two boundaries of the latter being 

 the sterno-mastoid in front and the clavicle below. The hyoid por- 

 tion of the muscle, divides the great anterior triangle into an inferior 

 carotid triangle situated below the muscle, and into a superior triangle 

 which lies above the muscle and is again subdivided by the digas- 

 tricus into the submaxillary triangle and the superior carotid triangle. 

 The other two boundaries of the inferior carotid triangle, are the 

 middle line of the neck in front and the anterior border of the sterno- 

 mastoid behind. The other boundaries of the superior carotid triangle 

 are the posterior belly of the digastricus muscle above and the ante- 

 rior border of the sterno-mastoid behind. 



Actions. The four muscles of this group are the depressors of 

 the os hyoides and larynx. The three former drawing these parts 

 downwards in the middle line, and the two omo-hyoidei regulating 

 their traction to the one or other side of the neck, according to the 

 position of the head. The omo-hyoid muscles by means of their con- 

 nection with the cervical fascia are rendered tensors of that portion of 

 the deep cervical fascia which covers in the lower part of the neck, 

 between the two sterno-mastoid muscles. 



Third Group. Elevators of the Os Hyoides. 

 Digastricus, 

 Stylo-hyoid, 

 Mylo-hyoid, 

 Genio-hyoid, 

 Genio-hyo -glossus. 



Dissection. These are best dissected by placing a high block be- 

 neath the neck, and throwing the head backwards. The integument 

 has been already dissected away, and the removal of the cellular tissue 

 and fat brings them clearly into view. 



The DIGASTRICUS (S/ 5 , twice, yxa-r^ belly) is a small muscle situ- 

 ated immediately beneath the side of the body of the lower jaw ; it is 

 fleshy at each extremity, and tendinous in the middle. It arises from 

 the digastric fossa, upon the inner side of the mastoid process of the 

 temporal bone, and is inserted into a depression on the inner side of 

 the lower jaw, close to the symphysis. The middle tendon is held in 

 connection with the body of the os hyoides by an aponeurotic loop, 

 through which it plays as through a pulley ; the loop being lubricated 

 by a synovial membrane. A thin layer of aponeurosis is given off 

 from the tendon of the digastricus at each side, which is connected 

 with the body of the os hyoides and forms a strong plane of fascia 



