ELEVATORS OF THE OS HYOIDES. 189 



triangle into an inferior carotid triangle situated below the muscle, and 

 into a superior triangle which lies above the muscle and is again subdi- 

 vided by the digastricus 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 tri- 

 angle are the posterior belly of the digastricus muscle above and the an- 

 terior 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 connexion 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, Genio-hyoid, 



Stylo-hyoid, Genio-hyo-glossus. 



Mylo-hyoid, 



Dissection. These are best dissected by placing a high block beneath 

 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 (5/g, twice, yaov^, belly) is a small muscle situated 

 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 connexion 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 between the anterior portions of the two muscles. 

 This fascia is called the supra-hyoidean. 



Relations. By its superficial surface with the platysma myoides, the 

 sterno-mastoid, the anterior fasciculus of the stylo-hyoid muscle, the 

 parotid gland, and submaxillary gland. By its deep surface with the 

 styloid muscles, the hyo-glossus, the mylo-hyoid muscle, the external 

 carotid artery, the lingual and the facial arteries, the internal carotid artery, 

 the jugular vein, and the hypoglossal nerve. The digastric muscle forms 

 the two inferior boundaries of the submaxillary triangle, the superior boun- 

 dary being the side of the body of the lower jaw. In the posterior half 

 of the submaxillary triangle are situated the submaxillary gland and the 

 facial artery. 



The STYLO-HYOIDEUS is a small and slender muscle situated in imme- 

 diate relation with the posterior belly of the digastricus muscle, being 

 pierced by its tendon. It arises from the middle of the styloid process, 

 and is inserted into the body of the os hyoides near the middle line. 



